src/obikmer2/src/cmd/phylo/args.rs

@@ -82,10 +82,10 @@ impl DistanceArg {
 /// `--free-loss`, `--no-ambiguity`, `--entropy`/`--entropy-sd`), Sankoff
 /// cost-matrix calibration (`--sankoff`, `--sankoff-ratio-ceiling`) and its
 /// TNT/PhyG/IQ-TREE exports (`--tnt`, `--phyg`, `--iqtree`/
-/// `--iqtree-min-freq`, `--sankoff-cost-scale`) — everything else
-/// sibling-annex-based (family overlap, ...) stays in `obikmer` until the
-/// rest of `obikphylo::siblings` is reconnected (see the project memory on
-/// this).
+/// `--iqtree-min-freq`, `--sankoff-cost-scale`), and Family Overlap
+/// (`--family-overlap`, `--min-shared-family`) — everything else
+/// sibling-annex-based stays in `obikmer` until the rest of
+/// `obikphylo::siblings` is reconnected (see the project memory on this).
 #[derive(Args)]
 pub struct PhyloArgs {
     /// Index directory
@@ -101,6 +101,23 @@ pub struct PhyloArgs {
     #[arg(long = "exclude-genome", value_name = "LABEL")]
     pub exclude_genome: Vec<String>,
 
+    /// Auto-exclude any genome whose mean shared-variable-family count
+    /// against every other genome (`FamilyOverlap::mean_row` — the same
+    /// per-row statistic `--family-overlap`'s own matrix shows) falls below
+    /// this threshold — same exclusion machinery as `--exclude-genome`,
+    /// applied on top of it rather than instead of it. Applies to the
+    /// `snp-*` `--distance`/`--pseudo-alignment`/`--sankoff` computations
+    /// below (all sibling-annex-based); does *not* affect the whole-index
+    /// `--distance` metrics (jaccard, hamming, bray-curtis, ...) or their
+    /// matrix/NJ/UPGMA output — a genome with too little SNP-family
+    /// coverage to trust is a different concern from one whose plain k-mer
+    /// profile is simply divergent. Requires the Family Overlap annex
+    /// (built on demand if missing, same as every other annex here — see
+    /// `obikphylo::siblings::extensions::SiblingExt::family_overlap`'s own
+    /// docs).
+    #[arg(long, value_name = "N")]
+    pub min_shared_family: Option<f64>,
+
     /// Build (or rebuild) the sibling-count/minorant annex — independent of
     /// the distance metric below, meant to be run routinely, ahead of any
     /// SNP-family distance computation that will later consume it.
@@ -122,6 +139,16 @@ pub struct PhyloArgs {
     #[arg(long)]
     pub sibling_hist: bool,
 
+    /// Write the Family Overlap matrix (CSV) — number of shared *variable*
+    /// families per genome pair, index-wide (`obikphylo::siblings::FamilyOverlap`).
+    /// Built on demand if missing (same as every other annex here); no
+    /// `--sibling-annex` prerequisite beyond that. Every genome is written,
+    /// unfiltered by `--exclude-genome`/`--min-shared-family` — a raw
+    /// coverage diagnostic, not a computation those exclusions are meant to
+    /// protect.
+    #[arg(long)]
+    pub family_overlap: bool,
+
     /// Write a per-family Shannon entropy report (CSV) — requires an
     /// already-built sibling annex (`--sibling-annex` first, in this
     /// invocation or an earlier one). Always a full, unsampled scan of

