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Fix multiple HNSW bugs causing intermittent search quality issues #371

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Merged
kriszyp merged 2 commits into from
Apr 17, 2026
Merged

Fix multiple HNSW bugs causing intermittent search quality issues #371

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73a712f
Fix multiple HNSW bugs causing intermittent search quality issues
kriszyp Apr 17, 2026
51d2db6
Update resources/indexes/HierarchicalNavigableSmallWorld.ts
kriszyp Apr 17, 2026
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30 changes: 23 additions & 7 deletions resources/indexes/HierarchicalNavigableSmallWorld.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -130,8 +130,8 @@ export class HierarchicalNavigableSmallWorld {
// Generate random level for this new element
const level = oldNode.level ?? Math.min(Math.floor(-Math.log(Math.random()) * this.mL), MAX_LEVEL);
let currentLevel = entryPoint.level;
if (level >= currentLevel) {
// if we are at this level or higher, make this the new entry point
if (level > currentLevel) {
// if we are at a higher level, make this the new entry point
if (typeof nodeId !== 'number') {
throw new Error('Invalid nodeId: ' + nodeId);
}
Expand Down Expand Up @@ -232,6 +232,19 @@ export class HierarchicalNavigableSmallWorld {
oldNode[l] = oldConnections;
}
oldConnections.splice(oldPosition, 1);
// update the distance in the reverse connection if the vector changed
if (oldConnection.distance !== distance) {
const neighborNode = updateNode(id, node);
if (neighborNode[l]) {
if (Object.isFrozen(neighborNode[l])) {
neighborNode[l] = neighborNode[l].slice();
}
const reverseIdx = neighborNode[l].findIndex(({ id: nid }) => nid === nodeId);
if (reverseIdx >= 0) {
neighborNode[l][reverseIdx] = { id: nodeId, distance };
}
}
}
} else {
// add new connection since this is truly a new connection now
this.addConnection(id, updateNode(id, node), nodeId, l, distance, updateNode, options);
Expand Down Expand Up @@ -360,7 +373,7 @@ export class HierarchicalNavigableSmallWorld {
const candidates = [
{
id: entryPointId,
distance: this.distance(queryVector, entryPoint.vector),
distance: distanceFunction(queryVector, entryPoint.vector),
node: entryPoint,
},
];
Expand Down Expand Up @@ -531,10 +544,13 @@ export class HierarchicalNavigableSmallWorld {
if (removedNode) {
// Remove the reverse connection if it exists
if (removedNode[level]) {
removedNode = updateNode(removed.id, removedNode);
removedNode[level] = removedNode[level].filter(({ id }) => id !== fromId);
if (level === 0 && removedNode[level].length === 0) {
logger.info?.('should not remove last connection', fromId, toId);
const filtered = removedNode[level].filter(({ id }) => id !== fromId);
if (level === 0 && filtered.length === 0) {
// don't remove the last connection at level 0 — it would orphan this node
logger.info?.('skipping removal of last connection', fromId, toId);
} else {
removedNode = updateNode(removed.id, removedNode);
removedNode[level] = filtered;
}
}
}
Expand Down
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