bytes, strings: add LastIndexByte

Currently the packages have the following index functions:

func Index(s, sep []byte) int
func IndexAny(s []byte, chars string) int
func IndexByte(s []byte, c byte) int
func IndexFunc(s []byte, f func(r rune) bool) int
func IndexRune(s []byte, r rune) int

func LastIndex(s, sep []byte) int
func LastIndexAny(s []byte, chars string) int
func LastIndexFunc(s []byte, f func(r rune) bool) int

Searching for the last occurrence of a byte is quite common
for string parsing algorithms (e.g. find the last paren on a line).
Also addition of LastIndexByte makes the set more orthogonal.

Change-Id: Ida168849acacf8e78dd70c1354bef9eac5effafe
Reviewed-on: https://go-review.googlesource.com/9500
Reviewed-by: Rob Pike <r@golang.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/strings/strings_test.go b/src/strings/strings_test.go
index ee0c260753..4e21deaecd 100644
--- a/src/strings/strings_test.go
+++ b/src/strings/strings_test.go
@@ -120,6 +120,23 @@ func TestLastIndex(t *testing.T)    { runIndexTests(t, LastIndex, "LastIndex", l
 func TestIndexAny(t *testing.T)     { runIndexTests(t, IndexAny, "IndexAny", indexAnyTests) }
 func TestLastIndexAny(t *testing.T) { runIndexTests(t, LastIndexAny, "LastIndexAny", lastIndexAnyTests) }
 
+func TestLastIndexByte(t *testing.T) {
+	testCases := []IndexTest{
+		{"", "q", -1},
+		{"abcdef", "q", -1},
+		{"abcdefabcdef", "a", len("abcdef")},      // something in the middle
+		{"abcdefabcdef", "f", len("abcdefabcde")}, // last byte
+		{"zabcdefabcdef", "z", 0},                 // first byte
+		{"a☺b☻c☹d", "b", len("a☺")},               // non-ascii
+	}
+	for _, test := range testCases {
+		actual := LastIndexByte(test.s, test.sep[0])
+		if actual != test.out {
+			t.Errorf("LastIndexByte(%q,%c) = %v; want %v", test.s, test.sep[0], actual, test.out)
+		}
+	}
+}
+
 var indexRuneTests = []struct {
 	s    string
 	rune rune