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# SuperKmer — implementation

## Memory layout

A super-kmer is stored as a 32-bit header followed by a byte-aligned nucleotide sequence (2 bits/base, nucleotide 0 at the MSB of the first byte):

Field | Bits | Role
--- | --- | ---
COUNT | 24 | Occurrence count (≤ 16 M)
NKMERS | 8 | Number of kmers (= seq_length − k + 1, range 1–255)

Bit layout (MSB to LSB): `[31:8] COUNT [7:0] NKMERS`

NKMERS is stored as a raw `u8` in kmer units, not nucleotides. The nucleotide length is recovered as `NKMERS + k − 1`. This avoids the awkward wrapping convention (`0 = 256`) that would be needed if nucleotide length were stored directly, and gains k−1 = 30 units of headroom:

unit | u8 covers | max nucleotides
--- | --- | ---
nucleotides | 255 nt | 225 kmers
kmers | 255 kmers | 285 nt

The public accessors:

    fn n_kmers(&self)            -> usize { (self.0 & 0xFF) as usize }
    fn seql(&self)               -> usize { self.n_kmers() + K - 1 }
    fn count(&self)              -> u32   { self.0 >> 8 }
    fn increment(&mut self)               { self.0 += 1 << 8; }
    fn add(&mut self, n: u32)             { self.0 += n << 8; }
    fn set_count(&mut self, n: u32)       { self.0 = (self.0 & 0xFF) | (n << 8); }

In practice, observed super-kmer lengths on metagenomic data (k=31) are below 55 nucleotides (≤ 25 kmers) — far from the 255-kmer cap. If a super-kmer ever exceeds 255 kmers, it is split with a k−1 nucleotide overlap, preserving all kmers without duplication (identical mechanism to partition-boundary splits).

The sequence is always stored in canonical form (lexicographic minimum of forward and reverse complement), with nucleotide 0 at the MSB of the first byte. The byte array can be hashed directly without any adjustment.

## ASCII encoding and decoding

Two lookup tables handle ASCII ↔ 2-bit conversion:

- `ENC: [u8; 32]` — indexed by `b & 0x1F` (lower 5 bits of the ASCII byte). Maps A/a→0, C/c→1, G/g→2, T/t and U/u→3; ambiguous bases and unknowns silently map to 0 (A). 32 entries, fits entirely in L1 cache. Upper- and lowercase are handled identically.
- `DEC4: [u32; 256]` — maps a packed byte (4 nucleotides) to 4 ASCII characters packed as a big-endian `u32`. 1 KB total, fits in L1 cache. One lookup per output byte yields 4 decoded characters.

Encoding 4 nucleotides into one byte:

    byte = ENC[c0 & 0x1F] << 6 | ENC[c1 & 0x1F] << 4 | ENC[c2 & 0x1F] << 2 | ENC[c3 & 0x1F]

Decoding one byte into 4 ASCII characters:

    DEC4[byte].to_be_bytes()   // [nuc0, nuc1, nuc2, nuc3] in ASCII

## Reverse complement

The reverse complement is computed in place with zero allocation in two steps.

Step 1 — byte swap with `REVCOMP4`. A 256-byte lookup table `REVCOMP4` maps each byte (4 nucleotides) to its reverse complement. Bytes are swapped from the outside in, applying `REVCOMP4` to each:

    const fn revcomp4(x: u8) -> u8 {
        let x = !x;                                       // complement all bases
        let x = (x >> 4) | (x << 4);                     // swap nibbles
        let x = ((x >> 2) & 0x33) | ((x & 0x33) << 2);  // swap 2-bit groups
        x
    }

`REVCOMP4` is 256 bytes (fits in L1 cache), computed at compile time. No endianness dependency — all operations are pure arithmetic on byte values.

Step 2 — realignment. After step 1, `padding = n × 8 − seql × 2` spurious bits (complements of the original padding A's) appear at the start of the array. They are flushed left using `BitSlice<u8, Msb0>::rotate_left(padding)` from the `bitvec` crate, which is SIMD-accelerated. The trailing `padding` bits are then zeroed:

    let seql = self.n_kmers() + k - 1;
    shift = n * 8 - seql * 2          // number of padding bits
    bits.rotate_left(shift)
    bits[len - shift..].fill(false)

`Msb0` ordering makes the bit layout hardware-independent.

