1.8 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 by bit manipulation in four steps, with no lookup table:
!!! abstract "Algorithm — Kmer reverse complement"
text procedure KmerRevcomp(kmer, k): x ← ~kmer -- complement all bases x ← swap_bytes(x) -- reverse byte order x ← ((x >> 4) & 0x0F0F0F0F0F0F0F0F) | ((x & 0x0F0F0F0F0F0F0F0F) << 4) -- swap nibbles within each byte x ← ((x >> 2) & 0x3333333333333333) | ((x & 0x3333333333333333) << 2) -- swap 2-bit pairs within each nibble return x << (64 - 2*k) -- re-align to MSB
The three reorder passes together reverse the order of all 2-bit base codes across the 64-bit word. The bitwise NOT in the first step complements each base (A↔T, C↔G). The final left shift clears the low 64−2k padding bits.
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.