src/obikmer2/src/cmd/phylo/mod.rs

@@ -151,6 +151,33 @@ pub fn run(args: PhyloArgs) {
         );
     }
 
+    // ── Family Overlap matrix (`--family-overlap`) ──────────────────────────────
+    if args.family_overlap {
+        let t = Stage::start("family_overlap");
+        let overlap = cache.family_overlap().unwrap_or_else(|e| {
+            eprintln!("error computing family overlap: {e}");
+            std::process::exit(1);
+        });
+        rep.push(t.stop());
+
+        let path = args.output.as_ref()
+            .map(|p| format!("{}_family_overlap.csv", p.display()))
+            .unwrap_or_else(|| "family_overlap.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);
+        }));
+        write!(f, "genome").unwrap();
+        for label in &labels { write!(f, ",{label}").unwrap(); }
+        writeln!(f).unwrap();
+        for (i, label) in labels.iter().enumerate() {
+            write!(f, "{label}").unwrap();
+            for j in 0..n { write!(f, ",{}", overlap.get(i, j)).unwrap(); }
+            writeln!(f).unwrap();
+        }
+        info!("family-overlap matrix → {path}");
+    }
+
     // ── Shannon entropy report (`--shannon`) ────────────────────────────────────
     if args.shannon {
         let path = args.output.as_ref()
@@ -166,6 +193,37 @@ pub fn run(args: PhyloArgs) {
         info!("entropy report → {path}");
     }
 
+    // ── `--min-shared-family` auto-exclusion ────────────────────────────────────
+    // Layered on top of `--exclude-genome`, not instead of it — a separate
+    // mask (not folded into `exclude_mask` itself) since it must reach
+    // `--pseudo-alignment`/`--sankoff`/`snp-*` `--distance` only, never the
+    // whole-index metrics' `kept`/`--shared-kmers` output (see
+    // `args::PhyloArgs::min_shared_family`'s own docs on why).
+    let snp_exclude_mask: Vec<bool> = match args.min_shared_family {
+        Some(threshold) => {
+            let overlap = cache.family_overlap().unwrap_or_else(|e| {
+                eprintln!("error computing family overlap: {e}");
+                std::process::exit(1);
+            });
+            let mut mask = exclude_mask.clone();
+            for (g, excluded) in mask.iter_mut().enumerate() {
+                if *excluded {
+                    continue;
+                }
+                let mean = overlap.mean_row(g);
+                if mean < threshold {
+                    info!(
+                        "auto-excluding {} (--min-shared-family: mean shared-family count {mean:.1} < {threshold})",
+                        labels[g]
+                    );
+                    *excluded = true;
+                }
+            }
+            mask
+        }
+        None => exclude_mask.clone(),
+    };
+
     // Shared by `--pseudo-alignment` and `--sankoff` — same activation rule:
     // either flag given activates entropy-biased sampling, the other
     // defaults to 1.0/0.5.
@@ -188,7 +246,7 @@ pub fn run(args: PhyloArgs) {
         info!("sampling SNP pseudo-alignment (target {subsample_n} site(s))");
         let t = Stage::start("pseudo_alignment");
         let alignment = cache
-            .snp_pseudo_alignment(subsample_n, args.free_loss, args.no_ambiguity, &exclude_mask, entropy_bias)
+            .snp_pseudo_alignment(subsample_n, args.free_loss, args.no_ambiguity, &snp_exclude_mask, entropy_bias)
             .unwrap_or_else(|e| {
                 eprintln!("error building pseudo-alignment: {e}");
                 std::process::exit(1);
@@ -223,7 +281,7 @@ pub fn run(args: PhyloArgs) {
                 subsample_n,
                 args.free_loss,
                 args.no_ambiguity,
-                &exclude_mask,
+                &snp_exclude_mask,
                 entropy_bias,
                 args.sankoff_ratio_ceiling,
             )
@@ -310,7 +368,7 @@ pub fn run(args: PhyloArgs) {
                     args.subsample,
                     args.free_loss,
                     args.no_ambiguity,
-                    &exclude_mask,
+                    &snp_exclude_mask,
                     entropy_bias,
                     args.gamma_shape,
                 )

src/obikphylo/src/siblings/algorithms/entropy.rs

@@ -16,7 +16,10 @@ use obikindex::{OKIError, OKIResult};
 
 use super::family_scan::{Selection, scan_layer_families};
 use super::minorant_selection::{is_non_monomorphic_minorant, non_monomorphic_reindex_layer};
-use crate::siblings::{ANNEX_FILE_NAME, ENTROPY_ANNEX_FILE_NAME, EntropyAnnex, EntropyAnnexBuilder, SiblingAnnex};
+use crate::siblings::{
+    ANNEX_FILE_NAME, ENTROPY_ANNEX_FILE_NAME, EntropyAnnex, EntropyAnnexBuilder,
+    FamilyOverlapAccumulator, SiblingAnnex,
+};
 
