From f6edf9245a6a286402086459d5bc6ae81efdeef1 Mon Sep 17 00:00:00 2001
From: Andrew Kane <andrew@ankane.org>
Date: Fri, 5 Dec 2025 15:26:49 -0800
Subject: [PATCH] Added support for adaptive beam search [skip ci]

---
 README.md       | 1 +
 src/hnswutils.c | 9 ++++++---
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/README.md b/README.md
index a108520..e6bdcc7 100644
--- a/README.md
+++ b/README.md
@@ -1282,6 +1282,7 @@ Thanks to:
 - [k-means++: The Advantage of Careful Seeding](https://theory.stanford.edu/~sergei/papers/kMeansPP-soda.pdf)
 - [Concept Decompositions for Large Sparse Text Data using Clustering](https://www.cs.utexas.edu/users/inderjit/public_papers/concept_mlj.pdf)
 - [Efficient and Robust Approximate Nearest Neighbor Search using Hierarchical Navigable Small World Graphs](https://arxiv.org/ftp/arxiv/papers/1603/1603.09320.pdf)
+- [Distance Adaptive Beam Search for Provably Accurate Graph-Based Nearest Neighbor Search](https://arxiv.org/pdf/2505.15636)
 
 ## History
 
diff --git a/src/hnswutils.c b/src/hnswutils.c
index 8e2a42c..65de3c8 100644
--- a/src/hnswutils.c
+++ b/src/hnswutils.c
@@ -829,6 +829,7 @@ HnswSearchLayer(char *base, HnswQuery * q, List *ep, int ef, int lc, Relation in
 	HnswUnvisited *unvisited = palloc(lm * sizeof(HnswUnvisited));
 	int			unvisitedLength;
 	bool		inMemory = index == NULL;
+	double		lambda = 0.12;
 
 	if (v == NULL)
 	{
@@ -884,7 +885,7 @@ HnswSearchLayer(char *base, HnswQuery * q, List *ep, int ef, int lc, Relation in
 		HnswSearchCandidate *f = HnswGetSearchCandidate(w_node, pairingheap_first(W));
 		HnswElement cElement;
 
-		if (c->distance > f->distance)
+		if (c->distance > f->distance * (1 + lambda))
 			break;
 
 		cElement = HnswPtrAccess(base, c->element);
@@ -904,8 +905,10 @@ HnswSearchLayer(char *base, HnswQuery * q, List *ep, int ef, int lc, Relation in
 			HnswSearchCandidate *e;
 			double		eDistance;
 			bool		alwaysAdd = wlen < ef;
+			double		fDistance;
 
 			f = HnswGetSearchCandidate(w_node, pairingheap_first(W));
+			fDistance = f->distance * (1 + lambda);
 
 			if (inMemory)
 			{
@@ -920,13 +923,13 @@ HnswSearchLayer(char *base, HnswQuery * q, List *ep, int ef, int lc, Relation in
 
 				/* Avoid any allocations if not adding */
 				eElement = NULL;
-				HnswLoadElementImpl(blkno, offno, &eDistance, q, index, support, inserting, alwaysAdd || discarded != NULL ? NULL : &f->distance, &eElement);
+				HnswLoadElementImpl(blkno, offno, &eDistance, q, index, support, inserting, alwaysAdd || discarded != NULL ? NULL : &fDistance, &eElement);
 
 				if (eElement == NULL)
 					continue;
 			}
 
-			if (eElement == NULL || !(eDistance < f->distance || alwaysAdd))
+			if (eElement == NULL || !(eDistance < fDistance || alwaysAdd))
 			{
 				if (discarded != NULL)
 				{