feat: add multi-genome SNP pseudo-alignment and CLI export

Introduces a `SnpAlignment` struct and helper methods to construct per-genome SNP pseudo-alignments from sibling k-mer data, filtering monomorphic families and encoding bases as IUPAC ambiguity codes. Exposes the type at the crate root for simplified imports. Adds a `--snp` CLI flag to compute and export these alignments as an IUPAC-coded FASTA file. Updates theory documentation to propose a multi-genome framing approach for joint phylogenetic inference, resolving pairwise correspondence ambiguities through positional homology and partial coverage thresholds. Bumps crate version to 1.1.40.
feat: add raw SNP distance calculation and CLI flag
2026-08-10 22:38:38 +02:00 · 2026-08-10 22:17:07 +02:00 · 2026-08-10 17:53:52 +02:00 · 2026-08-10 15:31:04 +02:00 · 2026-08-10 15:01:59 +02:00 · 2026-08-10 12:38:35 +02:00
22 changed files with 2578 additions and 33 deletions
@@ -9,6 +9,7 @@ data-stress
 ./**/*.json
 *.bin
 *.log
+*.csv
 Betula_exilis--IGA-24-33
 benchmark/genomes
 benchmark/simulated_data
@@ -92,18 +92,48 @@ For each genome:

 | Flag | Applies to | Meaning |
 |------|-----------|---------|
-| `--min-count N` | ingroup | k-mer present in at least N ingroup genomes |
-| `--max-count N` | ingroup | k-mer present in at most N ingroup genomes |
+| `--min-count N` | ingroup | k-mer present in at least N ingroup genomes (N may be negative, see below) |
+| `--max-count N` | ingroup | k-mer present in at most N ingroup genomes (N may be negative, see below) |
 | `--min-frac F` | ingroup | k-mer present in at least fraction F of ingroup genomes |
 | `--max-frac F` | ingroup | k-mer present in at most fraction F of ingroup genomes |
-| `--min-outgroup-count N` | outgroup | k-mer present in at least N outgroup genomes |
-| `--max-outgroup-count N` | outgroup | k-mer present in at most N outgroup genomes |
+| `--min-outgroup-count N` | outgroup | k-mer present in at least N outgroup genomes (N may be negative, see below) |
+| `--max-outgroup-count N` | outgroup | k-mer present in at most N outgroup genomes (N may be negative, see below) |
 | `--min-outgroup-frac F` | outgroup | k-mer present in at least fraction F of outgroup genomes |
 | `--max-outgroup-frac F` | outgroup | k-mer present in at most fraction F of outgroup genomes |
 | `--min-total-count N` | all genomes | sum of per-genome counts ≥ N (`filter` only) |
 | `--max-total-count N` | all genomes | sum of per-genome counts ≤ N (`filter` only) |
 | `--presence-threshold N` | all | per-genome count > N to be considered "present" (default 0) |

+### Negative counts — offset from group size
+
+The four integer count flags (`--min-count`, `--max-count`, `--min-outgroup-count`,
+`--max-outgroup-count`) accept **negative** values, interpreted as an offset counted
+down from the group size `n`, resolved at run time once `n` is known:
+
+| Value | Effective threshold |
+|-------|---------------------|
+| `N ≥ 0` | literal absolute count `N` |
+| `-x` (x > 0) | `max(1, n − x)` — "all but x" |
+
+`-1` literally means *all but one*, `-2` *all but two*, and so on. This expresses
+a quorum relative to the group size that a plain fraction cannot state exactly
+(e.g. "present in every genome except at most one" is `n−1`, which is `0.9` for
+`n = 10` but `0.857…` for `n = 7`).
+
+The threshold is **floored at 1**, never 0: the negative form always keeps
+constraining the group. Without the floor, `--min-count -1` on a singleton
+ingroup (`n = 1`) would resolve to `0` ("at least 0") and silently drop the
+constraint; the floor makes it `1` ("present in that one genome") instead.
+
+To express a count of `0` (e.g. "absent from the ingroup"), use the literal `0`,
+not a negative — `0` and `-0` are indistinguishable, so the offset form starts at
+`-1`.
+
+> **Edge case** — on an *empty* group (`n = 0`, e.g. a predicate matching no
+> genome), a negative count still resolves to `1`, an impossible constraint that
+> rejects every k-mer. This is consistent with an empty group letting nothing
+> through, but differs from the "no constraint" behaviour of the fraction flags.
+
 **Conditional defaults** — the defaults for `--min-frac` and `--max-outgroup-count` depend on two conditions:
 whether the corresponding group was declared, **and** whether any quorum flag for that group was explicitly set.

@@ -215,6 +245,17 @@ obikmer filter src --output dst \
  --max-outgroup-count 0
 ```

+Noise-tolerant core — keep k-mers present in *all but one* ingroup genome
+(`-1` = `n−1`) and absent from *all but one* of the outgroup:
+
+```sh
+obikmer filter src --output dst \
+  --ingroup  "genus=Betula" \
+  --outgroup "*" \
+  --min-count -1 \
+  --max-outgroup-count -1
+```
+
 To dump only k-mers specific to *Betula nana*:

 ```sh
@@ -347,11 +347,24 @@ Provided finalisations:
 | `relfreq_euclidean_dist_matrix()` | `√partial_relfreq_euclidean[i,j]` |
 | `hellinger_dist_matrix()` | `√partial_hellinger[i,j] / √2` |
 | `hellinger_euclidean_dist_matrix()` | `√partial_hellinger[i,j]` |
+| `threshold_mash_dist_matrix(k, t)` | Mash distance, derived from `threshold_jaccard_dist_matrix(t)` — no separate partial |

 ### BitPartials

 Required: `partial_jaccard() -> (Array2<u64>, Array2<u64>)`, `partial_hamming() -> Array2<u64>`. Both additive across layers and partitions.

+Provided finalisations also include `jaccard_dist_matrix()`, `hamming_dist_matrix()`, and `mash_dist_matrix(k)`.
+
+### Mash distance
+
+`mash_dist_matrix`/`threshold_mash_dist_matrix` add no new additive primitive: both are a pointwise transform of the existing Jaccard distance matrix, per the Mash mutation-rate estimator [@Mash-distances-doc; @Fan2015-mash-formula]:
+
+```
+D = -1/k · ln(2J / (1+J)),  J = 1 - d_jaccard
+```
+
+`J ≤ 0` (i.e. `d_jaccard ≥ 1`, no shared k-mers) maps to `D = 1` (maximal distance) rather than the `ln` singularity at `J = 0`.
+
 ---

