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>

4 files changed, 66 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
+}
diff --git a/src/bytes/bytes_test.go b/src/bytes/bytes_test.go
index 07280464d2..20d333217e 100644
--- a/src/bytes/bytes_test.go
+++ b/src/bytes/bytes_test.go
@@ -1976,6 +1976,27 @@ func TestCut(t *testing.T) {
 	}
 }
 
+func TestCutLast(t *testing.T) {
+	tests := []struct {
+		s, sep        string
+		before, after string
+		found         bool
+	}{
+		{"a/b/c", "/", "a/b", "c", true},
+		{"a//b//c", "//", "a//b", "c", true},
+		{"abc", "/", "abc", "", false},
+		{"abc", "", "abc", "", true},
+		{"", "", "", "", true},
+		{"/abc", "/", "", "abc", true},
+		{"abc/", "/", "abc", "", true},
+	}
+	for _, tt := range tests {
+		if before, after, found := CutLast([]byte(tt.s), []byte(tt.sep)); string(before) != tt.before || string(after) != tt.after || found != tt.found {
+			t.Errorf("CutLast(%q, %q) = %q, %q, %v; want %q, %q, %v", tt.s, tt.sep, before, after, found, tt.before, tt.after, tt.found)
+		}
+	}
+}
+
 var cutPrefixTests = []struct {
 	s, sep string
 	after  string
diff --git a/src/strings/strings.go b/src/strings/strings.go
index 367e0a8e24..70297f1e69 100644
--- a/src/strings/strings.go
+++ b/src/strings/strings.go
@@ -1289,3 +1289,14 @@ func CutPrefix(s, prefix string) (after string, found bool) {
 func CutSuffix(s, suffix string) (before string, found bool) {
 	return stringslite.CutSuffix(s, suffix)
 }
+
+// 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, "", false.
+func CutLast(s, sep string) (before, after string, found bool) {
+	if i := LastIndex(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):], true
+	}
+	return s, "", false
+}
diff --git a/src/strings/strings_test.go b/src/strings/strings_test.go
index edfeb0e813..4ff3a2a825 100644
--- a/src/strings/strings_test.go
+++ b/src/strings/strings_test.go
@@ -1826,6 +1826,27 @@ func TestCut(t *testing.T) {
 	}
 }
 
+func TestCutLast(t *testing.T) {
+	tests := []struct {
+		s, sep        string
+		before, after string
+		found         bool
+	}{
+		{"a/b/c", "/", "a/b", "c", true},
+		{"a//b//c", "//", "a//b", "c", true},
+		{"abc", "/", "abc", "", false},
+		{"abc", "", "abc", "", true},
+		{"", "", "", "", true},
+		{"/abc", "/", "", "abc", true},
+		{"abc/", "/", "abc", "", true},
+	}
+	for _, tt := range tests {
+		if before, after, found := CutLast(tt.s, tt.sep); before != tt.before || after != tt.after || found != tt.found {
+			t.Errorf("CutLast(%q, %q) = %q, %q, %v; want %q, %q, %v", tt.s, tt.sep, before, after, found, tt.before, tt.after, tt.found)
+		}
+	}
+}
+
 var cutPrefixTests = []struct {
 	s, sep string
 	after  string