Index: src/pkg/hash/adler32/adler32.go |
=================================================================== |
--- a/src/pkg/hash/adler32/adler32.go |
+++ b/src/pkg/hash/adler32/adler32.go |
@@ -3,7 +3,8 @@ |
// license that can be found in the LICENSE file. |
// Package adler32 implements the Adler-32 checksum. |
-// Defined in RFC 1950: |
+// |
+// 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 |
@@ -14,20 +15,22 @@ |
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. |
-type digest struct { |
- // invariant: (a < mod && b < mod) || a <= b |
- // invariant: a + b + 255 <= 0xffffffff |
- a, b uint32 |
-} |
+// The low 16 bits are s1, the high 16 bits are s2. |
+type digest uint32 |
-func (d *digest) Reset() { d.a, d.b = 1, 0 } |
+func (d *digest) Reset() { *d = 1 } |
// New returns a new hash.Hash32 computing the Adler-32 checksum. |
func New() hash.Hash32 { |
@@ -40,43 +43,36 @@ |
func (d *digest) BlockSize() int { return 1 } |
-// Add p to the running checksum a, b. |
-func update(a, b uint32, p []byte) (aa, bb uint32) { |
- for _, pi := range p { |
- a += uint32(pi) |
- 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 |
+// 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 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 |
+ return digest(s2<<16 | s1) |
} |
func (d *digest) Write(p []byte) (nn int, err error) { |
- d.a, d.b = update(d.a, d.b, p) |
+ *d = update(*d, p) |
return len(p), nil |
} |
-func (d *digest) Sum32() uint32 { return finish(d.a, d.b) } |
+func (d *digest) Sum32() uint32 { return uint32(*d) } |
func (d *digest) Sum(in []byte) []byte { |
- s := d.Sum32() |
+ 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 finish(update(1, 0, data)) } |
+func Checksum(data []byte) uint32 { return uint32(update(1, data)) } |