 /// Shannon entropy (bits) of one family's per-genome states, over the 15
 /// non-empty subsets of `{A,C,G,T}` actually observed among the genomes
@@ -83,31 +86,45 @@ pub(crate) fn family_entropy_4(genome_mask: &[u8]) -> Option<(f64, usize)> {
     Some((h, total as usize))
 }
 
-/// Builds one layer's entropy annex if it doesn't already exist — a no-op,
-/// cheap existence check, once it does. Bounded to non-monomorphic
-/// minorants only ([`non_monomorphic_reindex_layer`], a cheap annex-only
-/// pass — no cross-partition resolution): monomorphism is already known
-/// from the annex bits alone, so there is no reason to pay
+/// Builds one layer's entropy annex if it doesn't already exist, and/or
+/// feeds `overlap_acc` (`Some` iff the index-wide family-overlap annex is
+/// itself missing — see
+/// [`crate::siblings::FamilyOverlap`]'s own module docs) — a single shared
+/// `scan_layer_families` pass covers both, since both need the exact same
+/// per-family `genome_mask` resolution and there is no reason to pay for it
+/// twice just because only one of the two happens to be missing. A no-op
+/// (no scan at all) when entropy already exists for this layer *and*
+/// `overlap_acc` is `None`. Bounded to non-monomorphic minorants only
+/// ([`non_monomorphic_reindex_layer`], a cheap annex-only pass — no
+/// cross-partition resolution) whenever a scan does happen: monomorphism is
+/// already known from the annex bits alone, so there is no reason to pay
 /// `scan_layer_families`'s expensive per-genome resolution for the ~98% of
-/// minorants that are monomorphic only to discard the result.
+/// minorants that are monomorphic only to discard the result — true for
+/// family-overlap's own "variable families only" scope just as much as for
+/// entropy's.
 pub(crate) fn ensure_layer_entropy_annex(
     cache: &IndexCache,
     partition: usize,
     layer_idx: usize,
     n_genomes: usize,
     fast_mode: bool,
+    mut overlap_acc: Option<&mut FamilyOverlapAccumulator>,
 ) -> OKIResult<()> {
     let layer = cache
         .get_layer(partition, layer_idx)
         .expect("cache-consistent: caller already bounds layer_idx by n_layer(partition)");
     let path = layer.dir().join(ENTROPY_ANNEX_FILE_NAME);
-    if path.exists() {
+    let need_entropy = !path.exists();
+    if !need_entropy && overlap_acc.is_none() {
         return Ok(());
     }
 
     let reindex = non_monomorphic_reindex_layer(layer.dir())?;
-    let mut builder = EntropyAnnexBuilder::new(reindex.len(), &path).map_err(OKIError::Io)?;
     let eligible: HashSet<usize> = reindex.keys().copied().collect();
+    let mut builder = need_entropy
+        .then(|| EntropyAnnexBuilder::new(reindex.len(), &path))
+        .transpose()
+        .map_err(OKIError::Io)?;
 
     scan_layer_families(
         cache,
@@ -121,13 +138,19 @@ pub(crate) fn ensure_layer_entropy_annex(
                 mask.family_size() >= 2,
                 "Selection::Some(eligible) must only visit non-monomorphic minorants"
             );
-            if let Some((h15, _)) = family_entropy(genome_mask) {
-                let compact = reindex[&family_idx];
-                builder.set(compact, h15 as f32);
+            if let Some(builder) = builder.as_mut() {
+                if let Some((h15, _)) = family_entropy(genome_mask) {
+                    builder.set(reindex[&family_idx], h15 as f32);
+                }
+            }
+            if let Some(acc) = overlap_acc.as_deref_mut() {
+                acc.add_family(genome_mask);
             }
         },
     )?;
-    builder.close().map_err(OKIError::Io)?;
+    if let Some(builder) = builder {
+        builder.close().map_err(OKIError::Io)?;
+    }
 
