Eric Coissac
1.4 KiB
DNA encoding
2-bit nucleotide encoding
All nucleotides are encoded on 2 bits, MSB-first within each word. Nucleotides are numbered 0-based from the 5′ end across all sequence types:
| Base | Encoding |
|---|---|
| A | 00 |
| C | 01 |
| G | 10 |
| T | 11 |
The Watson-Crick complement of any base is its bitwise NOT on 2 bits: complement(base) = ~base & 0b11.
Kmer encoding
A kmer fits in a single u64. Nucleotide 0 occupies bits 63–62, nucleotide i occupies bits 63−2i and 62−2i, and the low 64−2k bits are zero. Extraction of nucleotide i (0 ≤ i < k): (kmer >> (62 - 2*i)) & 0b11.
Reverse complement is computed via a 16-bit lookup table (65 536 entries × 2 bytes = 128 KB, fits in L2 cache) storing the reverse-complement of every 8-base chunk.
!!! abstract "Algorithm — Kmer reverse complement"
text procedure KmerRevcomp(kmer, k): raw ← TABLE16[kmer & 0xFFFF] << 48 | TABLE16[(kmer >> 16) & 0xFFFF] << 32 | TABLE16[(kmer >> 32) & 0xFFFF] << 16 | TABLE16[(kmer >> 48) & 0xFFFF] return raw << (64 - 2*k)
The canonical form is the lexicographic minimum of the kmer and its reverse complement:
canonical(kmer) = min(kmer, revcomp(kmer))
This halves the kmer space and ensures strand-independent counting.