1 files changed, 78 insertions, 0 deletions
diff --git a/src/hash/adler32/adler32.go b/src/hash/adler32/adler32.go
new file mode 100644
index 000000000..7c80796bf
--- /dev/null
+++ b/src/hash/adler32/adler32.go
@@ -0,0 +1,78 @@
+// 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.
+
+// Package adler32 implements the Adler-32 checksum.
+//
+// It is 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"
+
+const (
+	// mod is the largest prime that is less than 65536.
+	mod = 65521
+	// nmax is the largest n such that
+	// 255 * n * (n+1) / 2 + (n+1) * (mod-1) <= 2^32-1.
+	// It is mentioned in RFC 1950 (search for "5552").
+	nmax = 5552
+)
+
+// The size of an Adler-32 checksum in bytes.
+const Size = 4
+
+// digest represents the partial evaluation of a checksum.
+// The low 16 bits are s1, the high 16 bits are s2.
+type digest uint32
+
+func (d *digest) Reset() { *d = 1 }
+
+// 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 }
+
+func (d *digest) BlockSize() int { return 1 }
+
+// Add p to the running checksum d.
+func update(d digest, p []byte) digest {
+	s1, s2 := uint32(d&0xffff), uint32(d>>16)
+	for len(p) > 0 {
+		var q []byte
+		if len(p) > nmax {
+			p, q = p[:nmax], p[nmax:]
+		}
+		for _, x := range p {
+			s1 += uint32(x)
+			s2 += s1
+		}
+		s1 %= mod
+		s2 %= mod
+		p = q
+	}
+	return digest(s2<<16 | s1)
+}
+
+func (d *digest) Write(p []byte) (nn int, err error) {
+	*d = update(*d, p)
+	return len(p), nil
+}
+
+func (d *digest) Sum32() uint32 { return uint32(*d) }
+
+func (d *digest) Sum(in []byte) []byte {
+	s := uint32(*d)
+	return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
+}
+
+// Checksum returns the Adler-32 checksum of data.
+func Checksum(data []byte) uint32 { return uint32(update(1, data)) }