     Ok(())
 }

src/obikphylo/src/siblings/algorithms/pairwise.rs

@@ -52,15 +52,9 @@ impl PairwiseTally {
         Self { n_genomes, pairs: vec![PairStats::default(); n_pairs] }
     }
 
-    /// Flat index of unordered pair `(i, j)`, `i != j`, into the upper
-    /// triangle — standard "row-major over `i < j`" packing: row `i`
-    /// contributes `n_genomes - 1 - i` entries (columns `i+1..n_genomes`),
-    /// so pair `(i, j)` sits at `sum_{r<i}(n_genomes-1-r) + (j-i-1)`.
+    /// See `crate::siblings::helpers::triangle_index`.
     fn flat_index(&self, i: usize, j: usize) -> usize {
-        let (i, j) = if i < j { (i, j) } else { (j, i) };
-        debug_assert!(j < self.n_genomes && i != j);
-        let n = self.n_genomes;
-        (i * (2 * n - i - 1)) / 2 + (j - i - 1)
+        crate::siblings::helpers::triangle_index(self.n_genomes, i, j)
     }
 
     #[inline]

src/obikphylo/src/siblings/algorithms/subsample.rs

@@ -142,7 +142,7 @@ pub(crate) fn sample_index(
     // annex (`EntropyWeights::open`, per layer, below) — build any missing
     // one now, once, rather than mid-walk.
     if entropy_bias.is_some() {
-        cache.ensure_entropy_annexes()?;
+        cache.ensure_entropy_and_overlap_annexes()?;
     }
 
     // Every cached layer's eligibility bitset, built up front — annex-only,

src/obikphylo/src/siblings/extensions/sibling_builder.rs

@@ -1,40 +1,53 @@
 //! [`SiblingBuilder`] — construction triggered *internally*, as a side
 //! effect of a [`crate::siblings::extensions::SiblingExt`] method (e.g.
-//! `shannon_entropy_csv` ensuring its layers' entropy annexes exist before
-//! reading them), never called directly from outside this crate. Unlike
-//! `SiblingExt` (explicit, CLI-facing actions), nothing here is meant to be
-//! its own CLI flag — same `pub`/`pub(crate)` split `obikindexer` uses for
-//! its own `IndexBuilder`/`PrivateBuilder`.
+//! `shannon_entropy_csv`/`family_overlap` ensuring their annexes exist
+//! before reading them), never called directly from outside this crate.
+//! Unlike `SiblingExt` (explicit, CLI-facing actions), nothing here is
+//! meant to be its own CLI flag — same `pub`/`pub(crate)` split
+//! `obikindexer` uses for its own `IndexBuilder`/`PrivateBuilder`.
 
 use obikidxcache::index_cache::IndexCache;
-use obikindex::OKIResult;
+use obikindex::{OKIError, OKIResult};
 use obisys::progress_bar;
 
 use crate::siblings::algorithms::{ensure_layer_entropy_annex, is_fast_mode};
+use crate::siblings::{FAMILY_OVERLAP_FILE_NAME, FamilyOverlapAccumulator};
 
 pub(crate) trait SiblingBuilder {
-    /// Build every cached layer's entropy annex that doesn't already exist
-    /// — a no-op, cheap existence check per layer, once every layer's
-    /// annex file exists. See `ensure_layer_entropy_annex`'s own docs for
-    /// why this is bounded to non-monomorphic minorants only.
-    fn ensure_entropy_annexes(&self) -> OKIResult<()>;
+    /// Build every cached layer's entropy annex, and the whole index's
+    /// family-overlap annex, that don't already exist — one call covers
+    /// both, whichever of the two a caller actually needs
+    /// (`shannon_entropy_csv`/entropy-biased sampling on one side,
+    /// `family_overlap`/`--min-shared-family` on the other): a missing
+    /// annex always triggers a scan that feeds both, per
+    /// `ensure_layer_entropy_annex`'s own docs. A no-op, cheap
+    /// existence-check pass, once both already exist everywhere.
+    fn ensure_entropy_and_overlap_annexes(&self) -> OKIResult<()>;
 }
 
 impl SiblingBuilder for IndexCache {
-    fn ensure_entropy_annexes(&self) -> OKIResult<()> {
+    fn ensure_entropy_and_overlap_annexes(&self) -> OKIResult<()> {
         let n_genomes = self.meta().genomes().len();
         let fast_mode = is_fast_mode(self);
 
