From b2a52bfb3763fc3d82f2d8c707b89e14ee4ecabb Mon Sep 17 00:00:00 2001
From: Eric Coissac <eric@coissac.eu>
Date: Sat, 23 May 2026 13:06:07 +0200
Subject: [PATCH] perf: optimize chunk_start for single-block indexing

Bypasses bitwise shift and mask operations when `block_bits == 0`, directly indexing `self.block_offsets[i]` instead. This eliminates unnecessary arithmetic overhead for single-block cases while preserving the original block-based offset calculation for larger block sizes.
---
 src/obiskio/src/unitig_index.rs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/obiskio/src/unitig_index.rs b/src/obiskio/src/unitig_index.rs
index eb3fa40..53bdaa5 100644
--- a/src/obiskio/src/unitig_index.rs
+++ b/src/obiskio/src/unitig_index.rs
@@ -271,6 +271,9 @@ impl UnitigFileReader {
     fn chunk_start(&self, i: usize) -> usize {
         assert!(!self.block_offsets.is_empty(),
             "random access requires UnitigFileReader::open(); use open_sequential() for iteration only");
+        if self.block_bits == 0 {
+            return self.block_offsets[i] as usize;
+        }
         let block = i >> self.block_bits;
         let rem   = i &  self.mask;
         let mut offset = self.block_offsets[block] as usize;