internal/bytealg: add generic LastIndexByte{,String}

To avoid duplicating them in net/netip and os and to allow these
packages automatically benefiting from future performance improvements
when optimized native LastIndexByte{,String} implementations are added.

For #36891

Change-Id: I4905a4742273570c2c36b867df57762c5bfbe1e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/522475
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/src/internal/bytealg/lastindexbyte_generic.go b/src/internal/bytealg/lastindexbyte_generic.go
new file mode 100644
index 0000000000..b905f53c2b
--- /dev/null
+++ b/src/internal/bytealg/lastindexbyte_generic.go
@@ -0,0 +1,23 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytealg
+
+func LastIndexByte(s []byte, c byte) int {
+	for i := len(s) - 1; i >= 0; i-- {
+		if s[i] == c {
+			return i
+		}
+	}
+	return -1
+}
+
+func LastIndexByteString(s string, c byte) int {
+	for i := len(s) - 1; i >= 0; i-- {
+		if s[i] == c {
+			return i
+		}
+	}
+	return -1
+}