-        let pb = progress_bar("entropy_annex", self.n_partition() as u64, "partitions");
+        let overlap_path = self.index().dir().join(FAMILY_OVERLAP_FILE_NAME);
+        let mut overlap_acc =
+            (!overlap_path.exists()).then(|| FamilyOverlapAccumulator::new(n_genomes));
+
+        let pb = progress_bar("entropy_overlap_annex", self.n_partition() as u64, "partitions");
         for part in self.partitions() {
             let n_layer = self.n_layer(part).unwrap_or(0);
             for l in 0..n_layer {
-                ensure_layer_entropy_annex(self, part, l, n_genomes, fast_mode)?;
+                ensure_layer_entropy_annex(self, part, l, n_genomes, fast_mode, overlap_acc.as_mut())?;
             }
             pb.inc(1);
         }
         pb.finish_and_clear();
 
+        if let Some(acc) = overlap_acc {
+            acc.write(&overlap_path).map_err(OKIError::Io)?;
+        }
+
         Ok(())
     }
 }

src/obikphylo/src/siblings/extensions/sibling_ext.rs

@@ -21,7 +21,7 @@ use crate::siblings::algorithms::{
     snp_distance, snp_pseudo_alignment,
 };
 use crate::siblings::extensions::SiblingBuilder;
-use crate::siblings::ENTROPY_ANNEX_FILE_NAME;
+use crate::siblings::{ENTROPY_ANNEX_FILE_NAME, FAMILY_OVERLAP_FILE_NAME, FamilyOverlap};
 
 pub trait SiblingExt {
     /// Build the sibling-count/minorant annex for every layer of every
@@ -154,6 +154,18 @@ pub trait SiblingExt {
         entropy_bias: Option<EntropyBias>,
         gamma_shape: Option<f64>,
     ) -> OKIResult<Array2<f64>>;
+
+    /// The Family Overlap annex — number of shared *variable* families per
+    /// genome pair, index-wide (see [`FamilyOverlap`]'s own module docs).
+    /// Built (alongside the entropy annex) if missing, same as every other
+    /// annex-backed method here. Feeds `--family-overlap`'s own matrix
+    /// output directly, and `--min-shared-family`'s auto-exclusion
+    /// (`FamilyOverlap::mean_row(g) < threshold` — same statistic
+    /// `--family-overlap`'s matrix shows, just averaged per row —
+    /// triggers adding `g` to the excluded-genome set consumed by
+    /// `snp_pseudo_alignment`/`sankoff_bundle`/`snp_distance`; this needs
+    /// no computation of its own beyond that check).
+    fn family_overlap(&self) -> OKIResult<FamilyOverlap>;
 }
 
 impl SiblingExt for IndexCache {
@@ -181,10 +193,10 @@ impl SiblingExt for IndexCache {
                 // Invalidate this layer's entropy annex, if any — it was
                 // built against the sibling annex's *previous*
                 // `family_idx` numbering (`non_monomorphic_reindex_layer`
-                // reads the sibling annex), now stale. `ensure_entropy_annexes`
-                // below only builds what's *missing*, so the old file must
-                // be removed here for it to be rebuilt rather than silently
-                // left stale.
+                // reads the sibling annex), now stale.
+                // `ensure_entropy_and_overlap_annexes` below only builds
+                // what's *missing*, so the old file must be removed here
+                // for it to be rebuilt rather than silently left stale.
                 let entropy_path = layer.dir().join(ENTROPY_ANNEX_FILE_NAME);
                 if entropy_path.exists() {
                     std::fs::remove_file(&entropy_path).map_err(OKIError::Io)?;
@@ -199,24 +211,36 @@ impl SiblingExt for IndexCache {
         pb.finish_and_clear();
         tracing::info!("sibling annex built — {total_slots} kmers across {n_parts} partitions");
 