 ## Temp-file-backed types
@@ -13,7 +13,7 @@
 | `query`     | Query an index with sequences and annotate matches |
 | `dump`      | Dump all indexed k-mers as CSV (kmer + per-genome counts or presence); supports the shared [kmer filtering](implementation/filtering.md) system; `--head N` limits output to the first N k-mers |
 | `annotate`  | Add or update genome metadata from a CSV file; or dump metadata as CSV |
-| `distance`  | Compute pairwise distance matrix between genomes; optionally build NJ/UPGMA trees; `--presence-threshold N` sets the minimum count to consider a k-mer present when computing Jaccard on count indexes (default 1) |
+| `distance`  | Compute pairwise distance matrix between genomes (`--metric jaccard\|mash\|hamming\|bray-curtis\|relfreq-bray-curtis\|euclidean\|relfreq-euclidean\|hellinger\|hellinger-euclidean`); optionally build NJ/UPGMA trees; `--presence-threshold N` sets the minimum count to consider a k-mer present when computing Jaccard/Mash on count indexes (default 1) |
 | `unitig`    | Build a global de Bruijn graph across all partitions and enumerate its unitigs as FASTA; supports the shared [kmer filtering](implementation/filtering.md) system |
 | `select`    | Project and/or aggregate genome columns into a new or in-place index; the column-axis counterpart of `filter` (see [select](implementation/select.md)) |
 | `estimate`  | Estimate approximate-index parameters (z, evidence bits, FP rates) before indexing |
@@ -241,3 +241,21 @@
 	volume = 33,
 	year = 2017,
 	bdsk-url-1 = {http://dx.doi.org/10.1093/bioinformatics/btw832}}
+
+@misc{Mash-distances-doc,
+	author = {{Marbl Lab}},
+	howpublished = {Mash documentation},
+	title = {Mash Distance},
+	url = {https://mash.readthedocs.io/en/latest/distances.html},
+	urldate = {2026-07-09},
+	year = 2026}
+
+@article{Fan2015-mash-formula,
+	author = {Fan, Huan and Ives, Anthony R and Surget-Groba, Yann and Cannon, Charles H},
+	doi = {10.1186/s12864-015-1647-5},
+	journal = {BMC Genomics},
+	number = 1,
+	title = {An assembly and alignment-free method of phylogeny reconstruction from next-generation sequencing data},
+	url = {https://doi.org/10.1186/s12864-015-1647-5},
+	volume = 16,
+	year = 2015}
@@ -0,0 +1,820 @@
+# Central-position SNP distance (discussion)
+
+Not implemented. Design discussion for a substitution-rate estimator that
+observes SNPs directly from paired-genome k-mer comparison, as an alternative
+to Mash's Poisson-Jaccard inference (see [obicompactvec](../implementation/obicompactvec.md)
+for the implemented Jaccard/Mash distances).
+
+## Motivation
+
+**Primary intent: restrict the comparison to what is actually comparable.**
+Mash's Jaccard is computed over the **union** of both genomes' k-mer content:
+anything not identically shared is folded into a single undifferentiated
+mass, whether the cause is a point substitution, a genuinely absent
+homologous region (lineage-specific content, gene-family expansion, HGT,
+genome-size asymmetry), or a diverged paralogous copy. The model then
+back-infers a single mutation rate from that mass, silently attributing
+non-homology to mutation. The central-SNP approach instead conditions every
+comparison on local, positive evidence of homology: a locus only enters the
+statistic if its `2m` flanking bases (`m = (k-1)/2`) are found intact in
+*both* genomes — genuinely absent or non-homologous content is excluded from
+the comparison entirely (neither numerator nor denominator), rather than
+silently counted as divergence. This is a conditioning on comparability, not
+just a richer summary statistic — see "Statistic and correspondence with
+`shared`" below for how it plays out against genome-size asymmetry and
+diverged gene families, and "Heterozygosity, ploidy, and consensus-assembly
+inputs" for the corresponding paralogy/heterozygosity filter.
+
+**Secondary benefit: access to the substitution's nature.** Because the
+central base of an odd-k window is directly observable once the flanks are
+confirmed conserved, this also yields more than a rate — the
+transition/transversion split — enabling classical corrected distances
+(Jukes-Cantor, Kimura 2-parameter, LogDet) that a single Jaccard scalar
+cannot support.
+
+## Statistic and correspondence with `shared`
+
+A genomic position `p` is covered by `k` overlapping k-mer windows. Requiring
+the substitution to sit at the window's **center** makes exactly one window
+per SNP eligible — a 1:1 correspondence between SNP and center-neighbor k-mer
+pair, avoiding the ~k-fold overcount of an any-position neighbor search.
+
+A locus with a fully conserved `k`-window (flanks **and** center) is an
+exact-shared k-mer at that locus; a locus with conserved flanks but a
+substituted center is a "central SNP". Both count each locus exactly once, in
+matching units:
+
+```
+p_hat[i,j] = SNP[i,j] / (SNP[i,j] + shared[i,j])
+```
+
+`p_hat` is `P(center substituted | 2m flanks conserved)`. `shared[i,j]` here
+is **not** the general-purpose `shared_kmers` matrix used by Jaccard/Mash
+(`--shared-kmers`, `BitPartials::partial_jaccard` /
+`CountPartials::partial_threshold_jaccard`) — that matrix counts raw k-mer
+identity with no per-genome copy-number constraint, whereas `p_hat`'s
+denominator applies the eligibility rule defined below (raw or
+paralogy-filtered). Both `SNP` and `shared` are accumulated by the same
+sweep, from the same per-locus candidate set (source k-mer + 3 variants),
+under the same eligibility rule — see "Locus eligibility" below, and
+"Heterozygosity, ploidy, and consensus-assembly inputs" for why the
+copy-number constraint matters and what it costs.
+
+**Canonical invariance**: for odd k, the central position maps to itself under
+reverse-complement (`m -> k-1-m = m`, base complemented). A transition maps to
+a transition, a transversion to a transversion — the transition/transversion
+split is well-defined in canonical space.
+
+### Definitions: family, and the canonical form of a family
+
+**Family.** The family of a k-mer `x` is the set of (up to) 4 k-mers sharing
+`x`'s `2m` flanking bases, differing only at the central base `m`. Membership
+is a property of the flank pattern, not of `x` itself: any of the 4 possible
+central substitutions belongs to the same family.
+
+**`central_canonical_neighbors()`** (`obikseq`, `CanonicalKmerOf::central_canonical_neighbors`)
+generates all 4 members from any one of them (observed or not), each
+independently canonicalised (`.canonical()`, i.e. `min(kmer, revcomp(kmer))`).
+This independent canonicalisation is necessary because a central substitution
+can flip which orientation is lexicographically smaller — two members of the
+same family can end up canonicalised in *different* orientations. Despite
+that, the **set** of 4 resulting canonical k-mers is invariant: calling
+`central_canonical_neighbors()` on any member of a family — present in the
+index or not — yields the same 4 values. This is relied upon throughout the
+rest of this document.
+
+**Canonical form of a family.** Because orientation can differ member to
+member, "which of the 4 is the reference" cannot be defined relative to
+*whichever member happened to be visited first*, nor relative to the
+minorant (see below) — both are data-dependent (they depend on what is
+actually observed), so using either as the reference would make the
+reference itself vary depending on what happens to be present in a given
+index. Instead: **the canonical form of a family is, by definition, the
+member whose own central base — read in its own already-canonical
+orientation — is `A`.** This is well-defined for every family, computed
+purely from the flank pattern, whether or not that specific member (or any
+member at all) is actually observed anywhere in the index. Concretely: call
+`central_canonical_neighbors()` on any member (observed or not) to get the
+family's 4 canonical forms; the one among them whose own centre nucleotide is
+`A` is the family's canonical form. The other 3 (`C`, `G`, `T`) are labelled
+relative to *that* fixed reference, not relative to the calling member's own
+orientation.
+
+**Consequence for the minorant.** With this fixed A-referenced labelling,
+`minorant` (the smallest raw encoding among the family's *observed* members,
+introduced further below) becomes directly computable rather than needing to
+be tracked as extra state: regenerate the family's 4 canonical forms from
+any member's own k-mer (cheap, no lookup), compare the raw encodings of
+whichever are marked present, and take the smallest. No separate stored bit
+is required — see Step 2b below, where this replaces the earlier
+minorant-bit design.
+
+## Locus eligibility: raw definition vs. paralogy filter
+
+For each k-mer `x` observed in genome A (source, one MPHF slot; the 3
+central-position variants generated as in the sweep below): check whether
+A's locus (flanks fixed) is resolvable in genome B under one of the 4
+central forms.
+
+**Raw / no model.** The locus counts in the denominator iff at least one of
+the 4 forms is present in B; it counts in the numerator iff the form found in
+B differs from A's own. No constraint on A's or B's own copy number at this
+locus. Open question, not resolved: what if **more than one** of the 4 forms
+is present in B simultaneously (ambiguous target — count once arbitrarily,
+count all, or drop)? The stringent filter below sidesteps the question by
+construction rather than answering it.
+
+**Stringent / paralogy-aware.** The locus counts only if exactly one of the
+4 forms is present in A **and** exactly one is present in B (`count == 1` at
+that slot too, when a count index is available, to also exclude same-allele
+duplicates that presence alone cannot see). This drops the raw definition's
+ambiguous-B case automatically, at the cost of also dropping heterozygous
+sites indiscriminately alongside true duplications (see "Heterozygosity,
+ploidy, and consensus-assembly inputs" below).
+
+**Rejected: parsimony-based multiset pairing for multiplicity > 1.** Rather
+than dropping ambiguous loci, pair identical alleles between A and B first
+(0-mutation explanation preferred), then take `min(unmatched_A, unmatched_B)`
+as inferred SNP pairs. Rejected on two grounds: (1) circularity — selecting
+pairs by minimal apparent divergence, then measuring divergence on those same
+pairs, deflates the estimate by construction, not a neutral heuristic; (2)
+the discriminating signal is a single base among 4 possible values, and the
+flanks are *already* guaranteed identical for every candidate by
+construction (that is how the locus was selected) — no information remains
+in a k-mer window to tell which copy in B truly corresponds to which copy in
+A once multiplicity > 1 on either side. Any pairing rule invents a
+correspondence the data cannot support. Multiplicity > 1 is treated as
+non-identifiable, not as a puzzle to solve with a heuristic.
+
+## Multi-genome framing: family as pseudo-alignment column
+
+**Idea.** Instead of resolving locus eligibility and correspondence one
+genome pair at a time, treat a family as a column of a pseudo multiple
+alignment across *all* genomes simultaneously: for each family, each genome
+has either a net single-copy state (`A`/`C`/`G`/`T`, when the genome carries
+exactly one of the 4 forms) or "missing" (`?`, multi-copy or absent). Flank
+conservation (the `2m` bases fixed by construction) supplies positional
+homology for free — the same role a real MSA would play, without alignment
+software, gap penalties, or progressive-alignment approximations. Stacking
+one such column per family, genomes as rows, produces a genuine SNP
+pseudo-alignment matrix, not just a bag of pairwise distances.
+
+**Precedent.** This is the same principle behind reference-free
+k-mer-based phylogenomics tools — SKA (Split K-mer Analysis, Harris 2018) and
+kSNP: split the k-mer around a variable center, use flank identity to call
+homologous columns across arbitrarily many genomes with no reference and no
+MSA step, then feed the resulting pseudo-alignment to standard phylogenetic
+tools. Landing on the same design independently is a good sign, not a
+coincidence.
+
+**Resolves the pairwise-correspondence problem, properly.** The "Rejected:
+parsimony-based multiset pairing" case above failed because, with only two
+genomes' cardinalities to look at, there is no external constraint to justify
+picking one correspondence between leftover alleles over another — `min(a,b)`
+is a lower bound dressed up as a point estimate (see the follow-up discussion
+on Felsenstein-style parsimony inconsistency: minimum-event explanations are
+systematically biased low whenever homoplasy/multiplicity is real, not
+noise-cancelling). With `N` genomes and many families jointly, the same
+question can be answered the way real phylogenetics answers it: ancestral
+state reconstruction / ML mapping over a tree estimated from the whole
+column set. The tree supplies the missing constraint that two isolated
+columns cannot — this is the principled way out, not a heuristic replacement
+for one.
+
+**Relation to what's already implemented.** `KmerIndex::raw_snp_distance`
+already computes, internally, per family, exactly this row — `single_form:
+Vec<Option<u8>>`, one entry per genome, `None` where ambiguous/absent —
+before immediately collapsing it into pairwise `snp[i,j]`/`shared[i,j]`
+tallies. The pivot this section proposes is small at the implementation
+level: stop collapsing early, and surface the per-family row as a first-class
+artifact (a `families x genomes` matrix). Pairwise raw p-distance becomes one
+projection of that matrix (what's computed today), not the primary object;
+downstream, the matrix itself could feed real phylogenetic tools (parsimony/
+ML, e.g. RAxML/IQ-TREE-style) instead of only NJ/UPGMA on a homemade
+pairwise-distance matrix.
+
+**Caveat: column completeness shrinks with `N`.** The probability that a
+family's flanks stay intact simultaneously across all `N` genomes decays with
+`N` (same ascertainment-bias mechanism as Bias 1 above, compounded over more
+genomes) — fully-resolved columns (no `?` anywhere) become rare as more
+genomes are added. Same missing-data situation any real multi-species
+alignment faces, and phylogenetic tools already handle it well; the practical
+implication is that columns should be allowed partial coverage (>=2 resolved
+genomes, not unanimous) rather than requiring every genome to be net
+single-copy at that locus.
+
+## Heterozygosity, ploidy, and consensus-assembly inputs
+
+A within-genome multiplicity signal (more than one of the 4 central forms
+present at a locus) is produced identically by two distinct causes:
+paralogous duplication and diploid/polyploid heterozygosity. K-mer data alone
+cannot distinguish them. The one real discriminator is sequencing depth
+(heterozygous site: total depth of the present forms ~= the genome's
+single-copy average; duplication: ~2x or more) — but that signal only exists
+if genome "counts" are raw-read depth (FASTQ input), not occurrence counts in
+an assembled FASTA, where per-locus depth is not preserved.
+
+**Magnitude is taxon- and mating-system-dependent, not universal.**
+Heterozygosity density: mammals ~1 site / 1-1.5 kb (~0.1%); highly
+outcrossing plants (maize, poplar) reported an order of magnitude higher
+(~1%); self-fertilising plants (*Arabidopsis thaliana*) near zero — but with
+a documented failure mode where segmental duplication masquerades as
+"pseudo-heterozygosity"; fungi split between haploid vegetative stages
+(non-issue) and dikaryotic Basidiomycetes, where two long-diverged haploid
+nuclei coexist without fusing. The estimator's target use case (closely
+related genomes, k=31) is exactly where the stringent filter above costs the
+least for low-heterozygosity taxa and the most for outcrossing/dikaryotic
+ones — no universal threshold; this is a scope caveat to document, not a
+problem to solve generically.
+
+**Why assembled-consensus inputs don't make measured distances wrong.**
+Phylogenetic inputs are near-universally assemblies, not raw reads, and
+assemblers collapse heterozygous sites to one consensus allele per
+position — effectively an arbitrary, largely uncorrelated-between-assemblies
+choice at each het site. This does not inject unbounded noise: standard
+population genetics gives `d_xy = d_a + (pi_A + pi_B)/2` — the expected
+pairwise difference between a random allele of population A and a random
+allele of population B equals the net (fixed) divergence `d_a` plus the
+average of the two populations' own within-population diversity `pi`.
+Consensus flattening realises exactly this random-allele draw, so the
+measured genome-to-genome distance is a `d_xy`-like quantity, not `d_a` —
+inflated by heterozygosity by a well-characterised additive term, not
+distorted unpredictably. The term is negligible when `pi << d_xy` (the common
+case for cross-species comparisons), and becomes material precisely in the
+two cases already flagged above: very closely related genomes (this
+estimator's explicit target) and highly heterozygous outcrossing organisms,
+where `pi` and `d_xy` are the same order of magnitude.
+
+Caveat: this assumes the flattening is uncorrelated with the phylogenetic
+signal — plausible for de novo assembly, not guaranteed for reference-guided
+assembly biased toward one allele (e.g. the reference's) at each het site,
+which would turn the noise term into a systematic bias toward the reference
+lineage. Not evaluated here.
+
+**Forward-looking implication, not part of the current design.** The
+multiplicity > 1 signal discarded by the stringent filter is a crude
+per-genome proxy for `pi` (under low background paralogy). If a `pi_hat` per
+genome were tallied alongside `SnpTally`, a `d_a` correction
+(`p_hat - mean(pi_hat_i, pi_hat_j)/2`, roughly) could recover an estimate
+closer to net divergence instead of `d_xy` — a possible extension, not
+scoped here.
+
+## Sufficient statistic: 4x4 base-pair tally
+
+Tabulating the joint distribution of `(center_i, center_j)` over conserved-flank
+loci, per genome pair, is sufficient for every downstream correction:
+
+| Estimator | Input | Formula |
+|---|---|---|
+| Raw p-distance | total off-diagonal / total | `p = SNP / (SNP + shared)` |
+| Jukes-Cantor | p | `d = -3/4 * ln(1 - 4p/3)` |
+| Kimura 2-parameter | transition rate P, transversion rate Q | `d = 1/2 ln(1/(1-2P-Q)) + 1/4 ln(1/(1-2Q))` |
+| LogDet/paralinear | full 4x4 + base-composition margins | `d ~= -1/4 ln det(F)`, robust to non-stationary base composition |
+
+JC/K2P need only the total and the transition/transversion split (the
+diagonal collapses to a single "shared" total). LogDet needs the full 4x4,
+already populated at no extra cost (see Step 1/2 below).
+
+Memory for the 4x4 tally: `n^2 * 16` counters. Trivial for the project's
+genome-scale use case (tens to hundreds of genomes); ~13 GB at n=10^4 — outside
+scope but worth flagging if n grows.
+
+## Biases (properties of the estimator, not defects)
+
+1. **Conserved-flank ascertainment bias.** Only SNPs with intact `2m`-base
+   flanks are visible; window-intact probability decays as `(1-p)^{2m}`. For
+   k=31 (2m=30): 0.74 at p=1%, 0.21 at p=5%, 0.04 at p=10%. This estimator
+   targets **closely related genomes**. Under rate heterogeneity across sites
+   (universal in practice), conserved flanks correlate with slow centers, so
+   `p_hat` underestimates the genome-wide average rate — it specifically
+   estimates the substitution rate of **conserved regions**.
+   Two distinct factors are at play here, not one: `P(centre of a given
+   window is a SNP) = p` exactly, **independent of k** — a direct restatement
+   of the raw per-site rate via the bijective window<->centre-position
+   correspondence (Statistic section above), not a k-dependent quantity.
+   `(1-p)^{2m}` is the *separate*, genuinely k-dependent ascertainment factor
+   (are the flanks also intact). The two multiply:
+   `P(usable window showing a central SNP) = p * (1-p)^{2m}` — e.g. at
+   p=1/31 (~3.2%), k=31: `p * (1-p)^30 ~= 0.0323 * 0.374 ~= 1.2%`, i.e. about
+   1 window in 83, not 1 in 31 (which is only the centre-mutated fraction,
+   before requiring intact flanks).
+2. **Bias toward isolated SNPs.** Two SNPs within k of each other disqualify
+   each other's flanks. Hypervariable regions are invisible by construction.
+3. **Indels are invisible.** A frameshift destroys k-mer matches in a block;
+   this channel captures substitutions only. Indel divergence shows up as lost
+   shared k-mers (lower Jaccard/Mash), not as SNP signal.
+4. **k-dependent specificity.** "A k-mer match implies common ancestry" is
+   quantitative. For a 3 Gbp genome, expected random flank-30 collisions
+   (k=31): `(3e9)^2 / 4^30 ~= 8` — negligible. At k=21: `(3e9)^2 / 4^20 ~= 2e6`
+   — no longer negligible. k=31 is safe; k<=21 is marginal to unreliable for
+   large genomes. The large k that guarantees homology is the same k that
+   shrinks the detectable-divergence window — an inherent tension.
+
+## Implementation: avoid materializing a de Bruijn graph
+
+A central-SNP pair is topologically a simple bubble in the colored de Bruijn
+graph (source/sink k-mer shared, two length-k branches differing only at the
+midpoint). Classical bubble-calling (Cortex/discoSNP-style) finds these, but
+requires the graph — nodes plus adjacency for ~10^9 colored k-mers — resident
+in memory. **Rejected**: prohibitive RAM for this project's scale.
+
+A naive per-pair generalisation of variant lookup across n genomes (query each
+non-shared k-mer's 3 central variants against every counterpart genome's
+index) costs `O(n^2 . N . 3)` random lookups, with the same k-mer's 3 variants
+regenerated and requeried once per counterpart genome — pure redundant work.
+**Rejected** as the basis for an n-genome design.
+
+## Implementation: sequential per-partition sweep (no scratch, no graph)
+
+`KmerIndex::distance()` already opens every partition's `presence_store`/
+`count_store` simultaneously, memory-mapped, into one `LayeredStore`
+(`distance.rs:73-77`). "Querying another partition" is therefore not a new
+I/O pattern to design — it is the same O(1) MPHF+evidence lookup the `query`
+command already performs at scale. This lets the SNP tally be computed with
+**no scratch files and no auxiliary graph**, by sweeping partitions once each
+as a source:
+
+1. For source partition `p`, enumerate its **distinct** k-mers (one per MPHF
+   slot; each already carries its full multi-genome presence/count vector —
+   no need to explode per (k-mer, genome) occurrence).
+2. For each, generate the 3 central-substitution variants and **canonicalise
+   each independently** (`min(kmer, revcomp)`, exactly as any normal query) —
+   this avoids the orientation edge case a masked-flank grouping would have
+   (a substitution that flips canonical orientation is handled correctly
+   because each variant is canonicalised on its own, not inferred from a
+   fixed-orientation flank key).
+3. Compute each variant's target partition `q` via its minimizer; batch/sort
+   the partition's outgoing variant queries by `q` for locality.
+4. Look up each variant in `q`'s already-mmap'd MPHF+evidence; on a hit,
+   combine the source's presence vector (base `a`) with the variant's
+   presence vector (base `b`): for every `i` carrying `a` and `j` carrying
+   `b`, `tally[i,j][a,b] += 1`.
+
+**Deduplication needs no persisted state.** Sweeping partitions in a fixed
+order `p = 0, 1, ..., P-1` and only acting on a variant when its target
+partition `q >= p` guarantees each unordered SNP pair is counted exactly
+once: a pair with `q < p` was already resolved earlier, when `q` was itself
+the source partition and `p` (being `>= q`) was a valid forward target. No
+cross-partition flag array is needed — the sweep order *is* the
+deduplication rule. Within the same partition (`q == p`), a lightweight
+transient tie-break suffices: either a `#slots(p)`-bit scratch flag reset per
+partition, or simply comparing the two k-mers' raw `u64` encodings and only
+counting when `kmer_source < kmer_variant` — no storage at all.
+
+This is a distinct computation stage, not a `partial_*` in the existing
+additive-by-partition sense: step 3-4 read across partition boundaries by
+construction, unlike the row-local `partial_jaccard`/`partial_threshold_jaccard`
+primitives. But it requires no new index files, permanent or scratch:
+`unitigs.bin`, `mphf.bin`, `evidence.bin`, and the presence/count columns are
+read as-is, and the only extra memory is the current partition's small
+outgoing-query batch (`#kmers(p) * 3`, released once `p` is done) plus the
+persistent `tally` accumulator (`n^2 * 16` counters, see above).
+
+**Outer loop (over source partitions `p`) must stay sequential.** Two
+independent reasons, not just one: (a) memory — the bounded-footprint claim
+above only holds with one partition's outgoing-query batch in flight; running
+`T` source partitions concurrently multiplies that batch by `T`, exactly the
+blowup the design avoids; (b) correctness — the `q >= p` deduplication rule
+requires partitions to be claimed as sources in a fixed order; running `p1 <
+p2` concurrently gives no guarantee `p1` has finished claiming its `q >= p1`
+targets before `p2` starts claiming its own, breaking the "counted exactly
+once" property.
+
+**Inner loop (target-partition lookups for a fixed `p`) parallelises safely.**
+Each lookup is an O(1) read against an already-mmap'd structure, independent
+of the others, with no growing allocation — no memory blowup, no ordering
+dependency between different `q`. The only shared mutable state is `tally`;
+give each worker thread a **thread-local partial tally** (fixed `n^2 * 16`
+size, independent of partition size) and merge into the global `tally` once
+`p`'s inner loop completes — the same reduce-then-merge pattern Rayon already
+uses elsewhere in this codebase to open partitions in parallel. Extra memory:
+`#threads * n^2 * 16`, negligible (~512 MB at n~1000, 32 threads) and
+unrelated to partition size.
+
+**Cost**: `3 * N_distinct` MPHF lookups total across the whole index (each
