bytes, strings: add CutByte

CutByte optimizes slicing operations for single-byte
separators, offering a more efficient alternative when only a single byte
is involved.

There is more discussion on https://golang.org/issue/67101.

Fixes golang#67101.

src/bytes/bytes.go

@@ -1336,6 +1336,19 @@ func Cut(s, sep []byte) (before, after []byte, found bool) {
 	return s, nil, false
 }
 
+// CutByte slices s around the first 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, cut returns s, nil, false.
+//
+// CutByte returns slices of the original slice s, not copies.
+func CutByte(s []byte, sep byte) (before, after []byte, found bool) {
+	if i := IndexByte(s, sep); i >= 0 {
+		return s[:i], s[i+1:], true
+	}
+	return s, nil, false
+}
+
 // Clone returns a copy of b[:len(b)].
 // The result may have additional unused capacity.
 // Clone(nil) returns nil.

src/bytes/bytes_test.go

@@ -1742,6 +1742,26 @@ func TestCut(t *testing.T) {
 	}
 }
 
+var cutByteTests = []struct {
+	s             string
+	sep           byte
+	before, after string
+	found         bool
+}{
+	{"abc", 'b', "a", "c", true},
+	{"abc", 'a', "", "bc", true},
+	{"abc", 'c', "ab", "", true},
+	{"", 'd', "", "", false},
+}
+
+func TestCutByte(t *testing.T) {
+	for _, tt := range cutByteTests {
+		if before, after, found := CutByte([]byte(tt.s), tt.sep); string(before) != tt.before || string(after) != tt.after || found != tt.found {
+			t.Errorf("Cut(%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

src/strings/strings.go

@@ -1315,6 +1315,17 @@ func Cut(s, sep string) (before, after string, found bool) {
 	return s, "", false
 }
 
+// CutByte slices s around the first 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, cut returns s, "", false.
+func CutByte(s string, sep byte) (before, after string, found bool) {
+	if i := IndexByte(s, sep); i >= 0 {
+		return s[:i], s[i+1:], true
+	}
+	return s, "", false
+}
+
 // CutPrefix returns s without the provided leading prefix string
 // and reports whether it found the prefix.
 // If s doesn't start with prefix, CutPrefix returns s, false.

src/strings/strings_test.go

@@ -1669,6 +1669,26 @@ func TestCut(t *testing.T) {
 	}
 }
 
+var cutByteTests = []struct {
+	s             string
+	sep           byte
+	before, after string
+	found         bool
+}{
+	{"abc", 'b', "a", "c", true},
+	{"abc", 'a', "", "bc", true},
+	{"abc", 'c', "ab", "", true},
+	{"", 'd', "", "", false},
+}
+
+func TestCutByte(t *testing.T) {
+	for _, tt := range cutByteTests {
+		if before, after, found := CutByte(tt.s, tt.sep); before != tt.before || after != tt.after || found != tt.found {
+			t.Errorf("Cut(%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