Eric Coissac
98c14aade9
Centralizes index configuration by storing a single `IndexMode` (`Exact`, `Approx`, or `Hybrid`) in `PartitionMeta`, eliminating per-layer metadata files. Introduces a `Hybrid` evidence mode and an `--approx` CLI flag to toggle between exact and probabilistic indexing. Refactors the build and query pipelines to dynamically dispatch based on the configured mode, deferring `.idx` generation to Pass 2 and only requiring it for Exact/Hybrid modes. Updates layer opening to load appropriate data structures, enforces strict parameter validation during merges, and clarifies performance trade-offs in documentation.
171 lines
8.9 KiB
Markdown
171 lines
8.9 KiB
Markdown
# Unitig-based MPHF evidence encoding
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## Role of unitigs in the index
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The MPHF maps each canonical kmer to an integer slot but provides no inverse: a slot index alone cannot reconstruct the kmer. The **evidence file** supplies this inverse: for each MPHF slot it stores a pointer into the unitig sequence file, from which k nucleotides can be extracted.
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Unitigs are the natural compact representation: a run of L nucleotides encodes L − k + 1 consecutive canonical kmers. The entire kmer set of a partition is reconstructible from its unitig binary file.
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---
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## Binary file formats
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### `unitigs.bin` — sequence chunks
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A sequence of binary records. Each record:
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```
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[u8: seql − k] [ceil(seql / 4) bytes: 2-bit packed nucleotides]
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```
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- `seql − k` (0–255): nucleotide length minus k, so `seql = byte[0] + k` and `n_kmers = byte[0] + 1`.
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- Packed nucleotides: A=00, C=01, G=10, T=11, MSB-first within each byte; last byte zero-padded.
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- Byte count for packed sequence: `ceil(seql / 4)`.
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Unitigs with more than `MAX_KMERS_PER_CHUNK = 256` k-mers are transparently split into overlapping chunks. Each chunk has at most 256 k-mers (= `seql − k + 1 ≤ 256`); consecutive chunks overlap by k−1 nucleotides so no kmer is lost:
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```
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chunk 1: nucleotides [0, MAX_KMERS_PER_CHUNK + k − 2] (256 kmers)
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chunk 2: nucleotides [256, end] (remaining kmers)
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overlap: k−1 nucleotides shared between the two chunks
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```
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### `unitigs.bin.idx` — block-sampled offset index
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```
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magic : 4 bytes = "UIX3"
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block_bits: u32 LE — granularity parameter (0–31)
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n_unitigs : u32 LE — total number of chunks in unitigs.bin
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n_kmers : u64 LE — total number of kmers across all chunks
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offsets : [u32 LE] — byte offsets into unitigs.bin, one per 2^block_bits chunks + sentinel
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```
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One offset entry is stored every `2^block_bits` chunks; the array is sentinel-terminated (last entry = file size). `DEFAULT_BLOCK_BITS = 0` stores one offset per chunk (exact table, no scan).
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### `evidence.bin` — per-slot MPHF evidence
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A flat array of u32 values, one per MPHF slot, no header:
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```
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bits [31:7] = chunk_id (25 bits)
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bits [6:0] = rank (7 bits, 0–127)
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```
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File size = `n_slots × 4` bytes. `chunk_id` is the 0-based index of the record in `unitigs.bin`; `rank` is the position of the canonical kmer within that chunk (counting only canonical kmers). Encoding: `raw = (chunk_id << 7) | (rank & 0x7F)`. Decoding: `chunk_id = raw >> 7`, `rank = raw & 0x7F`.
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---
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## Building and reading the index
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### `build_unitig_idx(path, block_bits)`
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Scans `unitigs.bin` sequentially: for each chunk at byte offset `offset`, if `chunk_count & mask == 0` (where `mask = (1 << block_bits) − 1`), appends `offset as u32` to `block_offsets`. After the scan, appends a sentinel (= total file size), then writes the `.idx` file. Called after the unitig file is fully written and closed.
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### `open()`, `open_sequential()`, `open_direct_access()`
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`UnitigFileReader` has three constructors:
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- `open(path)` — smart default: if `unitigs.bin.idx` exists, delegates to `open_direct_access`; otherwise delegates to `open_sequential`. Prefer this in call sites that don't require one specific mode.
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- `open_sequential(path)` — never reads `.idx`. Sequential iterators only; `chunk_start(i)` falls back to an O(i) mmap scan rather than panicking.
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- `open_direct_access(path)` — requires `.idx` to be present. Enables O(1) or O(2^block_bits) `chunk_start(i)`, used by `verify_canonical_kmer` at query time.
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`CanonicalKmerIter` — a clonable sequential iterator returned by `UnitigFileReader::iter_canonical_kmers()`. It holds an `Arc<Mmap>` so cloning resets the cursor to the start without reopening the file. This makes it usable with `par_bridge()` for parallel MPHF construction without random access.
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### `chunk_start(i)` — access modes
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When `.idx` is loaded (`open_direct_access`):
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- `block_bits = 0`: single array lookup, O(1).
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- `block_bits > 0`: lookup block, then scan ≤ 2^block_bits records, O(2^block_bits).