+partition swept once as source) — the same order of magnitude and the same
+operation as running `query` over the index's entire k-mer content against
+itself, three times. This is the tool's already-optimized regime, not a new
+I/O profile to validate.
+
+## Cheaper: subsampling
+
+Since the target is a ratio, restricting the source-partition sweep to a
+bottom-`s` hash sketch (only enumerate k-mers with `hash < threshold` as
+sources) divides the lookup count by the sampling factor without biasing
+`p_hat`. Mash-like tradeoff: rate estimated from a sample, not the full
+k-mer set.
+
+## Recommendation
+
+Sequential per-partition sweep (Route D): reuse the already-mmap'd
+per-partition MPHF/evidence/presence structures for O(1) variant lookups,
+dedup via the fixed sweep-order rule (`q >= p`, plus an in-partition
+tie-break), no scratch files, no graph materialisation. Both the SNP
+(off-diagonal) and shared (diagonal) counts are accumulated by this same
+sweep, under the locus-eligibility rule chosen (raw or paralogy-filtered) —
+not reused from the general-purpose `shared_kmers` matrix, whose raw-identity
+definition does not apply the same copy-number constraint. Distances (p, JC,
+K2P, LogDet) as finalisations of the resulting 4x4 tally, mirroring the
+`partial_* -> *_dist_matrix` pattern used for Jaccard/Mash/Bray-Curtis/etc.
+
+## Detailed implementation plan
+
+Grounded in the current codebase. File/type references are anchors, not
+prescriptions; adjust to reality when implementing.
+
+### Step 0 — new low-level primitives (`obikseq`)
+
+Two helpers do not yet exist and are prerequisites:
+
+1. **Central neighbours.** `CanonicalKmerOf<L>` already exposes
+   `left_canonical_neighbors()` / `right_canonical_neighbors()`
+   (`obikseq/src/kmer.rs`), each returning the 4 canonicalised neighbours at
+   an end position. Add `central_canonical_neighbors()` returning the 4
+   variants at position `m = (k-1)/2` (each independently canonicalised via
+   `.canonical()`). The 3 that differ from the source are the query variants;
+   skip the identity. Building on `nucleotide(i)` / the raw 2-bit layout keeps
+   it O(1).
+2. **Lone-k-mer minimiser.** Routing a *synthetic* variant to its partition
+   needs its minimiser, but `RollingStat` (`obiskbuilder/src/rolling_stat.rs`)
+   only computes minimisers incrementally along a sequence. Add a standalone
+   `minimizer(kmer) -> Minimizer` that scans the `k-m+1` m-mer windows
+   (`PackedSeq::mmer`, `obikseq/src/packed_seq.rs`), canonicalises each, and
+   takes the min by `seq_hash()` — the same selection `RollingStat` performs,
+   evaluated once. Partition index is then
+   `(minimizer.seq_hash() & (n_partitions - 1)) as usize`, exactly as
+   `QueryBatch::from_records` (`obikmer/src/cmd/query.rs:142`); `n_partitions`
+   is a power of two so the mask is valid.
+
+### Step 1 — the tally accumulator (`obikindex`)
+
+A `SnpTally` holding, per genome pair, the 4x4 joint count of central bases:
+`n * n * 4 * 4` `u64` (or a packed lower-triangular form since it is
+symmetric). Provide `merge(&mut self, other: &SnpTally)` for the thread-local
+reduce, and accessors yielding, per pair `(i,j)`: total off-diagonal (SNP),
+diagonal (shared, i.e. `p_hat`'s denominator minus SNP), transition count
+`P`, transversion count `Q`. The diagonal is always populated — it is not an
+optional LogDet-only extra, since `p_hat`'s denominator is no longer sourced
+from the external `shared_kmers` matrix (see "Locus eligibility" and
+"Statistic and correspondence with `shared`" above): the source k-mer's own
+presence/count vector, already in hand when it is enumerated, supplies the
+diagonal entry directly, at no extra lookup cost.
+
+### Step 2 — the sweep (`obikindex`, new `snp.rs`)
+
+Mirror `distance.rs`: open the presence or count store per partition. But
+instead of a per-partition `partial_*`, run the sequential source sweep:
+
+```text
+for p in 0..n_partitions:                       # OUTER — sequential
+    open source partition p's layers (QueryLayer-style, obikpartitionner)
+    enumerate distinct canonical k-mers of p (one per MPHF slot) with their
+        presence/count vectors                   # column-major, as query stage 2
+    par_iter over these source k-mers:           # INNER — rayon, thread-local tally
+        apply eligibility rule to the source's own vector (raw: none;         # diagonal
+            stringent: exactly one of the 4 forms present in each genome)     # gate
+        for i in genomes eligible with source base a:
+            for j in genomes eligible with source base a:
+                thread_tally[i,j][a,a] += 1        # diagonal — no extra lookup
+        for each of the 3 central variants:
+            q = partition_of(variant)
+            if q < p: continue                    # dedup: forward targets only
+            if q == p and variant <= source.raw(): continue   # in-partition tie-break
+            slot = layers[q].find_slot(variant)   # MphfLayer::find, mmap'd
+            if hit:
+                vb = variant presence/count vector
+                apply eligibility rule to vb (as above)
+                for i in eligible genomes with source base a:
+                    for j in eligible genomes with variant base b:
+                        thread_tally[i,j][a,b] += 1
+    merge thread-local tallies into global SnpTally
+```
+
+The inner lookup is precisely `QueryLayer::find_slot` +
+`col_value(g, slot)` (`obikpartitionner/src/query_layer.rs`) — reuse or factor
+out that path rather than reimplementing MPHF access. Enumerating "all distinct
+k-mers of a partition with their vectors" is the `dump`/`query` stage-2
+column-major scan already implemented in `dump_layer.rs` /
+`query_partition_with`; factor a reusable iterator if none fits.
+
+`presence_threshold` applies exactly as elsewhere: a genome "carries base b"
+iff its count at that slot is `>= presence_threshold` (trivially `>= 1` for
+presence indexes).
+
+### Open problem (unresolved, session end — not yet fully convinced)
+
+The `q >= p` / tie-break dedup rule in Step 2's pseudocode above is **flawed**
+for the stringent (paralogy-filtered) eligibility rule: it only ever brings
+two family members into view at once (the source and one looked-up variant),
+never all four simultaneously, and which subset gets compared depends on
+partition sweep order. "Exactly one of the 4 forms present in genome A" is a
+whole-family property and cannot be decided correctly from a sequence of
+pairwise, order-dependent glimpses — the pseudocode above needs revision, not
+just the eligibility gate bolted onto it as written.
+
+Direction discussed, **not yet settled**:
+
+1. **Every distinct source k-mer looks up all 3 variants unconditionally**
+   (drop the `q < p` skip entirely) so that every observed family member
+   independently gathers all 4 vectors (its own + whichever of the 3
+   variants exist) at once — a whole-family, order-independent view, computed
+   redundantly once per observed member. Same total lookup order of
+   magnitude as already budgeted (`3 * N_distinct`), just organised
+   differently (no lookup actually skipped, versus the original rule which
+   skipped roughly half).
+2. **Tie-break after gathering, not before**: only the member whose own
+   canonical encoding is the smallest *among the members actually observed*
+   (now known, since all were just looked up) writes to `SnpTally`; the
+   others silently discard their redundant computation. Deterministic,
+   order-independent — as a side effect this also removes the "outer loop
+   must stay sequential" constraint from the cost/parallelism discussion
+   above, since no step depends on partition processing order any more.
+3. **Proposed optimisation**: precompute, once at index build time, a
+   compact global (not per-genome) annex per MPHF slot — the count of
+   *other* family members observed anywhere in the dataset (0-3). Slots with
+   count 0 (majority under low divergence and few genomes, but see the
+   scaling caveat below) need no cross-lookup at all: eligibility reduces to
+   a local `count == 1` check at that single slot, and only slots with count
+   >= 1 enter the 3-lookup sweep machinery above. Revised (see Step 2b
+   below): minorant status *is* stored alongside the count after all, on 3
+   bits rather than 2 — it comes for free from the same lookups needed to
+   count siblings, and storing it lets the sweep discard non-minorant slots
+   without re-fetching anything.
+
+**Minorant/sibling-count relationship, worked out precisely.** "Minorant" is
+a one-way implication from sibling count, not an equivalence: `0 siblings
+=> minorant` (trivially — with no other observed member, the k-mer is by
+definition the smallest of the observed set, itself alone), and its
+contrapositive `not minorant => >= 1 sibling`. The converse does not hold:
+being the minorant says nothing about sibling count — a minorant can have 0,
+1, 2 or 3 siblings, all with larger encodings than itself. Consequence: this
+confirms, as a logical necessity rather than a heuristic, that a 0-sibling
+slot can always write its diagonal contribution with zero ambiguity and no
+lookup (it is unconditionally its own minorant) — but it gives no shortcut
+for the >= 1-sibling case, where minorant status still requires the actual
+comparison of gathered encodings; sibling count alone never determines it.
+
+**When to compute the annex, and cache invalidation.** Sibling count is a
+property of the whole set of columns (genomes/groups) currently in the
+index, not of any single genome — it cannot be computed correctly at
+mono-genome build time (a family may gain siblings, or its minorant may
+change, once more genomes are merged in later). Computing it eagerly at
+every `merge` would also waste work on intermediate merged states nobody
+ever queries. Instead: compute it lazily, on first `distance` call against a
+given index, and persist the result alongside that index for subsequent
+calls — the same lazy-derived-cache pattern `PersistentBitMatrix` already
+uses for `Columnar` -> `Packed`. This requires no explicit invalidation for
+`merge` or `filter` (`obikindex/src/merge.rs`, `obikmer/src/cmd/filter.rs`):
+both only ever write to a fresh `--output` directory, never mutate an input
+index in place, so a re-merged/re-filtered index is simply a new state with
+no annex yet. `select --in-place` (`select_layer.rs:139-235`) is the
+exception: it aggregates genome columns into groups (Any/All/None/Sum/Min/
+Max) by mutating the existing index's files without changing its location.
+It does not remove k-mer rows, but it can still change eligibility and
+sibling counts derived from those rows (e.g. a `Sum` over several
+single-copy genomes can read as multi-copy at the group level). Because it
+mutates in place, **`select --in-place` must explicitly invalidate (delete
+or mark stale) any cached sibling-count annex for that index** — the one
+operation in the current pipeline where this doesn't happen for free.
+
+Not yet convinced this is the right shape, and Step 2's pseudocode above has
+not been rewritten to match — flagged for the next pass rather than resolved
+here.
+
+### Step 2b — sibling-count / minorant annex (consolidated plan)
+
+Scope: only the precursor annex — not the SNP tally itself, whose Step 2
+sweep remains unresolved above. This piece is simpler than the sweep,
+because it writes to an independent per-slot value, not a shared
+cross-k-mer accumulator, so it needs no dedup/ownership logic at all at this
+stage.
+
+**Revised annex encoding — 4-bit presence mask, not 3-bit (minorant +
+count).** Superseded after settling the "canonical form of a family"
+definition above. The 3-bit design (1 minorant bit + 2-bit sibling count,
+§ below, kept for the historical record) had two problems: it discards
+*which* variants are present (only how many), so any future consumer
+(the SNP sweep, or a stats pass — see below) that needs to know which bases
+exist still has to regenerate and blindly re-query all 3 candidates; and
+the minorant bit's meaning was tied to whichever member was visited, not to
+a fixed reference. Storing instead a **4-bit mask** — one bit per base
+(A/C/G/T), set iff that member of the family (labelled relative to the
+family's fixed canonical form, i.e. the member with `A` at the centre — see
+above) is observed anywhere in the index — fixes both:
+- **Sibling count is derived, not stored**: `siblings = popcount(mask) - 1`.
+- **Minorant is derived, not stored**: regenerate the family's 4 canonical
+  forms from the slot's own k-mer (cheap, no lookup — see above), compare
+  the raw encodings of whichever bits are set in the mask, take the
+  smallest.
+- **A future consumer knows exactly which variants to (re-)query** —
+  `popcount(mask) - 1` lookups instead of always 3, and it knows *which*
+  3 (or fewer) to issue, not just how many hits to expect.
+- The all-zero value (no base present at all) is still logically
+  unreachable as a real result — the slot's *own* base is always present in
+  its own family — so it remains available as a free "not yet computed"
+  sentinel, exactly as before.
+
+1. **Primitive.** Reuse `central_canonical_neighbors()` from Step 0
+   unchanged — the 3 canonicalised central-substitution variants of a k-mer
+   (plus the identity, i.e. all 4 members of the family — see "Definitions"
+   above).
+2. **New annex type** (`obicompactvec`, alongside `bitmatrix.rs`): a 4-bit-
+   per-slot packed array (the presence mask above), one per partition — same
+   on-disk shape family as `PersistentBitMatrix`'s `Packed` variant, but
+   simpler (no per-genome columns, a single derived read-only value per
+   slot).
+   <details><summary>Superseded 3-bit design (historical)</summary>
+   3 bits, storing minorant status alongside sibling count directly, since
+   it came for free from the same lookups (point 3 below) — 5 real states
+   (not-minorant; minorant with 0/1/2/3 siblings) fit in 3 bits (8 states,
+   3 unused). This let the future SNP sweep discard a non-minorant slot
+   instantly, with no lookup at all. The otherwise-unreachable combination
+   "not-minorant + 0 siblings" (0 siblings always implies minorant) doubled
+   as the "not yet computed" sentinel. Replaced by the 4-bit mask above,
+   which subsumes this benefit (minorant still derivable, now for free at
+   read time rather than stored) while also fixing the "which variant"
+   blindness.
+   </details>
+3. **Computation pass** (`obikindex`, new `siblings.rs`): **one
+   `obipipeline` run per layer, iterated sequentially over the index's
+   layers** — settled after two false starts, worth recording both.
+   - *False start 1*: "fully parallel over every partition/slot at once,
+     no ordering at all". Correctness is fine with this (sibling count and
+     minorant are order-independent, unlike the old `q >= p` dedup they
+     replace), but it reintroduces, at a larger scale, exactly the
+     memory-blowup the original Step 2 sweep's sequential-outer-loop
+     constraint existed to prevent: scattering every source partition at
+     once multiplies the in-flight outgoing-query volume by the number of
+     partitions.
+   - *False start 2*: push the layer loop itself into the pipeline (source
+     = the index's layers, a first `Flat` stage expands each layer into
+     its k-mers). `obipipeline`'s scheduler already bounds memory on its
+     own — it dispatches every item through a **shared** worker pool at
+     each stage boundary (`scheduler.rs:217-372`, `dispatch()` into a
+     common `worker_tx` queue, any free worker picks up any pending item;
+     not "one worker owns a chunk end to end"), with a biased `Select`
+     that prioritises draining items already advanced in the chain over
+     admitting new source items (`scheduler.rs:271-282`: stage results
+     outrank the source, "vider le pipeline en priorité" / "dernier
+     recours" for new data) — so bounded channel `capacity` plus this
+     drain-first bias already caps in-flight work without any external
+     sequential discipline. Correct, but it means k-mers from several
+     layers can be completing concurrently, so the sink would need to
+     track several open per-layer annex-file writers at once — real,
+     avoidable complexity.
+   - **Settled design**: keep the layer loop external and sequential —
+     not for memory (the pipeline's own `capacity`/priority mechanism
+     already provides that, for free, regardless), but so each pipeline
+     run's sink targets exactly one layer's annex file, no concurrent
+     multi-writer bookkeeping. Per layer: source = that layer's distinct
+     k-mers; a `Flat` (1->N) stage generates the 3 central variants of a
+     k-mer, each tagged with its origin (local slot); a transform stage
+     routes each variant to its target partition (unchanged per-k-mer
+     minimiser); a transform stage performs the lookup (existence-only —
+     `find_slot` hit/miss, cheaper than the SNP sweep's full column
+     fetch); a final stage/sink folds each answer into its origin's
+     running state (below) and, once a layer's k-mers are all resolved,
+     flushes the completed array to that layer's annex file. Many small,
+     single-purpose stages on purpose, to let the scheduler interleave
+     them finely across many in-flight items — this deliberately does
+     **not** mirror how `obipipeline` is used elsewhere today: `query.rs`'s
+     `process_chunk` lumps parse+route+query+serialise into one closure
+     (`query.rs:325,743-758`), and `scatter.rs` only pipelines file-
+     reading/superkmer construction, routing partitions afterwards in a
+     plain sequential loop (`KmerPartition::write_batch`,
+     `partition.rs:140`) — both under-use the fine-grained scheduling the
+     mechanism offers, so they are not precedents to copy, only existing
+     (and arguably improvable, out of scope here) usages. Cross-partition
+     lookups (querying another layer's MPHF for a variant) remain
+     necessary as before — only the *output* side is kept single-layer.
+   - **Reconciliation**: processed at the granularity of one *answer batch
+     per destination partition*, not one source k-mer at a time — this is a
+     proper shuffle, not a per-k-mer wait. Each source partition `p` holds a
+     small array of running states `(minorant = true, siblings = 0)`, one
+     per local slot, initialised at scatter time and **persisting across
+     however many destination-partition batches answer it** (up to 3, one
+     per variant, not necessarily all from the same `q`). Every scattered
+     query carries an origin tag (source partition + local slot) so its
+     answer can be routed back. When target partition `q` returns its batch
+     (all answers for every query that named `q`, regardless of which source
+     k-mer or which source partition they came from), that batch is walked
+     once, locally, and each answer updates — via its origin tag — the
+     matching entry in *its* source partition's array: a miss changes
+     nothing; a hit does `siblings += 1`, and if the found sibling's own
+     encoding is smaller than the source's, `minorant = false`. A given
+     source k-mer's state is final only once every destination batch
+     concerning it has been folded in; its partition's array is flushed to
+     the persistent annex once complete. Commutative per entry, so the order
+     in which destination batches arrive and get folded in doesn't matter.
+
+   **Open optimisation, not adopted yet — real tradeoff, not a free win.**
+   Since looking up sibling `y` from `x`'s visit already yields everything
+   needed to fill `y`'s own annex entry too, one visit per *family* could in
+   principle replace one visit per *observed family member* — cutting this
+   pass's cost roughly by the average family size instead of paying
+   `3 * N_distinct` regardless. But it means threads processing different
+   source k-mers can end up writing the *same* sibling's slot concurrently —
+   the fully independent, ownership-free parallelism of the plan above is
+   deliberately traded away for this gain. It stays safe only because the
+   computed value for a given slot is deterministic regardless of who
+   computes it, so redundant concurrent writes converge to the same
+   value — correct as long as each write is atomic, no locking needed — but
+   it is a real design complexity increase over "every member redoes its
+   own 3 lookups independently," not a strict improvement to adopt by
+   default.
+4. **Trigger and caching** (`obikindex::KmerIndex`/`distance.rs`): compute
+   lazily on first `distance` call for an SNP-family metric against a given
+   index; check for an existing annex file first (mirrors
+   `PersistentBitMatrix::open()`'s auto-detect-and-fall-back,
+   `bitmatrix.rs:264-287`); if absent, run step 3 and persist; if present,
+   mmap and reuse.
+5. **Invalidation.** `merge` and `filter` always write to a fresh `--output`
+   directory (`obikindex/src/merge.rs`, `obikmer/src/cmd/filter.rs`) so a
+   re-merged/re-filtered index simply has no annex yet — nothing to
+   invalidate. `select --in-place` (`select_layer.rs:139-235`) mutates
+   columns of an existing index without changing its location, which can
+   change sibling counts without removing rows — it must explicitly delete
+   any cached annex for that index as part of its in-place rewrite.
+6. **Testing**: hand-built tiny indexes with known sibling counts (0-3);
+   order-independence (recompute twice on a static index, identical
+   result, given the fully-parallel no-ownership design); invalidation
+   (annex absent/correctly recomputed after `select --in-place`); once
+   Step 2's sweep is fixed, a regression check that sibling_count == 0
+   slots are never looked up cross-partition during the sweep.
+
+Cost: `3 * N_distinct` existence-only lookups, computed once per index
+state and amortised over every subsequent `distance` call that reuses the
+cached annex — cheaper per-lookup than the sweep itself (hit/miss only, no
+column fetch).
+
+### Step 3 — finalisation (`obikindex`)
+
+From the global `SnpTally` alone (diagonal and off-diagonal both populated by
+the sweep, see Step 1/2 — no dependency on the external `shared_kmers`
+matrix), derive n x n distance matrices, each a pure function of the
+accumulated counts (same shape as `jaccard_to_mash`):
+
+- `p_hat[i,j] = SNP / (SNP + shared)`
+- Jukes-Cantor, Kimura-2P (from `P`, `Q`), optionally LogDet (needs the
+  diagonal + base-composition margins).
+
+Guard the singularities (`p >= 3/4` for JC, `1-2P-Q <= 0` or `1-2Q <= 0` for
+K2P) by clamping to a max distance, as `jaccard_to_mash` clamps `J <= 0`.
+
+### Step 4 — surfacing (`obikindex` + `obikmer` CLI)
+
+These metrics do not fit `DistanceMetric`'s current `LayeredStore`-partial
+dispatch (they need the cross-partition sweep and produce a different
+intermediate). Two options, to decide:
+
+- **(a)** New `DistanceMetric` variants (`Pdistance`, `JukesCantor`,
+  `Kimura2P`, `LogDet`) whose `KmerIndex::distance` arm calls the sweep
+  (`snp.rs`) instead of the partial path, still returning `DistanceOutput`.
+  Keeps one CLI surface (`--metric jukes-cantor`), at the cost of a branch in
+  `distance()` that ignores the `LayeredStore` it built.
+- **(b)** A dedicated pathway (`KmerIndex::snp_distance`) and a distinct CLI
+  entry, if mixing a cross-partition sweep into the partition-local `distance`
+  command is judged architecturally muddy.
+
+Recommendation: (a) for user ergonomics (all pairwise distances under
+`distance`, all feeding NJ/UPGMA/`--shared-kmers` unchanged), but compute the
+sweep lazily only when an SNP-family metric is requested, so the existing
+metrics keep their partition-local fast path untouched.
+
+### Step 5 — subsampling flag
+
+Add `--snp-sample <fraction>` (or a bottom-`s` hash threshold): restrict the
+source-k-mer enumeration in Step 2 to `seq_hash(kmer) < threshold`. Divides
+lookups proportionally; `p_hat` is unbiased. Off by default (exact).
+
+### Testing
+
+- **Primitive unit tests**: `central_canonical_neighbors` on hand-checked
+  k-mers incl. palindrome-boundary cases; lone-k-mer `minimizer` against
+  `RollingStat`'s incremental result on the same k-mer.
+- **End-to-end tiny index**: two 1-genome indexes differing by a handful of
+  known isolated SNPs (transitions and transversions placed by hand), assert
+  exact `SNP`, `P`, `Q` counts and the resulting JC/K2P values.
+- **Dedup invariant**: assert the tally is identical regardless of genome/
+  partition order and that no pair is double-counted (compare against a
+  brute-force all-pairs reference on a small index).
+- **Subsampling**: `p_hat` within sampling error of the exact run.
+
+### Suggested phasing
+
+1. Step 0 primitives + their unit tests (self-contained, no distance wiring).
+   This also unblocks the long-declared-but-unimplemented `query --mismatch`
+   (`obikmer/src/cmd/query.rs:676`, currently a warning), which needs the same
+   neighbour + routing machinery.
+2. `SnpTally` + finalisation math with a brute-force (non-swept) reference
+   backend, validated on a tiny index.
+3. The real per-partition sweep (Step 2) behind the same finalisation; assert
+   it matches the brute-force backend.
+4. CLI surfacing (Step 4a) and NJ/UPGMA integration (already generic over the
+   matrix).
+5. Subsampling (Step 5).
+
+## References
+
+The Mash mutation-rate model this discussion contrasts with:
+[@Mash-distances-doc; @Fan2015-mash-formula].
@@ -36,6 +36,7 @@ nav:
      - Entropy filter: theory/entropy.md
      - Minimizer selection: theory/minimizer.md
      - Partitioning architecture: theory/indexing.md
+      - Central-position SNP distance (discussion): theory/evolutionary_distances.md
  - Implementation:
      - SuperKmer: implementation/superkmer.md
      - Kmer: implementation/kmer.md
@@ -1701,17 +1701,21 @@ dependencies = [
 "obikpartitionner",
 "obikseq",
 "obilayeredmap",
+ "obipipeline",
+ "obiread",
+ "obiskbuilder",
 "obiskio",
 "obisys",
 "rayon",
 "serde",
 "serde_json",
+ "tempfile",
 "tracing",
 ]

