From 736328f43c678a4f8132f8642ff275ae93983ff2 Mon Sep 17 00:00:00 2001
From: Yossef Hisham <93604359+yossev@users.noreply.github.com>
Date: Fri, 1 Aug 2025 21:52:44 +0300
Subject: [PATCH] Optimize prefix sum computation in Lucene99HnswVectorsReader

Replaced the two-step prefix sum loop in `Lucene99HnswVectorsReader` with a single-loop variant that avoids redundant memory access and improves performance.

Previous approach:
- Read first value separately.
- Then used previous buffer element + readVInt().

New approach:
- Accumulates sum in a single pass and assigns directly.

This change follows the suggestion from issue #14979 and has the same functional behavior with slightly better efficiency.
---
 .../lucene/codecs/lucene99/Lucene99HnswVectorsReader.java | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/lucene/core/src/java/org/apache/lucene/codecs/lucene99/Lucene99HnswVectorsReader.java b/lucene/core/src/java/org/apache/lucene/codecs/lucene99/Lucene99HnswVectorsReader.java
index faa885f7b2ea..a16e3dc8b06e 100644
--- a/lucene/core/src/java/org/apache/lucene/codecs/lucene99/Lucene99HnswVectorsReader.java
+++ b/lucene/core/src/java/org/apache/lucene/codecs/lucene99/Lucene99HnswVectorsReader.java
@@ -552,9 +552,11 @@ public void seek(int level, int targetOrd) throws IOException {
       arcCount = dataIn.readVInt();
       assert arcCount <= currentNeighborsBuffer.length : "too many neighbors: " + arcCount;
       if (arcCount > 0) {
-        currentNeighborsBuffer[0] = dataIn.readVInt();
-        for (int i = 1; i < arcCount; i++) {
-          currentNeighborsBuffer[i] = currentNeighborsBuffer[i - 1] + dataIn.readVInt();
+        // Faster prefix sum computation (see #14979)
+        int sum = 0;
+        for (int i = 0; i < arcCount; i++) {
+          sum += dataIn.readVInt();
+          currentNeighborsBuffer[i] = sum;
         }
       }
       arc = -1;