-        // Precompute the entropy annex too, now that every layer's fresh
-        // sibling annex is on disk (`ensure_entropy_annexes` reads it via
-        // `non_monomorphic_reindex_layer`) — same routine-rebuild footing
-        // as the sibling annex itself, rather than leaving it to be built
-        // lazily on first `--shannon`/`--entropy-bias` use.
-        tracing::info!("building entropy annex");
-        self.ensure_entropy_annexes()?;
+        // The family-overlap annex is index-level, not per-layer (see its
+        // own module docs) — any layer's sibling annex changing makes the
+        // whole thing stale, so it's invalidated once here rather than
+        // per-layer above.
+        let overlap_path = self.index().dir().join(FAMILY_OVERLAP_FILE_NAME);
+        if overlap_path.exists() {
+            std::fs::remove_file(&overlap_path).map_err(OKIError::Io)?;
+        }
+
+        // Precompute the entropy and family-overlap annexes too, now that
+        // every layer's fresh sibling annex is on disk
+        // (`non_monomorphic_reindex_layer`, which both depend on, reads
+        // it) — same routine-rebuild footing as the sibling annex itself,
+        // rather than leaving them to be built lazily on first
+        // `--shannon`/`--entropy-bias`/`--min-shared-family`/
+        // `--family-overlap` use.
+        tracing::info!("building entropy and family-overlap annexes");
+        self.ensure_entropy_and_overlap_annexes()?;
 
         Ok(())
     }
 
     fn shannon_entropy_csv(&self, path: &Path) -> OKIResult<()> {
-        // Entropy annexes aren't this method's own output, but building
-        // them is exactly the same full scan this method is about to run
-        // anyway — see `SiblingBuilder::ensure_entropy_annexes`'s own docs
+        // Entropy/family-overlap annexes aren't this method's own output,
+        // but building them is exactly the same full scan this method is
+        // about to run anyway — see
+        // `SiblingBuilder::ensure_entropy_and_overlap_annexes`'s own docs
         // for why that's a side effect worth taking here rather than a
         // separate, explicit step.
-        self.ensure_entropy_annexes()?;
+        self.ensure_entropy_and_overlap_annexes()?;
 
         let n_genomes = self.meta().genomes().len();
         let fast_mode = is_fast_mode(self);
@@ -311,4 +335,10 @@ impl SiblingExt for IndexCache {
     ) -> OKIResult<Array2<f64>> {
         snp_distance(self, kind, n, free_loss, no_ambiguity, excluded, entropy_bias, gamma_shape)
     }
+
+    fn family_overlap(&self) -> OKIResult<FamilyOverlap> {
+        self.ensure_entropy_and_overlap_annexes()?;
+        let path = self.index().dir().join(FAMILY_OVERLAP_FILE_NAME);
+        FamilyOverlap::open(&path).map_err(OKIError::Io)
+    }
 }