 [[package]]
 name = "obikmer"
-version = "1.1.39"
+version = "1.1.40"
 dependencies = [
 "clap",
 "csv",
@@ -7,6 +7,7 @@ mod intmatrix;
 mod layer_meta;
 mod meta;
 mod reader;
+mod siblingannex;
 mod tempbitvec;
 mod tempintvec;
 mod views;
@@ -18,6 +19,7 @@ pub use builder::PersistentCompactIntVecBuilder;
 pub use colgroup::{ColGroup, FilterMask, MatrixGroupOps, eval_filter_mask};
 pub use intmatrix::{PersistentCompactIntMatrix, PersistentCompactIntMatrixBuilder, pack_compact_int_matrix};
 pub use layer_meta::LayerMeta;
+pub use siblingannex::{FamilyMask, SiblingAnnex, SiblingAnnexBuilder};
 pub use reader::{PersistentCompactIntVec, Iter as CompactIntVecIter};
 pub use tempbitvec::{TempBitVec, TempBitVecBuilder};
 pub use tempintvec::{TempCompactIntVec, TempCompactIntVecBuilder};
@@ -0,0 +1,245 @@
+//! Family presence-mask annex: a compact, read-only-after-build, per-slot
+//! derived value used by the central-position SNP distance estimator (see
+//! `docmd/theory/evolutionary_distances.md`, "Step 2b" and "Definitions:
+//! family, and the canonical form of a family").
+//!
+//! One byte is stored per MPHF slot of a partition/layer, its low 4 bits
+//! encoding a **presence mask** for the slot's k-mer's "family" (the up to 4
+//! k-mers sharing the same flanks, differing only at the central base):
+//! bit `b` (`b` = 0..3, in the fixed A/C/G/T = 0/1/2/3 encoding already used
+//! for a single nucleotide) is set iff the family member whose *own* central
+//! base — in its own canonical orientation — is `b`, is observed anywhere in
+//! the current multi-genome index. This is a property of the whole index,
+//! not of any one genome.
+//!
+//! Both facts the earlier (superseded) 3-bit design stored explicitly are
+//! derived from the mask instead, not stored:
+//! - sibling count = `popcount(mask) - 1`;
+//! - minorant = regenerate the family's 4 canonical forms from the slot's
+//!   own k-mer (`CanonicalKmerOf::central_canonical_neighbors`, cheap, no
+//!   lookup), compare the raw encodings of whichever are set in the mask,
+//!   take the smallest — see `obikindex::siblings`.
+//!
+//! Mask value 0 is logically unreachable as a real result (a slot's own base
+//! is always present in its own family) and is reused as the "not yet
+//! computed" sentinel: annex files are pre-initialised to all-zero, and a
+//! real value is only ever written once, by the computation pass.
+//!
+//! Deliberately simpler than a true 4-bit pack (1 byte/slot instead of 4
+//! bits/slot): correctness and simplicity first, for a first implementation.
+//! Packing to 4 bits/slot is a pure storage-density follow-up, not a
+//! behavioural change, left for later.
+
+use std::fs::{File, OpenOptions};
+use std::io;
+use std::path::{Path, PathBuf};
+
+use memmap2::{Mmap, MmapMut};
+
+const MAGIC: [u8; 4] = *b"PSIB";
+
+// Header: magic(4) + _pad(4) + n(8) = 16 bytes. Data (1 byte/slot) follows.
+const HEADER_SIZE: usize = 16;
+
+/// A family presence mask: bit `b` set iff the member whose own canonical
+/// central base is `b` (0=A, 1=C, 2=G, 3=T) is observed in the index.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct FamilyMask(u8);
+
+impl FamilyMask {
+    /// The empty mask — never a valid *computed* result (a slot's own base
+    /// is always present in its own family) — used only to build up a mask
+    /// via repeated [`with`](Self::with) calls before storing it.
+    pub const EMPTY: FamilyMask = FamilyMask(0);
+
+    /// Set bit `base` (0=A, 1=C, 2=G, 3=T).
+    #[inline]
+    pub fn with(self, base: u8) -> Self {
+        debug_assert!(base < 4, "base out of range: {base}");
+        FamilyMask(self.0 | (1 << base))
+    }
+
+    /// Is the member with central base `base` (0..3) present?
+    #[inline]
+    pub fn has(self, base: u8) -> bool {
+        debug_assert!(base < 4, "base out of range: {base}");
+        self.0 & (1 << base) != 0
+    }
+
+    /// Number of family members observed anywhere in the index (1..=4).
+    #[inline]
+    pub fn family_size(self) -> u32 {
+        self.0.count_ones()
+    }
+
+    /// Number of *other* members observed (0..=3) — `family_size() - 1`.
+    #[inline]
+    pub fn siblings(self) -> u32 {
+        self.family_size() - 1
+    }
+
+    /// Raw bitmask (bit `b` = base `b` present) — for callers that build up
+    /// a mask via their own bit operations (e.g. concurrently, via an
+    /// `AtomicU8`) and only need the `FamilyMask` wrapper at the end.
+    #[inline]
+    pub fn bits(self) -> u8 {
+        self.0
+    }
+
+    /// Construct from a raw bitmask (only the low 4 bits are kept).
+    #[inline]
+    pub fn from_bits(bits: u8) -> Self {
+        FamilyMask(bits & 0b1111)
+    }
+
+    #[inline]
+    fn encode(self) -> u8 {
+        self.0
+    }
+
+    #[inline]
+    fn decode(byte: u8) -> Option<Self> {
+        if byte == 0 {
+            // Unreachable for a real result — reserved as the "not yet
+            // computed" sentinel.
+            return None;
+        }
+        Some(FamilyMask(byte & 0b1111))
+    }
+}
+
+// ── SiblingAnnex (reader) ───────────────────────────────────────────────────
+
+pub struct SiblingAnnex {
+    mmap: Mmap,
+    n: usize,
+    path: PathBuf,
+}
+
+impl SiblingAnnex {
+    pub fn open(path: &Path) -> io::Result<Self> {
+        let mmap = unsafe { Mmap::map(&File::open(path)?)? };
+        if mmap.len() < HEADER_SIZE {
+            return Err(io::Error::new(io::ErrorKind::InvalidData, "PSIB file too short"));
+        }
+        if mmap[0..4] != MAGIC {
+            return Err(io::Error::new(io::ErrorKind::InvalidData, "bad PSIB magic"));
+        }
+        let n = u64::from_le_bytes(mmap[8..16].try_into().unwrap()) as usize;
+        if mmap.len() < HEADER_SIZE + n {
+            return Err(io::Error::new(io::ErrorKind::InvalidData, "PSIB file truncated"));
+        }
+        Ok(Self { mmap, n, path: path.to_path_buf() })
+    }
+
+    pub fn path(&self) -> &Path { &self.path }
+    pub fn len(&self) -> usize { self.n }
+    pub fn is_empty(&self) -> bool { self.n == 0 }
+
+    /// `None` means the slot has not (yet) been computed — see module docs.
+    pub fn get(&self, slot: usize) -> Option<FamilyMask> {
+        FamilyMask::decode(self.mmap[HEADER_SIZE + slot])
+    }
+}
+
+// ── SiblingAnnexBuilder (writer) ────────────────────────────────────────────
+
+pub struct SiblingAnnexBuilder {
+    mmap: MmapMut,
+    n: usize,
+    path: PathBuf,
+}
+
+impl SiblingAnnexBuilder {
+    /// Create a new annex of `n` slots at `path`, pre-initialised to the
+    /// "not yet computed" sentinel (all-zero).
+    pub fn new(n: usize, path: &Path) -> io::Result<Self> {
+        let file_size = HEADER_SIZE + n;
+        let file = OpenOptions::new()
+            .read(true).write(true).create(true).truncate(true)
+            .open(path)?;
+        file.set_len(file_size as u64)?;
+        let mut mmap = unsafe { MmapMut::map_mut(&file)? };
+        mmap[0..4].copy_from_slice(&MAGIC);
+        mmap[4..8].copy_from_slice(&[0u8; 4]);
+        mmap[8..16].copy_from_slice(&(n as u64).to_le_bytes());
+        // Data region left at 0 by `set_len`/mmap — the sentinel value.
+        Ok(Self { mmap, n, path: path.to_path_buf() })
+    }
+
+    pub fn len(&self) -> usize { self.n }
+    pub fn is_empty(&self) -> bool { self.n == 0 }
+
+    pub fn get(&self, slot: usize) -> Option<FamilyMask> {
+        FamilyMask::decode(self.mmap[HEADER_SIZE + slot])
+    }
+
+    pub fn set(&mut self, slot: usize, mask: FamilyMask) {
+        // Redundant concurrent writes from independent recomputation paths
+        // converge to the same encoded byte for a given slot, so a plain
+        // store here is safe even without external synchronisation, as long
+        // as the byte write itself is atomic (true for a single aligned
+        // byte on every platform this project targets).
+        self.mmap[HEADER_SIZE + slot] = mask.encode();
+    }
+
+    pub fn close(self) -> io::Result<()> { self.mmap.flush() }
+
+    pub fn finish(self) -> io::Result<SiblingAnnex> {
+        let path = self.path.clone();
+        self.close()?;
+        SiblingAnnex::open(&path)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tempfile::tempdir;
+
+    #[test]
+    fn sentinel_is_zero_and_unset_slots_read_as_uncomputed() {
+        let dir = tempdir().unwrap();
+        let path = dir.path().join("test.psib");
+        let builder = SiblingAnnexBuilder::new(4, &path).unwrap();
+        for slot in 0..4 {
+            assert_eq!(builder.get(slot), None);
+        }
+        builder.close().unwrap();
+    }
+
+    #[test]
+    fn roundtrip_all_valid_masks() {
+        let dir = tempdir().unwrap();
+        let path = dir.path().join("test.psib");
+        let mut builder = SiblingAnnexBuilder::new(4, &path).unwrap();
+
+        let masks = [
+            FamilyMask::EMPTY.with(0),                   // just A: family size 1
+            FamilyMask::EMPTY.with(0).with(3),            // A + T: size 2
+            FamilyMask::EMPTY.with(1).with(2).with(3),    // C+G+T: size 3
+            FamilyMask::EMPTY.with(0).with(1).with(2).with(3), // all 4
+        ];
+        for (slot, mask) in masks.iter().enumerate() {
+            builder.set(slot, *mask);
+        }
+        let annex = builder.finish().unwrap();
+        for (slot, mask) in masks.iter().enumerate() {
+            assert_eq!(annex.get(slot), Some(*mask));
+        }
+        assert_eq!(annex.get(0).unwrap().siblings(), 0);
+        assert_eq!(annex.get(1).unwrap().siblings(), 1);
+        assert_eq!(annex.get(2).unwrap().siblings(), 2);
+        assert_eq!(annex.get(3).unwrap().siblings(), 3);
+        assert_eq!(annex.get(3).unwrap().family_size(), 4);
+    }
+
+    #[test]
+    fn has_reflects_individual_bits() {
+        let mask = FamilyMask::EMPTY.with(0).with(2);
+        assert!(mask.has(0));
+        assert!(!mask.has(1));
+        assert!(mask.has(2));
+        assert!(!mask.has(3));
+    }
+}
@@ -1,5 +1,16 @@
 use ndarray::{Array1, Array2};

