bytes,strings: add CutLast

Fixes #71151

Change-Id: I3b3d49c35b0fa2c1220d3f39bbd506cc072b52b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/764601
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/src/bytes/bytes.go b/src/bytes/bytes.go
index 98f78f10c3..2bd1284296 100644
--- a/src/bytes/bytes.go
+++ b/src/bytes/bytes.go
@@ -1422,3 +1422,16 @@ func CutSuffix(s, suffix []byte) (before []byte, found bool) {
 	}
 	return s[:len(s)-len(suffix)], true
 }
+
+// CutLast slices s around the last instance of sep,
+// returning the text before and after sep.
+// The found result reports whether sep appears in s.
+// If sep does not appear in s, CutLast returns s, nil, false.
+//
+// CutLast returns slices of the original slice s, not copies.
+func CutLast(s, sep []byte) (before, after []byte, found bool) {
+	if i := LastIndex(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):], true
+	}
+	return s, nil, false
+}