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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// This package implements the Adler-32 checksum.
// Defined in RFC 1950:
// Adler-32 is composed of two sums accumulated per byte: s1 is
// the sum of all bytes, s2 is the sum of all s1 values. Both sums
// are done modulo 65521. s1 is initialized to 1, s2 to zero. The
// Adler-32 checksum is stored as s2*65536 + s1 in most-
// significant-byte first (network) order.
package adler32
import (
"hash"
"os"
)
const (
mod = 65521
)
// The size of an Adler-32 checksum in bytes.
const Size = 4
// digest represents the partial evaluation of a checksum.
type digest struct {
// invariant: (a < mod && b < mod) || a <= b
// invariant: a + b + 255 <= 0xffffffff
a, b uint32
}
func (d *digest) Reset() { d.a, d.b = 1, 0 }
// New returns a new hash.Hash32 computing the Adler-32 checksum.
func New() hash.Hash32 {
d := new(digest)
d.Reset()
return d
}
func (d *digest) Size() int { return Size }
// Add p to the running checksum a, b.
func update(a, b uint32, p []byte) (aa, bb uint32) {
for i := 0; i < len(p); i++ {
a += uint32(p[i])
b += a
// invariant: a <= b
if b > (0xffffffff-255)/2 {
a %= mod
b %= mod
// invariant: a < mod && b < mod
} else {
// invariant: a + b + 255 <= 2 * b + 255 <= 0xffffffff
}
}
return a, b
}
// Return the 32-bit checksum corresponding to a, b.
func finish(a, b uint32) uint32 {
if b >= mod {
a %= mod
b %= mod
}
return b<<16 | a
}
func (d *digest) Write(p []byte) (nn int, err os.Error) {
d.a, d.b = update(d.a, d.b, p)
return len(p), nil
}
func (d *digest) Sum32() uint32 { return finish(d.a, d.b) }
func (d *digest) Sum() []byte {
p := make([]byte, 4)
s := d.Sum32()
p[0] = byte(s >> 24)
p[1] = byte(s >> 16)
p[2] = byte(s >> 8)
p[3] = byte(s)
return p
}
// Checksum returns the Adler-32 checksum of data.
func Checksum(data []byte) uint32 { return finish(update(1, 0, data)) }
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