+/// Convert a Jaccard distance matrix (`1 - J`) into a Mash distance matrix, per
+/// https://mash.readthedocs.io/en/latest/distances.html:
+/// `D = -1/k * ln(2J / (1+J))`.
+fn jaccard_to_mash(d_jaccard: &Array2<f64>, k: usize) -> Array2<f64> {
+    d_jaccard.mapv(|d| {
+        let j = 1.0 - d;
+        if j <= 0.0 { 1.0 }
+        else { -1.0 / k as f64 * (2.0 * j / (1.0 + j)).ln() }
+    })
+}
+
 // ── Column-level weight statistic — total count or presence count per column.
 /// Additive across layers and partitions; used as denominator in normalised distances.
 ///
@@ -74,6 +85,12 @@ pub trait CountPartials: ColumnWeights {
        m
    }

+    /// Mash distance (https://mash.readthedocs.io/en/latest/distances.html), derived
+    /// from the presence-threshold Jaccard distance.
+    fn threshold_mash_dist_matrix(&self, k: usize, threshold: u32) -> Array2<f64> {
+        jaccard_to_mash(&self.threshold_jaccard_dist_matrix(threshold), k)
+    }
+
    fn relfreq_bray_dist_matrix(&self) -> Array2<f64> {
        let global = self.col_weights();
        let mut m = self.partial_relfreq_bray(&global).mapv(|v| 1.0 - v);
@@ -126,6 +143,12 @@ pub trait BitPartials: ColumnWeights {
        m
    }

