mime: Remove an allocation in word decoding.

This fixes a TODO in (*WordDecoder).Decode by replacing a call to strings.Split with
simple prefix/suffix checking and some custom slicing.

Benchmark results:

benchmark                    old ns/op     new ns/op     delta
BenchmarkQEncodeWord-8       740           693           -6.35%
BenchmarkQDecodeWord-8       1291          727           -43.69%
BenchmarkQDecodeHeader-8     1194          767           -35.76%

benchmark                    old allocs     new allocs     delta
BenchmarkQEncodeWord-8       1              1              +0.00%
BenchmarkQDecodeWord-8       3              2              -33.33%
BenchmarkQDecodeHeader-8     3              2              -33.33%

benchmark                    old bytes     new bytes     delta
BenchmarkQEncodeWord-8       48            48            +0.00%
BenchmarkQDecodeWord-8       128           48            -62.50%
BenchmarkQDecodeHeader-8     128           48            -62.50%

Change-Id: I2d6844c75ec2e2b79be2e49b7fc4ca320b7e84e5
Reviewed-on: https://go-review.googlesource.com/14432
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/mime/encodedword.go

@@ -194,20 +194,32 @@ type WordDecoder struct {
 
 // Decode decodes an RFC 2047 encoded-word.
 func (d *WordDecoder) Decode(word string) (string, error) {
-	fields := strings.Split(word, "?") // TODO: remove allocation?
-	if len(fields) != 5 || fields[0] != "=" || fields[4] != "=" || len(fields[2]) != 1 {
+	if !strings.HasPrefix(word, "=?") || !strings.HasSuffix(word, "?=") || strings.Count(word, "?") != 4 {
 		return "", errInvalidWord
 	}
+	word = word[2 : len(word)-2]
 
-	content, err := decode(fields[2][0], fields[3])
+	// split delimits the first 2 fields
+	split := strings.IndexByte(word, '?')
+	// the field after split must only be one byte
+	if word[split+2] != '?' {
+		return "", errInvalidWord
+	}
+
+	// split word "UTF-8?q?ascii" into "UTF-8", 'q', and "ascii"
+	charset := word[:split]
+	encoding := word[split+1]
+	text := word[split+3:]
+
+	content, err := decode(encoding, text)
 	if err != nil {
 		return "", err
 	}
 
 	buf := getBuffer()
 	defer putBuffer(buf)
 
-	if err := d.convert(buf, fields[1], content); err != nil {
+	if err := d.convert(buf, charset, content); err != nil {
 		return "", err
 	}