src/obikphylo/src/siblings/family_overlap.rs

@@ -0,0 +1,167 @@
+//! Family Overlap annex — the number of shared *variable* families
+//! (`FamilyMask::family_size() >= 2`) per genome pair, summed across the
+//! **whole index**. See `DevDocMD/theory/evolutionary_distances.md`,
+//! "Family Overlap as an index-level annex", for the design discussion:
+//! unlike [`crate::siblings::SiblingAnnex`]/[`crate::siblings::EntropyAnnex`]
+//! (sharded per layer because they hold one entry per k-mer — potentially
+//! billions, impossible to keep in memory), this structure's size depends
+//! only on `n_genomes²`, never on index size, so there is no memory-pressure
+//! reason to shard it. It lives once, at the index root
+//! (`IndexCache::index().dir()`), not inside any layer directory.
+//!
+//! Built alongside the entropy annex, in the very same
+//! `scan_layer_families` sweep — see
+//! `algorithms::entropy::ensure_layer_entropy_annex`'s own docs: both need
+//! the identical per-family `genome_mask` resolution, so paying for it once
+//! and feeding both is (almost) free, whichever of the two triggered the
+//! sweep. The only case that still costs a full rescan for just one of them
+//! is a pre-existing index where the other annex already happens to be
+//! present — accepted as unavoidable, not worth special-casing.
+
+use std::fs::File;
+use std::io;
+use std::path::Path;
+
+use memmap2::Mmap;
+
+use crate::siblings::helpers::triangle_index;
+
+const MAGIC: [u8; 4] = *b"POVL";
+
+// Header: magic(4) + _pad(4) + n_genomes(8) = 16 bytes. Data (8 bytes/pair,
+// upper triangle) follows.
+const HEADER_SIZE: usize = 16;
+
+pub(crate) const FAMILY_OVERLAP_FILE_NAME: &str = "family_overlap.povl";
+
+/// Read-only, mmap-backed view of an already-built Family Overlap annex.
+pub struct FamilyOverlap {
+    mmap: Mmap,
+    n_genomes: usize,
+}
+
+impl FamilyOverlap {
+    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, "POVL file too short"));
+        }
+        if mmap[0..4] != MAGIC {
+            return Err(io::Error::new(io::ErrorKind::InvalidData, "bad POVL magic"));
+        }
+        let n_genomes = u64::from_le_bytes(mmap[8..16].try_into().unwrap()) as usize;
+        let n_pairs = n_genomes.saturating_sub(1) * n_genomes / 2;
+        if mmap.len() < HEADER_SIZE + n_pairs * 8 {
+            return Err(io::Error::new(io::ErrorKind::InvalidData, "POVL file truncated"));
+        }
+        Ok(Self { mmap, n_genomes })
+    }
+
+    pub fn n_genomes(&self) -> usize {
+        self.n_genomes
+    }
+
+    /// Number of variable families shared by genomes `i` and `j` — `0` for
+    /// `i == j` (not stored; a genome trivially "shares" every one of its
+    /// own families with itself, not a meaningful statistic here).
+    pub fn get(&self, i: usize, j: usize) -> u64 {
+        if i == j {
+            return 0;
+        }
+        let off = HEADER_SIZE + triangle_index(self.n_genomes, i, j) * 8;
+        u64::from_le_bytes(self.mmap[off..off + 8].try_into().unwrap())
+    }
+
+    /// Mean shared-family count of genome `i` against every other genome —
+    /// `--min-shared-family`'s own statistic (see
+    /// `crate::siblings::extensions::SiblingExt::family_overlap`'s docs).
+    /// `0.0` for a single-genome index (no other genome to average
+    /// against).
+    pub fn mean_row(&self, i: usize) -> f64 {
+        if self.n_genomes <= 1 {
+            return 0.0;
+        }
+        let sum: u64 = (0..self.n_genomes).filter(|&j| j != i).map(|j| self.get(i, j)).sum();
+        sum as f64 / (self.n_genomes - 1) as f64
+    }
+}
+
+/// In-memory accumulator, filled while sweeping every layer of the index
+/// (one `add_family` call per variable family visited, from whichever
+/// `scan_layer_families` pass is already running for the entropy annex —
+/// see the module docs), written once as a whole file when the sweep
+/// completes. Small enough (`O(n_genomes²)`) that there is no reason to
+/// build it incrementally on disk the way `SiblingAnnexBuilder`/
+/// `EntropyAnnexBuilder` do for their own, much larger, per-k-mer data.
+pub(crate) struct FamilyOverlapAccumulator {
+    n_genomes: usize,
+    counts: Vec<u64>,
+}
+
+impl FamilyOverlapAccumulator {
+    pub(crate) fn new(n_genomes: usize) -> Self {
+        let n_pairs = n_genomes.saturating_sub(1) * n_genomes / 2;
+        Self { n_genomes, counts: vec![0u64; n_pairs] }
+    }
+
+    /// One variable family's contribution: every pair of genomes both
+    /// carrying it (`genome_mask[g] != 0`) gets its count incremented once.
+    pub(crate) fn add_family(&mut self, genome_mask: &[u8]) {
+        for i in 0..self.n_genomes {
+            if genome_mask[i] == 0 {
+                continue;
+            }
+            for j in (i + 1)..self.n_genomes {
+                if genome_mask[j] != 0 {
+                    self.counts[triangle_index(self.n_genomes, i, j)] += 1;
+                }
+            }
+        }
+    }
+
+    pub(crate) fn write(&self, path: &Path) -> io::Result<()> {
+        let mut buf = Vec::with_capacity(HEADER_SIZE + self.counts.len() * 8);
+        buf.extend_from_slice(&MAGIC);
+        buf.extend_from_slice(&[0u8; 4]);
+        buf.extend_from_slice(&(self.n_genomes as u64).to_le_bytes());
+        for &c in &self.counts {
+            buf.extend_from_slice(&c.to_le_bytes());
+        }
+        std::fs::write(path, buf)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn roundtrip_and_mean_row() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("test.povl");
+
+        let mut acc = FamilyOverlapAccumulator::new(3);
+        acc.add_family(&[1, 1, 0]); // genomes 0,1 share this family
+        acc.add_family(&[1, 1, 1]); // all three share this one
+        acc.add_family(&[0, 1, 1]); // genomes 1,2 share this family
+        acc.write(&path).unwrap();
+
+        let overlap = FamilyOverlap::open(&path).unwrap();
+        assert_eq!(overlap.n_genomes(), 3);
+        assert_eq!(overlap.get(0, 1), 2);
+        assert_eq!(overlap.get(0, 2), 1);
+        assert_eq!(overlap.get(1, 2), 2);
+        assert_eq!(overlap.get(0, 0), 0);
+        assert_eq!(overlap.get(1, 0), overlap.get(0, 1)); // symmetric
+        assert!((overlap.mean_row(0) - 1.5).abs() < 1e-12); // (2+1)/2
+    }
+
+    #[test]
+    fn single_genome_mean_row_is_zero() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("test.povl");
+        FamilyOverlapAccumulator::new(1).write(&path).unwrap();
+        let overlap = FamilyOverlap::open(&path).unwrap();
+        assert_eq!(overlap.mean_row(0), 0.0);
+    }
+}