+    /// Mash distance (https://mash.readthedocs.io/en/latest/distances.html), derived
+    /// from the Jaccard distance.
+    fn mash_dist_matrix(&self, k: usize) -> Array2<f64> {
+        jaccard_to_mash(&self.jaccard_dist_matrix(), k)
+    }
+
    fn hamming_dist_matrix(&self) -> Array2<u64> {
        self.partial_hamming()
    }
@@ -10,6 +10,8 @@ obiskio          = { path = "../obiskio" }
 obisys           = { path = "../obisys" }
 obicompactvec    = { path = "../obicompactvec" }
 obilayeredmap    = { path = "../obilayeredmap" }
+obiskbuilder     = { path = "../obiskbuilder" }
+obipipeline      = { path = "../obipipeline" }
 ndarray          = "0.16"
 rayon             = "1"
 crossbeam-channel = "0.5"
@@ -19,6 +21,10 @@ indicatif        = "0.17"
 tracing          = "0.1.44"
 hwlocality       = { version = "1.0.0-alpha.11", features = ["vendored"], optional = true }

+[dev-dependencies]
+obiread  = { path = "../obiread" }
+tempfile = "3"
+
 [features]
 default = ["numa"]
 numa    = ["hwlocality"]
@@ -14,6 +14,8 @@ pub enum DistanceMetric {
    Jaccard,
    /// Hamming distance (number of differing kmer positions) on presence/absence data.
    Hamming,
+    /// Mash distance on presence/absence data (Jaccard-derived mutation-rate estimate).
+    Mash,
    /// Bray-Curtis dissimilarity on raw counts.
    BrayCurtis,
    /// Bray-Curtis dissimilarity normalised by per-genome total counts.
@@ -84,6 +86,7 @@ impl KmerIndex {
                DistanceMetric::Hellinger         => CountPartials::hellinger_dist_matrix(&global),
                DistanceMetric::HellingerEuclidean => CountPartials::hellinger_euclidean_dist_matrix(&global),
                DistanceMetric::Jaccard           => CountPartials::threshold_jaccard_dist_matrix(&global, presence_threshold),
+                DistanceMetric::Mash              => CountPartials::threshold_mash_dist_matrix(&global, self.kmer_size(), presence_threshold),
                DistanceMetric::Hamming           => {
                    return Err(OKIError::InvalidInput(
                        "Hamming is only available for presence/absence indexes".into(),
@@ -108,6 +111,7 @@ impl KmerIndex {

            let matrix = match metric {
                DistanceMetric::Jaccard => BitPartials::jaccard_dist_matrix(&global),
+                DistanceMetric::Mash    => BitPartials::mash_dist_matrix(&global, self.kmer_size()),
                DistanceMetric::Hamming => {
                    BitPartials::hamming_dist_matrix(&global).mapv(|v| v as f64)
                }
@@ -9,6 +9,7 @@ mod numa;
 mod rebuild;
 mod reindex;
 mod select;
+mod siblings;
 mod stats;

 pub use error::{OKIError, OKIResult};
@@ -18,3 +19,4 @@ pub use merge::MergeMode;
 pub use meta::{validate_label, GenomeInfo, IndexConfig, IndexMeta, META_FILENAME};
 pub use state::{IndexState, SENTINEL_COUNTED, SENTINEL_INDEXED, SENTINEL_SCATTERED};
 pub use stats::IndexBitsPerKmer;
+pub use siblings::{RawSnpDistanceOutput, SiblingAnnexStats, SnpAlignment};
@@ -1,6 +1,6 @@
 [package]
 name = "obikmer"
-version = "1.1.39"
+version = "1.1.40"
 edition = "2024"

