From 8aef2201d817203a3ebd666457912efe3c5e68e8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E9=80=8D=E9=81=A5?= <caowenbo@shizhuang-inc.com>
Date: Tue, 21 Nov 2023 18:45:37 +0800
Subject: [PATCH] perf: Optimize the performance of union method, avoid
 repeated expansion

---
 intersect.go | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/intersect.go b/intersect.go
index cf6cab3d..90312b88 100644
--- a/intersect.go
+++ b/intersect.go
@@ -144,8 +144,14 @@ func Difference[T comparable](list1 []T, list2 []T) ([]T, []T) {
 // Union returns all distinct elements from given collections.
 // result returns will not change the order of elements relatively.
 func Union[T comparable](lists ...[]T) []T {
-	result := []T{}
-	seen := map[T]struct{}{}
+	var capLen int
+
+	for _, list := range lists {
+		capLen += len(list)
+	}
+
+	result := make([]T, 0, capLen)
+	seen := make(map[T]struct{}, capLen)
 
 	for _, list := range lists {
 		for _, e := range list {