Algorithm — Super-kmer canonisation

    procedure SuperKmerCanonical(seq, SEQL):
        for i ← 0 to SEQL − 1:
            fwd ← nucleotide(seq, i)
            rev ← complement(nucleotide(seq, SEQL − 1 − i))
            if fwd < rev: return seq                          -- forward is canonical
            if fwd > rev: return SuperKmerRevcomp(seq, SEQL) -- revcomp is canonical
        return seq                                            -- palindrome: either orientation valid

## Minimizer sliding window

Super-kmers are built by `SuperKmerIter` (crate `obiskbuilder`), which maintains the current minimizer with a monotonic deque over a sliding window of W = k − m + 1 m-mer positions.

Each deque entry stores:

Field | Type | Purpose
--- | --- | ---
`position` | usize | 0-based start of this m-mer in the segment
`canonical` | u64 | right-aligned canonical m-mer value (lex-min of fwd and rc); used as partition key
`hash` | u64 | \(H(\text{canonical})\) — ordering key for random minimizer selection

The hash \(H\) is the seeded splitmix64 finalizer (see Minimizer selection):

    fn hash_mmer(canonical: u64) -> u64 {
        let x = canonical ^ 0x9e3779b97f4a7c15;   // seed: eliminates fixed point at 0
        let x = x ^ (x >> 30);
        let x = x.wrapping_mul(0xbf58476d1ce4e5b9);
        let x = x ^ (x >> 27);
        let x = x.wrapping_mul(0x94d049bb133111eb);
        x ^ (x >> 31)
    }

On each new nucleotide, once the window is full, the deque is updated:

Algorithm — minimizer deque update

    procedure UpdateMinimizer(deque, position, canonical, hash, k, received):
        -- pop dominated entries from the back
        while deque.back.hash ≥ hash:
            deque.pop_back()
        deque.push_back({position, canonical, hash})

        -- evict expired entries from the front
        while deque.front.position + k < received:
            deque.pop_front()

The front of the deque is always the current minimizer. Because the deque is maintained in strictly increasing hash order, each entry is popped at most once — O(1) amortized per nucleotide.

A super-kmer boundary is emitted when the minimizer changes: `deque.front.hash ≠ prev_hash`. The `canonical` field of the front entry is not used for boundary detection — that uses the hash alone. The canonical value is stored so that the partition key \(H(\text{canonical})\) can be recomputed independently at routing time from the stored `minimizer_pos`, without inheriting the minimum-order-statistic bias (see Minimizer selection — partition key independence).

## Kmer extraction

A k-mer is extracted from a super-kmer with `SuperKmer::kmer(i, k)`, which returns a `Kmer` — a left-aligned `u64` newtype (see Kmer implementation):

    pub fn kmer(&self, i: usize, k: usize) -> Result<Kmer, KmerError>

The bit slice `seq[i*2 .. (i+k)*2]` (Msb0 order) is loaded as a big-endian `u64` via `bitvec::load_be`, then left-shifted to produce the canonical left-aligned layout. One call — no loop, no allocation.

Algorithm — Super-kmer reverse complement

    procedure SuperKmerRevcomp(seq, SEQL):
        seql  ← NKMERS + k − 1               -- nucleotide length
        n     ← ⌈seql / 4⌉                  -- number of bytes
        shift ← n × 8 − seql × 2            -- padding bits to flush

        -- step 1: swap bytes outside-in, applying REVCOMP4 to each (256-byte L1 table)
        lo ← 0 ; hi ← n − 1
        while lo < hi:
            seq[lo], seq[hi] ← REVCOMP4[seq[hi]], REVCOMP4[seq[lo]]
            lo ← lo + 1 ; hi ← hi − 1
        if lo == hi: seq[lo] ← REVCOMP4[seq[lo]]

        -- step 2: left-rotate entire bit array by shift, zero trailing bits (SIMD via bitvec)
        if shift > 0:
            bits.rotate_left(shift)
            bits[n×8 − shift .. n×8].fill(0)