 [[bin]]
@@ -3,13 +3,15 @@ use std::path::PathBuf;

 use clap::Args;
 use kodama::{Method, linkage};
-use obikindex::{DistanceMetric, KmerIndex};
+use obifastwrite::{JsonVal, write_record};
+use obikindex::{DistanceMetric, KmerIndex, RawSnpDistanceOutput, SiblingAnnexStats, SnpAlignment};
 use speedytree::{DistanceMatrix, Hybrid, NeighborJoiningSolver, to_newick};
 use tracing::info;

 #[derive(clap::ValueEnum, Clone, Copy, Debug)]
 pub enum MetricArg {
    Jaccard,
+    Mash,
    Hamming,
    BrayCurtis,
    #[value(name = "relfreq-bray-curtis")]
@@ -26,6 +28,7 @@ impl From<MetricArg> for DistanceMetric {
    fn from(m: MetricArg) -> Self {
        match m {
            MetricArg::Jaccard            => DistanceMetric::Jaccard,
+            MetricArg::Mash               => DistanceMetric::Mash,
            MetricArg::Hamming            => DistanceMetric::Hamming,
            MetricArg::BrayCurtis         => DistanceMetric::BrayCurtis,
            MetricArg::RelfreqBrayCurtis  => DistanceMetric::RelfreqBrayCurtis,
@@ -62,7 +65,37 @@ pub struct DistanceArgs {
    #[arg(long)]
    pub upgma: bool,

+    /// Build the sibling-count/minorant annex on this (multi-genome) index
+    /// — see `docmd/theory/evolutionary_distances.md`, Step 2b. Construction
+    /// only; does not by itself compute or write any statistics.
+    #[arg(long)]
+    pub sibling_annex: bool,
+
+    /// Tally the sibling-count distribution (CSV) of an already-built annex
+    /// (run with `--sibling-annex` first, in this invocation or an earlier
+    /// one). A separate, occasional diagnostic pass — not run every time the
+    /// annex itself is (re)built.
+    #[arg(long)]
+    pub sibling_stats: bool,
+
+    /// Compute the raw p-distance restricted to loci that are single-copy
+    /// in both genomes of each pair (an already-built sibling annex is
+    /// required — run with `--sibling-annex` first, in this invocation or
+    /// an earlier one). A quick way to test the central-position SNP
+    /// estimator against a real index; not the full `SnpTally` design.
+    #[arg(long)]
+    pub raw_snp_distance: bool,
+
+    /// Write a SNP-only pseudo-alignment (FASTA, IUPAC-coded) from an
+    /// already-built sibling annex — one row per genome, one column per
+    /// variable family (monomorphic families skipped), no flanking
+    /// sequence. See `docmd/theory/evolutionary_distances.md`,
+    /// "Multi-genome framing: family as pseudo-alignment column".
+    #[arg(long)]
+    pub snp: bool,
+
    /// Output prefix: <prefix>_dist.csv, <prefix>_shared.csv,
+    /// <prefix>_siblings.csv, <prefix>_rawsnp.csv, <prefix>_snp.fasta,
    /// <prefix>_nj.nwk, <prefix>_upgma.nwk.
    /// If omitted, the distance matrix is written to stdout.
    #[arg(short, long)]
@@ -78,6 +111,51 @@ pub fn run(args: DistanceArgs) {

    let labels: Vec<String> = idx.meta().genomes.iter().map(|g| g.label.clone()).collect();
    let n = labels.len();
+
+    // ── Sibling-count/minorant annex (independent of the distance metric) ──
+    // Construction (`--sibling-annex`) and stats (`--sibling-stats`) are
+    // deliberately decoupled: the annex is meant to be (re)built routinely,
+    // the distribution only occasionally, on demand.
+    if args.sibling_annex {
+        info!("building sibling-count/minorant annex");
+        idx.build_sibling_annex().unwrap_or_else(|e| {
+            eprintln!("error building sibling annex: {e}");
+            std::process::exit(1);
+        });
+    }
+    if args.sibling_stats {
+        let stats = idx.sibling_annex_stats().unwrap_or_else(|e| {
+            eprintln!("error computing sibling-annex stats: {e}");
+            std::process::exit(1);
+        });
+        write_sibling_stats_csv(&stats, &labels, &args.output);
+    }
+    if args.raw_snp_distance {
+        let result = idx.raw_snp_distance().unwrap_or_else(|e| {
+            eprintln!("error computing raw SNP distance: {e}");
+            std::process::exit(1);
+        });
+        write_raw_snp_distance_csv(&result, &labels, &args.output);
+    }
+    if args.snp {
+        let alignment = idx.snp_pseudo_alignment().unwrap_or_else(|e| {
+            eprintln!("error computing SNP pseudo-alignment: {e}");
+            std::process::exit(1);
+        });
+        write_snp_fasta(&alignment, &labels, &args.output);
+    }
+
+    // `--sibling-annex`/`--sibling-stats`/`--raw-snp-distance`/`--snp` are
+    // their own operation, not a modifier on top of a distance-metric
+    // computation — a metric was never requested by asking for any of them,
+    // so there is nothing for the rest of this function to compute. Not a
+    // historical accident to keep: stop here rather than always also
+    // running a Jaccard (or whichever `--metric` defaults to) pass and
+    // printing an unrequested matrix.
+    if args.sibling_annex || args.sibling_stats || args.raw_snp_distance || args.snp {
+        return;
+    }
+
    info!(
        "computing {:?} distances for {} genome(s)",
        args.metric, n
@@ -189,6 +267,103 @@ pub fn run(args: DistanceArgs) {
    }
 }

+// ── Family-size distribution → CSV ──────────────────────────────────────────
+//
+// Each row is a family (the up-to-4 k-mers sharing flanks, differing only at
+// the centre), counted once — at its minorant — regardless of how many of
+// its members are observed. Family size 1..4 (not "sibling count" 0..3):
+// see `docmd/theory/evolutionary_distances.md`, "Definitions".
+
+fn write_sibling_stats_csv(stats: &SiblingAnnexStats, labels: &[String], output: &Option<PathBuf>) {
+    // One row per genome (4 columns, family size 1-4: number of families of
+    // that size for which the genome carries at least one member), plus a
+    // `global` row — the actual deduplicated family-size histogram
+    // (`stats.counts`), NOT a sum of the per-genome columns (a family shared
+    // by several genomes would otherwise be counted once per genome it
+    // appears in, inflating the total beyond the real family count).
+    let path = output.as_ref()
+        .map(|p| format!("{}_siblings.csv", p.display()))
+        .unwrap_or_else(|| "siblings.csv".into());
+    let mut f = BufWriter::new(std::fs::File::create(&path).unwrap_or_else(|e| {
+        eprintln!("error creating {path}: {e}");
+        std::process::exit(1);
+    }));
+    writeln!(f, "genome,1,2,3,4").unwrap();
+    for (label, counts) in labels.iter().zip(stats.per_genome.iter()) {
+        writeln!(f, "{label},{},{},{},{}", counts[0], counts[1], counts[2], counts[3]).unwrap();
+    }
+    writeln!(
+        f, "global,{},{},{},{}",
+        stats.counts[0], stats.counts[1], stats.counts[2], stats.counts[3],
+    ).unwrap();
+    let total: u64 = stats.counts.iter().sum();
+    info!("family-size distribution → {path} (total {total} famil{})",
+        if total == 1 { "y" } else { "ies" });
+}
+
+// ── Raw single-copy SNP distance → CSV ──────────────────────────────────────
+//
+// p_hat[i,j] = snp[i,j] / (snp[i,j] + shared[i,j]) over loci single-copy in
+// both i and j — see `RawSnpDistanceOutput` / `KmerIndex::raw_snp_distance`.
+// A single file: the distance matrix, with an eligible-loci count alongside
+// each value so a 0/0 pair (no eligible locus at all) is distinguishable
+// from a genuinely identical pair.
+
+fn write_raw_snp_distance_csv(result: &RawSnpDistanceOutput, labels: &[String], output: &Option<PathBuf>) {
+    let path = output.as_ref()
+        .map(|p| format!("{}_rawsnp.csv", p.display()))
+        .unwrap_or_else(|| "rawsnp.csv".into());
+    let mut f = BufWriter::new(std::fs::File::create(&path).unwrap_or_else(|e| {
+        eprintln!("error creating {path}: {e}");
+        std::process::exit(1);
+    }));
+    let n = labels.len();
+    write!(f, "genome").unwrap();
+    for g in labels { write!(f, ",{g}").unwrap(); }
+    writeln!(f).unwrap();
+    for (i, g) in labels.iter().enumerate() {
+        write!(f, "{g}").unwrap();
+        for j in 0..n {
+            let snp = result.snp[[i, j]];
+            let shared = result.shared[[i, j]];
+            let eligible = snp + shared;
+            if eligible == 0 {
+                write!(f, ",NA").unwrap();
+            } else {
+                write!(f, ",{:.6}", snp as f64 / eligible as f64).unwrap();
+            }
+        }
+        writeln!(f).unwrap();
+    }
+    info!("raw single-copy SNP distance matrix → {path}");
+}
+
+// ── SNP-only pseudo-alignment → FASTA ───────────────────────────────────────
+//
+// One record per genome, IUPAC-coded, no flanking sequence — see
+// `SnpAlignment` / `KmerIndex::snp_pseudo_alignment`. Uses the project's
+// existing FASTA writer (`obifastwrite::write_record`) rather than
+// hand-rolling one.
+
+fn write_snp_fasta(alignment: &SnpAlignment, labels: &[String], output: &Option<PathBuf>) {
+    let path = output.as_ref()
+        .map(|p| format!("{}_snp.fasta", p.display()))
+        .unwrap_or_else(|| "snp.fasta".into());
+    let mut f = BufWriter::new(std::fs::File::create(&path).unwrap_or_else(|e| {
+        eprintln!("error creating {path}: {e}");
+        std::process::exit(1);
+    }));
+    let n_sites = alignment.sequences.first().map(|s| s.len()).unwrap_or(0);
+    for (label, seq) in labels.iter().zip(alignment.sequences.iter()) {
+        write_record(seq, label, &[("n_sites", JsonVal::Num(n_sites as u64))], &mut f).unwrap_or_else(|e| {
+            eprintln!("error writing {path}: {e}");
+            std::process::exit(1);
+        });
+    }
+    info!("SNP pseudo-alignment → {path} ({n_sites} site{})",
+        if n_sites == 1 { "" } else { "s" });
+}
+
 // ── UPGMA Newick from kodama dendrogram ───────────────────────────────────────

 fn upgma_to_newick(dendro: &kodama::Dendrogram<f64>, names: &[String]) -> String {
@@ -151,12 +151,14 @@ pub struct FilterArgs {
    pub outgroup: Vec<String>,

    /// Minimum number of ingroup genomes containing the k-mer
-    #[arg(long)]
-    pub min_count: Option<usize>,
+    /// (negative: offset from group size, e.g. -1 = all but one)
+    #[arg(long, allow_hyphen_values = true)]
+    pub min_count: Option<isize>,

    /// Maximum number of ingroup genomes containing the k-mer
-    #[arg(long)]
-    pub max_count: Option<usize>,
+    /// (negative: offset from group size, e.g. -1 = all but one)
+    #[arg(long, allow_hyphen_values = true)]
+    pub max_count: Option<isize>,

    /// Minimum fraction of ingroup genomes containing the k-mer [0.0–1.0]
    /// (default 1.0 when --ingroup is set, 0.0 otherwise)
@@ -168,13 +170,15 @@ pub struct FilterArgs {
    pub max_frac: Option<f64>,

    /// Minimum number of outgroup genomes containing the k-mer
-    #[arg(long)]
-    pub min_outgroup_count: Option<usize>,
+    /// (negative: offset from outgroup size, e.g. -1 = all but one)
+    #[arg(long, allow_hyphen_values = true)]
+    pub min_outgroup_count: Option<isize>,