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When `.idx` is absent (`open_sequential`): `chunk_start(i)` performs an O(i) sequential mmap scan from offset 0. No panic — the function degrades gracefully. This degraded path is used by `find_strict()` on Approx layers (sequential scan of all canonical kmers).
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### Decoding a kmer from slot `s`
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```rust
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let (chunk_id, rank) = evidence.decode(s); // u32 → (chunk_id: u32, rank: u8)
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let kmer = unitigs.raw_kmer(chunk_id, rank); // 2-bit packed slice → left-aligned u64
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```
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Two memory accesses: one 4-byte read from `evidence.bin`, one packed-bit extraction from `unitigs.bin` via the mmap. The retrieved sequence is already canonical (only canonical kmers are inserted into the De Bruijn graph).
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---
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## Field widths and capacity
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| field | bits | range | capacity check (*B. nana*, 256 partitions) |
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|------------|------|---------------|---------------------------------------------|
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| `seql − k` | 8 | 0–255 | max `n_kmers` per chunk = 256 = `MAX_KMERS_PER_CHUNK` |
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| `rank` | 7 | 0–127 | observed max ~46 kmers/chunk; structural max k−m+1 = 21 |
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| `chunk_id` | 25 | 0–33 554 431 | avg U ≈ 275 k chunks/partition |
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The rank field is 7 bits (max 127) even though chunks can contain up to 256 k-mers, because rank counts only canonical kmers within the chunk, and the canonical kmer count is at most half the total.
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---
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## Evidence bit-cost
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Strategy B (chunk_id + rank) is the implemented strategy. For *B. nana* (k=31, 256 partitions, P ≈ 10.4 M unique kmers/partition, U ≈ 275 k chunks/partition, m_u ≈ 37.9 kmers/chunk):
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| field | theoretical cost | value |
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|------------|-------------------------|---------|
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| chunk_id | ⌈log₂ U⌉ | 19 bits |
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| rank | ⌈log₂ m_u⌉ (≈ fixed) | 6 bits |
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| **stored** | aligned u32 | **32 bits/slot** |
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The u32 layout is chosen for alignment and simplicity; no bit-addressing arithmetic is needed.
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Comparison with strategy A (global nucleotide offset): `⌈log₂(P · (1 + (k−1)/m_u))⌉ = 25 bits`. Strategy A is theoretically 2 bits cheaper; strategy B's advantage is **locality** (decoding touches one chunk's cache lines) and a bounded, constant-width rank field independent of partition size.
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---
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## Unitig decomposition non-determinism
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The unitig extraction from `GraphDeBruijn` is **not deterministic**: two runs on identical input can produce different unitig counts and sequences while covering exactly the same canonical kmer set.
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The hash map (`hashbrown::HashMap` with `Xxh3Builder`) has run-dependent iteration order. The `start_iter` first pass emits every node where `can_extend_left` is false — this includes true dead-ends and branch points (nodes with ≥2 left neighbours). When a branch point is encountered before its upstream neighbours, it claims the downstream chain and those upstream neighbours later produce length-k degenerate unitigs. When upstream neighbours appear first, they extend through the branch point.
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**Example** — fork topology (k = 31):
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```
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A → B ← C
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↓
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D
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```
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B has two left neighbours, so `can_extend_left = false`. Two valid tilings:
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| iteration order | unitigs | count |
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|---|---|---|
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| A first | ABD, C | 2 |
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| B first | BD, A, C | 3 |
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Both cover the same 4 canonical kmers. Pure cycles are unaffected: all cycle nodes have both extensions present, so none are emitted in the first pass; each cycle produces exactly one unitig regardless of entry point (only the cut point varies).
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This non-determinism is benign for MPHF construction: the MPHF is built from the kmer set, which is identical across tilings.
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---
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## Partition-size tradeoff
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Measured on *B. nana* (k=31, m=11), summing across all partitions:
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| N partitions | m_u |
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|---|---|
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| 1 | 41.89 |
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| 16 | 38.19 |
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| 256 | 37.90 |
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| 1 024 | 37.89 |
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`m_u` is set by De Bruijn graph topology (heterozygosity, repeats, sequencing errors), not partition count. The variation from 1 to 1024 partitions is under 10%; within 16–1024 it is under 1%. Unitigs provide ~3.1× nucleotide compaction over super-kmers at 256 partitions.
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Evidence cost decreases by 1 bit/kmer with each doubling of partition count (via `log₂ U = log₂(P/m_u)`). The sequence storage term `2 · (1 + (k−1)/m_u) ≈ 3.6 bits/kmer` is approximately constant.
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---
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## Alternative: fingerprint evidence
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`evidence.bin` can be replaced by `fingerprint.bin` at index build time (`--approx`) or after the fact (`reindex --approx`). The fingerprint stores b bits per MPHF slot (the low b bits of `kmer.seq_hash()`); verification becomes a single bitfield comparison instead of a unitig dereference. False-positive rate per k-mer query: 1/2^b. With the Findere z parameter, z consecutive k-mers must all match, reducing the effective window FP rate to 1/2^(b·z) while skipping z−1 of every z k-mers. No `.idx` file is written or read in approx mode.
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See [Approximate evidence (Findere fingerprint)](evidence_elimination.md) for the full design and CLI parameters.
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