Crate packed_seq

Types and traits to iterate over (packed) input data.

The main type is PackedSeqVec, that holds a sequence of 2-bit packed DNA bases. PackedSeq is a non-owned slice of packed data.

To make libraries depending on this crate more generic, logic is encapsulated in the Seq and SeqVec traits. Seq is a non-owned slice of characters that can be iterated, while SeqVec is the corresponding owned type.

These traits serve two purposes:

1. They encapsulate the packing/unpacking of characters between ASCII and the possibly different in-memory format.
2. They allow efficiently iterating over 8 chunks of a sequence in parallel using SIMD instructions.

Sequence types

The traits are implemented for three types.

Plain ASCII sequences

With &[u8]: Seq and Vec<u8>: SeqVec, the ASCII characters (or arbitrary u8 values, really) of any input slice can be iterated.

ASCII-encoded DNA sequences

The [AsciiSeq: Seq] and [AsciiSeqVec: SeqVec] types store a DNA sequence of ACTGactg characters. When iterated, these are returned as 0123 values, with the mapping A=0, C=1, T=2, G=3.

Any other characters are silently mapped to 0123 using (c>>1) & 3, but this should not be relied upon.

Packed DNA

The [PackedSeq: Seq] and [PackedSeqVec: SeqVec] types store a packed DNA sequence, encoded as 4 bases per byte. Each ACTG base is stored as 0123 as above and four of these 2-bit values fill a byte.

Use PackedSeqVec::from_ascii to construct a PackedSeqVec. Currently this relies on the pext instruction for good performance on x86.

Parallel iterators

This library enables iterating 8 chunks of a sequence at the same time using SIMD instructions. The Seq::par_iter_bp functions return a wide::u32x8 that contains the 2-bit or 8-bit values of the next character in each chunk in a u32 for 8 SIMD lanes.

This is used in the simd-minimizers crate, and explained in more detail in the corresponding preprint.

Context

The context parameter determines how much adjacent chunks overlap. When context=1, they are disjoint. When context=k, adjacent chunks overlap by k-1 characters, so that each k-mer is present in exactly one chunk. Thus, this can be used to iterate all k-mers, where the first k-1 characters in each chunk are used to initialize the first k-mer.

Delayed iteration

This crate also provides Seq::par_iter_bp_delayed and Seq::par_iter_bp_delayed_2 functions. Like Seq::par_iter_bp, these split the input into 8 chunks and stream over the chunks in parallel. But instead of just returning a single character, they also return a second (and third) character, that is delay positions behind the new character (at index idx - delay). This way, k-mers can be enumerated by setting delay=k and then mapping e.g. |(add, remove)| kmer = (kmer<<2) ^ add ^ (remove << (2*k)).

Example

use packed_seq::{SeqVec, Seq, AsciiSeqVec, PackedSeqVec, pack_char};
// Plain ASCII sequence.
let seq = b"ACTGCAGCGCATATGTAGT";
// ASCII DNA sequence.
let ascii_seq = AsciiSeqVec::from_ascii(seq);
// Packed DNA sequence.
let packed_seq = PackedSeqVec::from_ascii(seq);
assert_eq!(ascii_seq.len(), packed_seq.len());
// Iterate the ASCII characters.
let characters: Vec<u8> = seq.iter_bp().collect();
assert_eq!(characters, seq);

// Iterate the bases with 0..4 values.
let bases: Vec<u8> = seq.iter().copied().map(pack_char).collect();
assert_eq!(bases, vec![0,1,2,3,1,0,3,1,3,1,0,2,0,2,3,2,0,3,2]);
let ascii_bases: Vec<u8> = ascii_seq.as_slice().iter_bp().collect();
assert_eq!(ascii_bases, bases);
let packed_bases: Vec<u8> = ascii_seq.as_slice().iter_bp().collect();
assert_eq!(packed_bases, bases);

// Iterate over 8 chunks at the same time.
let seq = b"AAAACCTTGGTTACTG"; // plain ASCII sequence
// chunks:  ^ ^ ^ ^ ^ ^ ^ ^
let (par_iter, padding) = seq.as_slice().par_iter_bp(1);
let mut par_iter_u8 = par_iter.map(|x| x.as_array_ref().map(|c| c as u8));
assert_eq!(par_iter_u8.next(), Some(*b"AACTGTAT"));
assert_eq!(par_iter_u8.next(), Some(*b"AACTGTCG"));
assert_eq!(par_iter_u8.next(), None);

Feature flags

• epserde enables derive(epserde::Epserde) for PackedSeqVec and AsciiSeqVec, and adds its SerializeInner and DeserializeInner traits to SeqVec.
• pyo3 enables derive(pyo3::pyclass) for PackedSeqVec and AsciiSeqVec.

Structs

AsciiSeq

A &[u8] representing an ASCII-encoded DNA sequence of ACGTacgt.

AsciiSeqVec

A Vec<u8> representing an ASCII-encoded DNA sequence of ACGTacgt.

PackedSeq

A 2-bit packed non-owned slice of DNA bases.

PackedSeqVec

A 2-bit packed owned sequence of DNA bases.

u32x8

A SIMD vector containing 8 u32s.

Constants

L

The number of lanes in a u32x8.

Traits

Seq

A non-owned slice of characters.

SeqVec

An owned sequence. Can be constructed from either ASCII input or the underlying non-owning Seq type.

Functions

complement_base

Complement a 2-bit base: 0<>2 and 1<>3.

complement_base_simd

Complement 8 lanes of 2-bit bases: 0<>2 and 1<>3.

complement_char

Complement an ASCII character: A<>T and C<>G.

pack_char

Pack an ASCII ACTGactg character into its 2-bit representation.

unpack_base

Unpack a 2-bit DNA base into the corresponding ACTG character.