strings: improve explode and correct comment

Merges explodetests into splittests which already contain
some of the tests that cover explode.

Adds a test to cover the utf8.RuneError branch in explode.

name      old time/op  new time/op  delta
Split1-2  14.9ms ± 0%  14.2ms ± 0%  -4.06%  (p=0.000 n=47+49)

Change-Id: I00f796bd2edab70e926ea9e65439d820c6a28254
Reviewed-on: https://go-review.googlesource.com/21609
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/strings/strings.go

@@ -12,32 +12,25 @@ import (
 	"unicode/utf8"
 )
 
-// explode splits s into an array of UTF-8 sequences, one per Unicode character (still strings) up to a maximum of n (n < 0 means no limit).
-// Invalid UTF-8 sequences become correct encodings of U+FFF8.
+// explode splits s into a slice of UTF-8 strings,
+// one string per Unicode character up to a maximum of n (n < 0 means no limit).
+// Invalid UTF-8 sequences become correct encodings of U+FFFD.
 func explode(s string, n int) []string {
-	if n == 0 {
-		return nil
-	}
 	l := utf8.RuneCountInString(s)
-	if n <= 0 || n > l {
+	if n < 0 || n > l {
 		n = l
 	}
 	a := make([]string, n)
-	var size int
-	var ch rune
-	i, cur := 0, 0
-	for ; i+1 < n; i++ {
-		ch, size = utf8.DecodeRuneInString(s[cur:])
+	for i := 0; i < n-1; i++ {
+		ch, size := utf8.DecodeRuneInString(s)
+		a[i] = s[:size]
+		s = s[size:]
 		if ch == utf8.RuneError {
 			a[i] = string(utf8.RuneError)
-		} else {
-			a[i] = s[cur : cur+size]
 		}
-		cur += size
 	}
-	// add the rest, if there is any
-	if cur < len(s) {
-		a[i] = s[cur:]
+	if n > 0 {
+		a[n-1] = s
 	}
 	return a
 }