encoding/hex: change lookup table from string to array

name            old time/op  new time/op  delta
Encode/256-4     431ns ± 2%   391ns ± 2%   -9.36%  (p=0.000 n=8+8)
Encode/1024-4   1.68µs ± 0%  1.51µs ± 0%   -9.91%  (p=0.001 n=7+7)
Encode/4096-4   6.68µs ± 0%  6.03µs ± 1%   -9.69%  (p=0.000 n=8+8)
Encode/16384-4  27.0µs ± 1%  24.0µs ± 0%  -11.03%  (p=0.000 n=8+7)

Change-Id: I6994e02f77797349c4e188377d84f97dffe98399
Reviewed-on: https://go-review.googlesource.com/27254Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/hex/hex.go

@@ -12,7 +12,10 @@ import (
 	"io"
 )
 
-const hextable = "0123456789abcdef"
+var hextable = [16]byte{
+	'0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
+	'a', 'b', 'c', 'd', 'e', 'f',
+}
 
 // EncodedLen returns the length of an encoding of n source bytes.
 func EncodedLen(n int) int { return n * 2 }

src/encoding/hex/hex_test.go

@@ -6,6 +6,7 @@ package hex
 
 import (
 	"bytes"
+	"fmt"
 	"testing"
 )
 
@@ -151,3 +152,18 @@ var expectedHexDump = []byte(`00000000  1e 1f 20 21 22 23 24 25  26 27 28 29 2a
 00000010  2e 2f 30 31 32 33 34 35  36 37 38 39 3a 3b 3c 3d  |./0123456789:;<=|
 00000020  3e 3f 40 41 42 43 44 45                           |>?@ABCDE|
 `)
+
+var sink []byte
+
+func BenchmarkEncode(b *testing.B) {
+	for _, size := range []int{256, 1024, 4096, 16384} {
+		src := bytes.Repeat([]byte{2, 3, 5, 7, 9, 11, 13, 17}, size/8)
+		sink = make([]byte, 2*size)
+
+		b.Run(fmt.Sprintf("%v", size), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				Encode(sink, src)
+			}
+		})
+	}
+}