From 3ca52f4c319e56986586e3d519924876320b29fb Mon Sep 17 00:00:00 2001
From: Alan Donovan <adonovan@google.com>
Date: Thu, 16 Feb 2023 11:02:12 -0500
Subject: [PATCH] slices: add in-place Reverse function

Fixes #58565

Change-Id: I583f8380c12386178fb18e553322bbb019d9fae0
Reviewed-on: https://go-review.googlesource.com/c/go/+/468855
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Shay Nehmad <dude500@gmail.com>
---
 api/next/58565.txt        |  1 +
 doc/go1.21.html           | 10 ++++++++++
 src/slices/slices.go      |  7 +++++++
 src/slices/slices_test.go | 28 ++++++++++++++++++++++++++++
 4 files changed, 46 insertions(+)
 create mode 100644 api/next/58565.txt

diff --git a/api/next/58565.txt b/api/next/58565.txt
new file mode 100644
index 0000000000..88ae7e52b1
--- /dev/null
+++ b/api/next/58565.txt
@@ -0,0 +1 @@
+pkg slices, func Reverse[$0 interface{}]([]$0) #58565
diff --git a/doc/go1.21.html b/doc/go1.21.html
index 3e63016e02..743e2a3e28 100644
--- a/doc/go1.21.html
+++ b/doc/go1.21.html
@@ -668,6 +668,16 @@ Do not send CLs removing the interior tags from such phrases.
   </dd>
 </dl><!-- spec -->
 
+<dl id="slices"><dt><a href="/pkg/slices/">slices</a></dt>
+  <dd>
+    <p><!-- https://go.dev/issue/45955 -->
+      The new <a href="/pkg/slices/"><code>slices</code></a> package
+      provides many common operations on slices, using generic
+      functions that work with slices of any element type.
+    </p>
+  </dd>
+</dl>
+
 <dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
   <dd>
     <p><!-- https://go.dev/issue/56102, CL 451356 -->
diff --git a/src/slices/slices.go b/src/slices/slices.go
index 837863bacc..7de00b342f 100644
--- a/src/slices/slices.go
+++ b/src/slices/slices.go
@@ -445,3 +445,10 @@ func startIdx[S ~[]E, E any](haystack, needle S) int {
 	// TODO: what if the overlap is by a non-integral number of Es?
 	panic("needle not found")
 }
+
+// Reverse reverses the elements of the slice in place.
+func Reverse[E any](s []E) {
+	for i, j := 0, len(s)-1; i < j; i, j = i+1, j-1 {
+		s[i], s[j] = s[j], s[i]
+	}
+}
diff --git a/src/slices/slices_test.go b/src/slices/slices_test.go
index 2f3a03bd9f..a99299321f 100644
--- a/src/slices/slices_test.go
+++ b/src/slices/slices_test.go
@@ -623,6 +623,34 @@ func TestClip(t *testing.T) {
 	}
 }
 
+func TestReverse(t *testing.T) {
+	even := []int{3, 1, 4, 1, 5, 9} // len = 6
+	Reverse(even)
+	if want := []int{9, 5, 1, 4, 1, 3}; !Equal(even, want) {
+		t.Errorf("Reverse(even) = %v, want %v", even, want)
+	}
+
+	odd := []int{3, 1, 4, 1, 5, 9, 2} // len = 7
+	Reverse(odd)
+	if want := []int{2, 9, 5, 1, 4, 1, 3}; !Equal(odd, want) {
+		t.Errorf("Reverse(odd) = %v, want %v", odd, want)
+	}
+
+	words := strings.Fields("one two three")
+	Reverse(words)
+	if want := strings.Fields("three two one"); !Equal(words, want) {
+		t.Errorf("Reverse(words) = %v, want %v", words, want)
+	}
+
+	singleton := []string{"one"}
+	Reverse(singleton)
+	if want := []string{"one"}; !Equal(singleton, want) {
+		t.Errorf("Reverse(singeleton) = %v, want %v", singleton, want)
+	}
+
+	Reverse[string](nil)
+}
+
 // naiveReplace is a baseline implementation to the Replace function.
 func naiveReplace[S ~[]E, E any](s S, i, j int, v ...E) S {
 	s = Delete(s, i, j)
-- 
2.44.0