Add IndexFunc and LastIndexFunc.

Change TrimRight and TrimLeft to use these functions.
Incidentally fix minor bug in TrimRight.
Add some test cases for this.
YMMV whether it's worth saving the closure allocation.

R=r, r2
CC=golang-dev, hoisie, rsc
https://golang.org/cl/1198044

src/pkg/strings/strings.go

@@ -328,43 +328,24 @@ func ToTitleSpecial(_case unicode.SpecialCase, s string) string {
 // TrimLeftFunc returns a slice of the string s with all leading
 // Unicode code points c satisfying f(c) removed.
 func TrimLeftFunc(s string, f func(r int) bool) string {
-	start, end := 0, len(s)
-	for start < end {
-		wid := 1
-		rune := int(s[start])
-		if rune >= utf8.RuneSelf {
-			rune, wid = utf8.DecodeRuneInString(s[start:end])
-		}
-		if !f(rune) {
-			return s[start:]
-		}
-		start += wid
+	i := indexFunc(s, f, false)
+	if i == -1 {
+		return ""
 	}
-	return s[start:]
+	return s[i:]
 }
 
 // TrimRightFunc returns a slice of the string s with all trailing
 // Unicode code points c satisfying f(c) removed.
 func TrimRightFunc(s string, f func(r int) bool) string {
-	start, end := 0, len(s)
-	for start < end {
-		wid := 1
-		rune := int(s[end-wid])
-		if rune >= utf8.RuneSelf {
-			// Back up & look for beginning of rune. Mustn't pass start.
-			for wid = 2; start <= end-wid && !utf8.RuneStart(s[end-wid]); wid++ {
-			}
-			if start > end-wid { // invalid UTF-8 sequence; stop processing
-				return s[start:end]
-			}
-			rune, wid = utf8.DecodeRuneInString(s[end-wid : end])
-		}
-		if !f(rune) {
-			return s[0:end]
-		}
-		end -= wid
+	i := lastIndexFunc(s, f, false)
+	if i >= 0 && s[i] >= utf8.RuneSelf {
+		_, wid := utf8.DecodeRuneInString(s[i:])
+		i += wid
+	} else {
+		i++
 	}
-	return s[0:end]
+	return s[0:i]
 }
 
 // TrimFunc returns a slice of the string s with all leading
@@ -373,6 +354,75 @@ func TrimFunc(s string, f func(r int) bool) string {
 	return TrimRightFunc(TrimLeftFunc(s, f), f)
 }
 
+// IndexFunc returns the index into s of the first Unicode
+// code point satisfying f(c), or -1 if none do.
+func IndexFunc(s string, f func(r int) bool) int {
+	return indexFunc(s, f, true)
+}
+
+// LastIndexFunc returns the index into s of the last
+// Unicode code point satisfying f(c), or -1 if none do.
+func LastIndexFunc(s string, f func(r int) bool) int {
+	return lastIndexFunc(s, f, true)
+}
+
+// indexFunc is the same as IndexFunc except that if
+// truth==false, the sense of the predicate function is
+// inverted. We could use IndexFunc directly, but this
+// way saves a closure allocation.
+func indexFunc(s string, f func(r int) bool, truth bool) int {
+	start := 0
+	for start < len(s) {
+		wid := 1
+		rune := int(s[start])
+		if rune >= utf8.RuneSelf {
+			rune, wid = utf8.DecodeRuneInString(s[start:])
+		}
+		if f(rune) == truth {
+			return start
+		}
+		start += wid
+	}
+	return -1
+}
+
+// lastIndexFunc is the same as LastIndexFunc except that if
+// truth==false, the sense of the predicate function is
+// inverted. We could use IndexFunc directly, but this
+// way saves a closure allocation.
+func lastIndexFunc(s string, f func(r int) bool, truth bool) int {
+	end := len(s)
+	for end > 0 {
+		start := end - 1
+		rune := int(s[start])
+		if rune >= utf8.RuneSelf {
+			// Back up & look for beginning of rune. Mustn't pass start.
+			for start--; start >= 0; start-- {
+				if utf8.RuneStart(s[start]) {
+					break
+				}
+			}
+			if start < 0 {
+				return -1
+			}
+			var wid int
+			rune, wid = utf8.DecodeRuneInString(s[start:end])
+
+			// If we've decoded fewer bytes than we expected,
+			// we've got some invalid UTF-8, so make sure we return
+			// the last possible index in s.
+			if start+wid < end && f(utf8.RuneError) == truth {
+				return end - 1
+			}
+		}
+		if f(rune) == truth {
+			return start
+		}
+		end = start
+	}
+	return -1
+}
+
 func makeCutsetFunc(cutset string) func(rune int) bool {
 	return func(rune int) bool { return IndexRune(cutset, rune) != -1 }
 }