Added strings.FieldsFunc, a generalization of strings.Fields in style of the...

Added strings.FieldsFunc, a generalization of strings.Fields in style of the strings.Trim*Func functions.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/824051

src/pkg/strings/strings.go

@@ -172,12 +172,19 @@ func SplitAfter(s, sep string, n int) []string {
 // Fields splits the string s around each instance of one or more consecutive white space
 // characters, returning an array of substrings of s or an empty list if s contains only white space.
 func Fields(s string) []string {
+	return FieldsFunc(s, unicode.IsSpace)
+}
+
+// FieldsFunc splits the string s at each run of Unicode code points c satifying f(c)
+// and returns an array of slices of s. If no code points in s satisfy f(c), an empty slice
+// is returned.
+func FieldsFunc(s string, f func(int) bool) []string {
 	// First count the fields.
 	n := 0
 	inField := false
 	for _, rune := range s {
 		wasInField := inField
-		inField = !unicode.IsSpace(rune)
+		inField = !f(rune)
 		if inField && !wasInField {
 			n++
 		}
@@ -188,7 +195,7 @@ func Fields(s string) []string {
 	na := 0
 	fieldStart := -1 // Set to -1 when looking for start of field.
 	for i, rune := range s {
-		if unicode.IsSpace(rune) {
+		if f(rune) {
 			if fieldStart >= 0 {
 				a[na] = s[fieldStart:i]
 				na++

src/pkg/strings/strings_test.go

@@ -222,6 +222,22 @@ func TestFields(t *testing.T) {
 	}
 }
 
+func TestFieldsFunc(t *testing.T) {
+	pred := func(c int) bool { return c == 'X' }
+	var fieldsFuncTests = []FieldsTest{
+		FieldsTest{"", []string{}},
+		FieldsTest{"XX", []string{}},
+		FieldsTest{"XXhiXXX", []string{"hi"}},
+		FieldsTest{"aXXbXXXcX", []string{"a", "b", "c"}},
+	}
+	for _, tt := range fieldsFuncTests {
+		a := FieldsFunc(tt.s, pred)
+		if !eq(a, tt.a) {
+			t.Errorf("FieldsFunc(%q) = %v, want %v", tt.s, a, tt.a)
+		}
+	}
+}
+
 
 // Test case for any function which accepts and returns a single string.
 type StringTest struct {