91 lines
2.2 KiB
Go
91 lines
2.2 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package adler32 implements the Adler-32 checksum.
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//
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// It is defined in RFC 1950:
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// Adler-32 is composed of two sums accumulated per byte: s1 is
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// the sum of all bytes, s2 is the sum of all s1 values. Both sums
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// are done modulo 65521. s1 is initialized to 1, s2 to zero. The
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// Adler-32 checksum is stored as s2*65536 + s1 in most-
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// significant-byte first (network) order.
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package adler32
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import "hash"
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const (
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// mod is the largest prime that is less than 65536.
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mod = 65521
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// nmax is the largest n such that
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// 255 * n * (n+1) / 2 + (n+1) * (mod-1) <= 2^32-1.
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// It is mentioned in RFC 1950 (search for "5552").
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nmax = 5552
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)
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// The size of an Adler-32 checksum in bytes.
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const Size = 4
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// digest represents the partial evaluation of a checksum.
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// The low 16 bits are s1, the high 16 bits are s2.
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type digest uint32
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func (d *digest) Reset() { *d = 1 }
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// New returns a new hash.Hash32 computing the Adler-32 checksum.
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// Its Sum method will lay the value out in big-endian byte order.
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func New() hash.Hash32 {
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d := new(digest)
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d.Reset()
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return d
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}
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func (d *digest) Size() int { return Size }
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func (d *digest) BlockSize() int { return 4 }
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// Add p to the running checksum d.
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func update(d digest, p []byte) digest {
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s1, s2 := uint32(d&0xffff), uint32(d>>16)
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for len(p) > 0 {
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var q []byte
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if len(p) > nmax {
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p, q = p[:nmax], p[nmax:]
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}
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for len(p) >= 4 {
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s1 += uint32(p[0])
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s2 += s1
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s1 += uint32(p[1])
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s2 += s1
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s1 += uint32(p[2])
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s2 += s1
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s1 += uint32(p[3])
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s2 += s1
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p = p[4:]
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}
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for _, x := range p {
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s1 += uint32(x)
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s2 += s1
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}
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s1 %= mod
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s2 %= mod
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p = q
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}
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return digest(s2<<16 | s1)
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}
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func (d *digest) Write(p []byte) (nn int, err error) {
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*d = update(*d, p)
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return len(p), nil
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}
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func (d *digest) Sum32() uint32 { return uint32(*d) }
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func (d *digest) Sum(in []byte) []byte {
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s := uint32(*d)
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return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
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}
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// Checksum returns the Adler-32 checksum of data.
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func Checksum(data []byte) uint32 { return uint32(update(1, data)) }
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