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utf8.String: Slice(i,j)
R=rsc CC=golang-dev https://golang.org/cl/2225048
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@ -11,6 +11,8 @@ package utf8
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// O(N) in the length of the string, but the overhead is less than always
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// scanning from the beginning.
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// If the string is ASCII, random access is O(1).
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// Unlike the built-in string type, String has internal mutable state and
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// is not thread-safe.
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type String struct {
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str string
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numRunes int
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@ -55,6 +57,39 @@ func (s *String) IsASCII() bool {
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return s.width == 0
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}
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// Slice returns the string sliced at rune positions [i:j].
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func (s *String) Slice(i, j int) string {
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// ASCII is easy. Let the compiler catch the indexing error if there is one.
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if j < s.nonASCII {
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return s.str[i:j]
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}
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if i < 0 || j > s.numRunes || i > j {
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panic(sliceOutOfRange)
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}
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if i == j {
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return ""
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}
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// For non-ASCII, after At(i), bytePos is always the position of the indexed character.
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var low, high int
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switch {
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case i < s.nonASCII:
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low = i
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case i == s.numRunes:
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low = len(s.str)
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default:
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s.At(i)
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low = s.bytePos
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}
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switch {
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case j == s.numRunes:
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high = len(s.str)
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default:
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s.At(j)
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high = s.bytePos
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}
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return s.str[low:high]
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}
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// At returns the rune with index i in the String. The sequence of runes is the same
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// as iterating over the contents with a "for range" clause.
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func (s *String) At(i int) int {
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@ -163,4 +198,5 @@ func (err error) String() string {
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func (err error) RunTimeError() {
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}
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var outOfRange = error("utf8.String: index out of Range")
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var outOfRange = error("utf8.String: index out of range")
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var sliceOutOfRange = error("utf8.String: slice index out of range")
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@ -68,3 +68,42 @@ func TestRandomAccess(t *testing.T) {
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}
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}
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}
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func TestRandomSliceAccess(t *testing.T) {
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for _, s := range testStrings {
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if len(s) == 0 || s[0] == '\x80' { // the bad-UTF-8 string fools this simple test
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continue
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}
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runes := []int(s)
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str := NewString(s)
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if str.RuneCount() != len(runes) {
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t.Error("%s: expected %d runes; got %d", s, len(runes), str.RuneCount())
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break
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}
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for k := 0; k < randCount; k++ {
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i := rand.Intn(len(runes))
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j := rand.Intn(len(runes) + 1)
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if i > j { // include empty strings
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continue
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}
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expect := string(runes[i:j])
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got := str.Slice(i, j)
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if got != expect {
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t.Errorf("%s[%d:%d]: expected %q got %q", s, i, j, expect, got)
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}
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}
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}
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}
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func TestLimitSliceAccess(t *testing.T) {
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for _, s := range testStrings {
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str := NewString(s)
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if str.Slice(0, 0) != "" {
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t.Error("failure with empty slice at beginning")
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}
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nr := RuneCountInString(s)
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if str.Slice(nr, nr) != "" {
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t.Error("failure with empty slice at end")
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}
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}
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}
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