From 550de86135149c2fdd51fddd8093134c39c6f220 Mon Sep 17 00:00:00 2001
From: Dennis Gursky <81270766+dennygursky@users.noreply.github.com>
Date: Wed, 24 Mar 2021 10:04:37 -0700
Subject: [PATCH] Set Intersection #performance (#28183)

* Set Intersection #performance

Intersection is faster for sets of different sizes if one iterates over the shorter set and checks the presence of an element in a larger one. For an edge case consider `s` having 1M entries and `other` no entries at all. In this case original code will iterate over 1M entries in `s` not finding anything and then returning an empty result set. In the patched code the iteration won't happen at all and result is returned immediately.

This change is inspired by profiling a relatively large terraform configuration, where the time to validate was sped up 4x with this change.
---
 dag/set.go      | 15 ++++++++-------
 dag/set_test.go | 28 ++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+), 7 deletions(-)

diff --git a/dag/set.go b/dag/set.go
index fc16e801b..2e5d85b2d 100644
--- a/dag/set.go
+++ b/dag/set.go
@@ -38,17 +38,18 @@ func (s Set) Include(v interface{}) bool {
 // Intersection computes the set intersection with other.
 func (s Set) Intersection(other Set) Set {
 	result := make(Set)
-	if s == nil {
+	if s == nil || other == nil {
 		return result
 	}
-	if other != nil {
-		for _, v := range s {
-			if other.Include(v) {
-				result.Add(v)
-			}
+	// Iteration over a smaller set has better performance.
+	if other.Len() < s.Len() {
+		s, other = other, s
+	}
+	for _, v := range s {
+		if other.Include(v) {
+			result.Add(v)
 		}
 	}
-
 	return result
 }
 
diff --git a/dag/set_test.go b/dag/set_test.go
index 36bd6a65b..d59eacfbd 100644
--- a/dag/set_test.go
+++ b/dag/set_test.go
@@ -119,3 +119,31 @@ func TestSetCopy(t *testing.T) {
 	}
 
 }
+
+func makeSet(n int) Set {
+	ret := make(Set, n)
+	for i := 0; i < n; i++ {
+		ret.Add(i)
+	}
+	return ret
+}
+
+func BenchmarkSetIntersection_100_100000(b *testing.B) {
+	small := makeSet(100)
+	large := makeSet(100000)
+
+	b.ResetTimer()
+	for n := 0; n < b.N; n++ {
+		small.Intersection(large)
+	}
+}
+
+func BenchmarkSetIntersection_100000_100(b *testing.B) {
+	small := makeSet(100)
+	large := makeSet(100000)
+
+	b.ResetTimer()
+	for n := 0; n < b.N; n++ {
+		large.Intersection(small)
+	}
+}