eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2024-08-26
17:1
17:25
10.4230/LIPIcs.WABI.2024.17
article
A*PA2: Up to 19× Faster Exact Global Alignment
Groot Koerkamp, Ragnar
1
https://orcid.org/0000-0002-2091-1237
ETH Zurich, Switzerland
Motivation. Pairwise alignment is at the core of computational biology. Most commonly used exact methods are either based on O(ns) band doubling or O(n+s²) diagonal transition, where n is the sequence length and s the number of errors. However, as the length of sequences has grown, these exact methods are often replaced by approximate methods based on e.g. seed-and-extend and heuristics to bound the computed region. We would like to develop an exact method that matches the performance of these approximate methods.
Recently, Astarix introduced the A* shortest path algorithm with the seed heuristic for exact sequence-to-graph alignment. A*PA adapted and improved this for pairwise sequence alignment and achieves near-linear runtime when divergence (error rate) is low, at the cost of being very slow when divergence is high.
Methods. We introduce A*PA2, an exact global pairwise aligner with respect to edit distance. The goal of A*PA2 is to unify the near-linear runtime of A*PA on similar sequences with the efficiency of dynamic programming (DP) based methods. Like Edlib, A*PA2 uses Ukkonen’s band doubling in combination with Myers' bitpacking. A*PA2 1) uses large block sizes inspired by Block Aligner, 2) extends this with SIMD (single instruction, multiple data), 3) introduces a new profile for efficient computations, 4) introduces a new optimistic technique for traceback based on diagonal transition, 5) avoids recomputation of states where possible, and 6) applies the heuristics developed in A*PA and improves them using pre-pruning.
Results. With the first 4 engineering optimizations, A*PA2-simple has complexity O(ns) and is 6× to 8× faster than Edlib for sequences ≥ 10 kbp. A*PA2-full also includes the heuristic and is often near-linear in practice for sequences with small divergence. The average runtime of A*PA2 is 19× faster than the exact aligners BiWFA and Edlib on >500 kbp long ONT (Oxford Nanopore Technologies) reads of a human genome having 6% divergence on average. On shorter ONT reads of 11% average divergence the speedup is 5.6× (avg. length 11 kbp) and 0.81× (avg. length 800 bp). On all tested datasets, A*PA2 is competitive with or faster than approximate methods.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol312-wabi2024/LIPIcs.WABI.2024.17/LIPIcs.WABI.2024.17.pdf
Edit distance
Pairwise alignment
A*
Shortest path
Dynamic programming