src/obikphylo/src/siblings/helpers.rs

@@ -20,3 +20,17 @@ pub(super) fn is_minorant(kmer: CanonicalKmer, mask: FamilyMask, k: usize) -> bo
         other == kmer || !mask.has(central_base(other, k)) || kmer.raw() <= other.raw()
     })
 }
+
+/// Flat index of unordered genome pair `(i, j)`, `i != j`, into an
+/// upper-triangle-packed `n`-genome array — standard "row-major over `i <
+/// j`" packing: row `i` contributes `n - 1 - i` entries (columns
+/// `i+1..n`), so pair `(i, j)` sits at `sum_{r<i}(n-1-r) + (j-i-1)`. Shared
+/// by every per-genome-pair structure that only stores the upper triangle
+/// (`PairwiseTally`, `FamilyOverlap`) rather than each re-deriving the same
+/// formula.
+#[inline]
+pub(super) fn triangle_index(n: usize, i: usize, j: usize) -> usize {
+    let (i, j) = if i < j { (i, j) } else { (j, i) };
+    debug_assert!(j < n && i != j);
+    (i * (2 * n - i - 1)) / 2 + (j - i - 1)
+}

src/obikphylo/src/siblings/mod.rs

@@ -22,12 +22,15 @@
 pub mod algorithms;
 mod entropy_annex;
 pub mod extensions;
+mod family_overlap;
 mod helpers;
 pub mod iter;
 mod siblingannex;
 
 pub(crate) use entropy_annex::ENTROPY_ANNEX_FILE_NAME;
 pub use entropy_annex::{EntropyAnnex, EntropyAnnexBuilder};
+pub(crate) use family_overlap::{FAMILY_OVERLAP_FILE_NAME, FamilyOverlapAccumulator};
+pub use family_overlap::FamilyOverlap;
 pub use iter::{
     MinorantBatchIter, MinorantIter, SiblingBatchIter, SiblingEntry, SiblingIter, SiblingLayerExt,
 };