From f5774b580b32cf19eaf92892f83afe10573659b9 Mon Sep 17 00:00:00 2001
From: Robert Sayre <sayrer@gmail.com>
Date: Fri, 23 Jan 2026 12:59:51 -0800
Subject: [PATCH] Pool result buffer in nfaBuffers to reduce allocations

This change adds a resultBuf field to nfaBuffers and a matchesInto()
method to matchSet. Instead of allocating a new slice for each match
result, we reuse the pooled buffer by appending matches to it. The
buffer is reset to length 0 before each use while preserving capacity.
---
 core_matcher.go |  2 +-
 match_set.go    |  9 +++++++++
 nfa.go          | 10 ++++++----
 3 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/core_matcher.go b/core_matcher.go
index 142ad4a..891c00b 100644
--- a/core_matcher.go
+++ b/core_matcher.go
@@ -177,7 +177,7 @@ func (m *coreMatcher) matchesForFields(fields []Field, bufs *nfaBuffers) ([]X, e
 	for i := 0; i < len(fields); i++ {
 		tryToMatch(fields, i, cmFields.state, matches, bufs)
 	}
-	return matches.matches(), nil
+	return matches.matchesInto(bufs.resultBuf[:0]), nil
 }
 
 // tryToMatch tries to match the field at fields[index] to the provided state. If it does match and generate
diff --git a/match_set.go b/match_set.go
index 1da255e..712d684 100644
--- a/match_set.go
+++ b/match_set.go
@@ -46,3 +46,12 @@ func (m *matchSet) matches() []X {
 	}
 	return matches
 }
+
+// matchesInto appends matches to the provided buffer and returns it.
+// This avoids allocating a new slice for the result.
+func (m *matchSet) matchesInto(buf []X) []X {
+	for x := range m.set {
+		buf = append(buf, x)
+	}
+	return buf
+}
diff --git a/nfa.go b/nfa.go
index 1bdca1d..af8ce9d 100644
--- a/nfa.go
+++ b/nfa.go
@@ -74,14 +74,16 @@ type nfaBuffers struct {
 	buf1, buf2 []*faState
 	eClosure   *epsilonClosure
 	matches    *matchSet
+	resultBuf  []X
 }
 
 func newNfaBuffers() *nfaBuffers {
 	return &nfaBuffers{
-		buf1:     make([]*faState, 0, 16),
-		buf2:     make([]*faState, 0, 16),
-		eClosure: newEpsilonClosure(),
-		matches:  newMatchSet(),
+		buf1:      make([]*faState, 0, 16),
+		buf2:      make([]*faState, 0, 16),
+		eClosure:  newEpsilonClosure(),
+		matches:   newMatchSet(),
+		resultBuf: make([]X, 0, 16),
 	}
 }