    /// Maximum number of outgroup genomes containing the k-mer
-    /// (default 0 when --outgroup is set, no constraint otherwise)
-    #[arg(long)]
-    pub max_outgroup_count: Option<usize>,
+    /// (default 0 when --outgroup is set, no constraint otherwise;
+    /// negative: offset from outgroup size, e.g. -1 = all but one)
+    #[arg(long, allow_hyphen_values = true)]
+    pub max_outgroup_count: Option<isize>,

    /// Minimum fraction of outgroup genomes containing the k-mer [0.0–1.0]
    #[arg(long)]
@@ -239,12 +243,12 @@ pub fn matching_genome_indices(pred_str: &str, genomes: &[GenomeInfo]) -> Result

 pub struct GroupFilterParams {
    pub threshold:          u32,
-    pub min_count:          Option<usize>,
-    pub max_count:          Option<usize>,
+    pub min_count:          Option<isize>,
+    pub max_count:          Option<isize>,
    pub min_frac:           Option<f64>,
    pub max_frac:           Option<f64>,
-    pub min_outgroup_count: Option<usize>,
-    pub max_outgroup_count: Option<usize>,
+    pub min_outgroup_count: Option<isize>,
+    pub max_outgroup_count: Option<isize>,
    pub min_outgroup_frac:  Option<f64>,
    pub max_outgroup_frac:  Option<f64>,
 }
@@ -279,12 +283,20 @@ pub fn build_group_filter(
    let default_min_frac           = if !ingroup_preds.is_empty()  && !ingroup_quorum_explicit  { 1.0      } else { 0.0      };
    let default_max_outgroup_count = if !outgroup_preds.is_empty() && !outgroup_quorum_explicit { 0        } else { out_size };

-    let min_count          = p.min_count.unwrap_or(0);
-    let max_count          = p.max_count.unwrap_or(in_size);
+    // Resolve a signed count: negative means an offset from the group size
+    // (e.g. -1 = all but one), floored at 1 so the negative form always keeps
+    // constraining the group — even a singleton group, where n-1 would be 0
+    // and would otherwise drop the constraint entirely.
+    let resolve = |v: isize, size: usize| -> usize {
+        if v < 0 { (size as isize + v).max(1) as usize } else { v as usize }
+    };
+
+    let min_count          = p.min_count.map(|v| resolve(v, in_size)).unwrap_or(0);
+    let max_count          = p.max_count.map(|v| resolve(v, in_size)).unwrap_or(in_size);
    let min_frac           = p.min_frac.unwrap_or(default_min_frac);
    let max_frac           = p.max_frac.unwrap_or(1.0);
-    let min_outgroup_count = p.min_outgroup_count.unwrap_or(0);
-    let max_outgroup_count = p.max_outgroup_count.unwrap_or(default_max_outgroup_count);
+    let min_outgroup_count = p.min_outgroup_count.map(|v| resolve(v, out_size)).unwrap_or(0);
+    let max_outgroup_count = p.max_outgroup_count.map(|v| resolve(v, out_size)).unwrap_or(default_max_outgroup_count);
    let min_outgroup_frac  = p.min_outgroup_frac.unwrap_or(0.0);
    let max_outgroup_frac  = p.max_outgroup_frac.unwrap_or(1.0);

@@ -341,6 +341,27 @@ impl<L: KmerLength> CanonicalKmerOf<L> {
        ]
    }

+    /// Return the four central canonical neighbours (each already canonical),
+    /// substituting the base at the middle position `m = (L::len()-1)/2`
+    /// (well-defined for odd `L::len()`). Each of the 4 substitutions is
+    /// canonicalised independently — this correctly handles the case where a
+    /// substitution flips the canonical orientation, unlike inferring the
+    /// variant from a fixed-orientation flank key. One of the 4 equals
+    /// `self`'s own canonical form (the identity substitution); callers that
+    /// only want the 3 genuine variants should skip it.
+    pub fn central_canonical_neighbors(&self) -> [CanonicalKmerOf<L>; 4] {
+        let k = L::len();
+        let m = (k - 1) / 2;
+        let shift = KMER_BITS - 2 - 2 * m;
+        let cleared = self.0 & !((0b11 as RawKmer) << shift);
+        [
+            KmerOf::<L>(cleared | ((0 as RawKmer) << shift), PhantomData).canonical(),
+            KmerOf::<L>(cleared | ((1 as RawKmer) << shift), PhantomData).canonical(),
+            KmerOf::<L>(cleared | ((2 as RawKmer) << shift), PhantomData).canonical(),
+            KmerOf::<L>(cleared | ((3 as RawKmer) << shift), PhantomData).canonical(),
+        ]
+    }
+
    /// Return the inner value as a raw [`KmerOf<L>`].
    #[inline]
    pub fn into_kmer(self) -> KmerOf<L> {
@@ -210,4 +210,46 @@ mod tests {
        check!(31);
        check!(32);
    }
+
+    // ── central_canonical_neighbors ─────────────────────────────────────────
+
+    #[test]
+    fn central_canonical_neighbors_hand_checked_k3() {
+        // k=3, centre = index 1. For "ACG", every one of the 4 central
+        // substitutions ("AAG","ACG","AGG","ATG") happens to stay in forward
+        // orientation when canonicalised (verified by hand: each is already
+        // lexicographically <= its own reverse complement), so this case
+        // exercises the substitution logic without the RC-flip edge case.
+        let ck = KmerOf::<ConstLen<3>>::from_ascii(b"ACG").unwrap().canonical();
+        let neighbours = ck.central_canonical_neighbors();
+        let ascii: Vec<Vec<u8>> = neighbours.iter().map(|n| n.to_ascii()).collect();
+        assert_eq!(ascii, vec![b"AAG".to_vec(), b"ACG".to_vec(), b"AGG".to_vec(), b"ATG".to_vec()]);
+        // The identity substitution (centre unchanged) must reproduce `ck`.
+        assert!(neighbours.contains(&ck));
+    }
+
+    #[test]
+    fn central_canonical_neighbors_identity_present_for_various_k() {
+        macro_rules! check {
+            ($n:expr) => {{
+                let ck = KmerOf::<ConstLen<$n>>::from_ascii(&make_seq::<$n>())
+                    .unwrap()
+                    .canonical();
+                let neighbours = ck.central_canonical_neighbors();
+                assert!(
+                    neighbours.contains(&ck),
+                    "identity substitution missing from central_canonical_neighbors for k={}",
+                    $n
+                );
+                // Every returned neighbour must itself already be canonical.
+                for n in &neighbours {
+                    assert_eq!(n.into_kmer().canonical(), *n, "neighbour not canonical for k={}", $n);
+                }
+            }};
+        }
+        check!(1);
+        check!(3);
+        check!(5);
+        check!(31);
+    }
 }
@@ -10,7 +10,8 @@ pub mod stream_iter;
 mod scratch;

 pub(crate) mod encoding;
-pub(crate) mod rolling_stat;
+#[allow(missing_docs)]
+pub mod rolling_stat;

 pub use iter::SuperKmerIter;
 pub use scratch::SuperKmerScratch;
@@ -196,13 +196,6 @@ impl RollingStat {
            .map(|raw| Minimizer::from_raw_unchecked(raw << (64 - self.m * 2)))
    }

-    pub fn entropy(&self, order: usize) -> Option<f64> {
-        if !self.ready() {
-            return None;
-        }
-        Some(self.entropy.entropy(order))
-    }
-
    pub fn normalized_entropy(&self) -> Option<f64> {
        if !self.ready() {
            return None;
Author	SHA1	Message	Date
Eric Coissac	f5e508ed33	feat: add multi-genome SNP pseudo-alignment and CLI export Release / create-release (push) Successful in 5m58s Details Release / build-macos-arm64 (push) Successful in 2m47s Details Release / build-linux-x86_64 (push) Successful in 8m50s Details CI / build (pull_request) Canceled after 5m42s Details Introduces a `SnpAlignment` struct and helper methods to construct per-genome SNP pseudo-alignments from sibling k-mer data, filtering monomorphic families and encoding bases as IUPAC ambiguity codes. Exposes the type at the crate root for simplified imports. Adds a `--snp` CLI flag to compute and export these alignments as an IUPAC-coded FASTA file. Updates theory documentation to propose a multi-genome framing approach for joint phylogenetic inference, resolving pairwise correspondence ambiguities through positional homology and partial coverage thresholds. Bumps crate version to 1.1.40.	2026-08-10 22:38:38 +02:00
Eric Coissac	49f329edd5	feat: add raw SNP distance calculation and CLI flag Exposes RawSnpDistanceOutput and implements KmerIndex::raw_snp_distance() to compute pairwise single-copy locus counts under a paralogy-aware rule. The implementation leverages ndarray for parallel matrix aggregation, producing raw p-distance matrices for sanity-checking. A --raw-snp-distance CLI flag is added to export results as CSV, mapping zero-eligible pairs to NA.	2026-08-10 22:17:07 +02:00
Eric Coissac	1a470eab9e	Refactor k-mer sibling tracking to compact bitmask and on-demand counts Replaces the explicit `SiblingInfo` struct and 3-bit minorant flags with a derived 4-bit presence mask (`FamilyMask`) that tracks observed bases per family. This eliminates redundant file I/O overhead by introducing a `PartitionCache` for batch lookups, simplifies serialization, and updates all downstream builders, stats computation, and tests to operate on the new bitmask representation. Adjusts CLI output to report deduplicated family sizes instead of histograms, ignores generated CSV files, and updates documentation to reflect the fixed canonical reference and new theory.	2026-08-10 17:53:52 +02:00
Eric Coissac	ba990a48a0	feat: add obipipeline for concurrent sibling annex stats Add the `obipipeline` crate and replace sequential scatter/gather logic with a concurrent pipeline using `Flat` and `Transform` stages. Introduce `SiblingAnnexStats` API to compute distributions, and add CLI flags to `distance.rs` for constructing the annex and exporting statistics as CSV.	2026-08-10 15:31:04 +02:00
Eric Coissac	ea914bb536	feat: implement per-k-mer sibling counts and central neighbor generation Introduce the siblingannex module in obicompactvec to store per-slot minorant flags and sibling counts in a memory-mapped annex file. Add a scatter-gather pipeline in obikindex to compute these values across index layers and write them to .psib files. Implement central_canonical_neighbors in obikseq for generating strand-aware k-mer variants around the middle base. Expose rolling statistics in obiskbuilder and update dependency graphs accordingly.	2026-08-10 15:01:59 +02:00
Eric Coissac	8bc6d533e5	feat: support negative count filters as group size offsets Updates CLI parsing to accept negative integers for count filters, interpreting them as offsets from the group size (e.g., `-1` means all but one). A resolution closure enforces a floor of 1 to prevent unconstrained filtering on small groups. Additionally, refines evolutionary distance documentation to condition comparisons on local homology, replacing union-based Jaccard with a self-contained `SnpTally`. This unified approach streamlines SNP and shared count computation, incorporates paralogy and heterozygosity handling, and enables direct derivation of corrected distance matrices without external dependencies.	2026-08-10 12:38:35 +02:00
Eric Coissac	45df9919e5	docs: add central-position SNP distance estimator spec Introduces a design specification for inferring substitution rates directly from k-mers with conserved flanks. The document details a memory-efficient implementation that computes 4x4 base-pair tallies using existing MPHF structures, enabling classical corrections without de Bruijn graph materialization. Updates MkDocs navigation to include the new theory page.	2026-07-10 09:49:49 +02:00
Eric Coissac	2610a4af79	feat: add Mash distance metric and rolling entropy support Implement the Mash distance metric across the CLI, index, and compact vector traits. This includes adding a `Mash` variant to the `DistanceMetric` enum and `MetricArg` CLI argument, implementing the conversion from Jaccard distances using the standard mutation-rate estimator formula, and updating documentation with supported metrics and algorithmic references. Additionally, add an `entropy` method to rolling statistics for computing order-specific entropy.	2026-07-09 11:40:48 +02:00
coissac	dc3392865f	Merge pull request 'Push qowsvpqmoukq' (#61 ) from push-qowsvpqmoukq into main Reviewed-on: #61	2026-07-08 18:05:42 +00:00