Optimize base58 decoding.

This change introduces an autogenerated base58 digit table to remove
the need to find the index of a character in the modified base58
alphabet each time.  Additionally, it removes some unnecessary big
integer allocations to cut down on the GC churn.

Before:
BenchmarkBase58Encode         20          64998995 ns/op           0.08 MB/s
BenchmarkBase58Decode         50          35965928 ns/op           0.19 MB/s

Now:
BenchmarkBase58Encode         20          64644351 ns/op           0.08 MB/s
BenchmarkBase58Decode        200           7914748 ns/op           0.86 MB/s

base58/alphabet.go

@@ -0,0 +1,49 @@
+// Copyright (c) 2015 Conformal Systems LLC.
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+// AUTOGENERATED by genalphabet.go; do not edit.
+
+package base58
+
+const (
+	// alphabet is the modified base58 alphabet used by Bitcoin.
+	alphabet = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
+
+	alphabetIdx0 = '1'
+)
+
+var b58 = [256]byte{
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 0, 1, 2, 3, 4, 5, 6,
+	7, 8, 255, 255, 255, 255, 255, 255,
+	255, 9, 10, 11, 12, 13, 14, 15,
+	16, 255, 17, 18, 19, 20, 21, 255,
+	22, 23, 24, 25, 26, 27, 28, 29,
+	30, 31, 32, 255, 255, 255, 255, 255,
+	255, 33, 34, 35, 36, 37, 38, 39,
+	40, 41, 42, 43, 255, 44, 45, 46,
+	47, 48, 49, 50, 51, 52, 53, 54,
+	55, 56, 57, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+	255, 255, 255, 255, 255, 255, 255, 255,
+}

base58/base58.go

@@ -1,4 +1,4 @@
-// Copyright (c) 2013-2014 Conformal Systems LLC.
+// Copyright (c) 2013-2015 Conformal Systems LLC.
 // Use of this source code is governed by an ISC
 // license that can be found in the LICENSE file.
 
@@ -6,11 +6,9 @@ package base58
 
 import (
 	"math/big"
-	"strings"
 )
 
-// alphabet is the modified base58 alphabet used by Bitcoin.
+//go:generate go run genalphabet.go
-const alphabet = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
 
 var bigRadix = big.NewInt(58)
 var bigZero = big.NewInt(0)
@@ -20,16 +18,15 @@ func Decode(b string) []byte {
 	answer := big.NewInt(0)
 	j := big.NewInt(1)
 
+	scratch := new(big.Int)
 	for i := len(b) - 1; i >= 0; i-- {
-		tmp := strings.IndexAny(alphabet, string(b[i]))
+		tmp := b58[b[i]]
-		if tmp == -1 {
+		if tmp == 255 {
 			return []byte("")
 		}
-		idx := big.NewInt(int64(tmp))
+		scratch.SetInt64(int64(tmp))
-		tmp1 := big.NewInt(0)
+		scratch.Mul(j, scratch)
-		tmp1.Mul(j, idx)
+		answer.Add(answer, scratch)
-
-		answer.Add(answer, tmp1)
 		j.Mul(j, bigRadix)
 	}
 
@@ -37,7 +34,7 @@ func Decode(b string) []byte {
 
 	var numZeros int
 	for numZeros = 0; numZeros < len(b); numZeros++ {
-		if b[numZeros] != alphabet[0] {
+		if b[numZeros] != alphabetIdx0 {
 			break
 		}
 	}
@@ -65,7 +62,7 @@ func Encode(b []byte) string {
 		if i != 0 {
 			break
 		}
-		answer = append(answer, alphabet[0])
+		answer = append(answer, alphabetIdx0)
 	}
 
 	// reverse

base58/genalphabet.go

@@ -0,0 +1,79 @@
+// Copyright (c) 2015 Conformal Systems LLC.
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+//+build ignore
+
+package main
+
+import (
+	"bytes"
+	"io"
+	"log"
+	"os"
+	"strconv"
+)
+
+var (
+	start = []byte(`// Copyright (c) 2015 Conformal Systems LLC.
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+// AUTOGENERATED by genalphabet.go; do not edit.
+
+package base58
+
+const (
+	// alphabet is the modified base58 alphabet used by Bitcoin.
+	alphabet = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
+
+	alphabetIdx0 = '1'
+)
+
+var b58 = [256]byte{`)
+
+	end = []byte(`}`)
+
+	alphabet = []byte("123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz")
+	tab      = []byte("\t")
+	invalid  = []byte("255")
+	comma    = []byte(",")
+	space    = []byte(" ")
+	nl       = []byte("\n")
+)
+
+func write(w io.Writer, b []byte) {
+	_, err := w.Write(b)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func main() {
+	fi, err := os.Create("alphabet.go")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer fi.Close()
+
+	write(fi, start)
+	write(fi, nl)
+	for i := byte(0); i < 32; i++ {
+		write(fi, tab)
+		for j := byte(0); j < 8; j++ {
+			idx := bytes.IndexByte(alphabet, i*8+j)
+			if idx == -1 {
+				write(fi, invalid)
+			} else {
+				write(fi, strconv.AppendInt(nil, int64(idx), 10))
+			}
+			write(fi, comma)
+			if j != 7 {
+				write(fi, space)
+			}
+		}
+		write(fi, nl)
+	}
+	write(fi, end)
+	write(fi, nl)
+}