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, 10 insertions, 0 deletions

diff --git a/src/strings/strings.go b/src/strings/strings.go
index c6085f51d5..7b8a6b536b 100644
--- a/src/strings/strings.go
+++ b/src/strings/strings.go
@@ -271,6 +271,16 @@ func LastIndexAny(s, chars string) int {
 	return -1
 }
 
+// LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
+func LastIndexByte(s string, c byte) int {
+	for i := len(s) - 1; i >= 0; i-- {
+		if s[i] == c {
+			return i
+		}
+	}
+	return -1
+}
+
 // Generic split: splits after each instance of sep,
 // including sepSave bytes of sep in the subarrays.
 func genSplit(s, sep string, sepSave, n int) []string {