Make adler32 cleaner.

R=rsc
APPROVED=rsc
DELTA=22  (9 added, 6 deleted, 7 changed)
OCL=26498
CL=26500

src/lib/hash/adler32.go

@@ -13,44 +13,46 @@ package adler32
 
 import "os"
 
+const (
+	mod = 65521;
+)
+
 // 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;
-	n int;
 }
 
-const (
-	mod = 65521;
-	maxIter = 5552;  // max mod-free iterations before would overflow uint32
-)
-
 // NewDigest creates a new Digest.
 func NewDigest() *Digest {
-	return &Digest{1, 0, 0};
+	return &Digest{1, 0};
 }
 
 // Write updates the Digest with the incremental checksum generated by p.
 // It returns the number of bytes written; err is always nil.
 func (d *Digest) Write(p []byte) (nn int, err *os.Error) {
-	a, b, n := d.a, d.b, d.n;
+	a, b := d.a, d.b;
 	for i := 0; i < len(p); i++ {
 		a += uint32(p[i]);
 		b += a;
-		n++;
-		if n == maxIter {
+		// invariant: a <= b
+		if b > (0xffffffff - 255) / 2 {
 			a %= mod;
 			b %= mod;
-			n = 0;
+			// invariant: a < mod && b < mod
+		} else {
+			// invariant: a + b + 255 <= 2 * b + 255 <= 0xffffffff
 		}
 	}
-	d.a, d.b, d.n = a, b, n;
+	d.a, d.b = a, b;
 	return len(p), nil
 }
 
 // Sum32 returns the 32-bit Adler-32 checksum of the data written to the Digest.
 func (d *Digest) Sum32() uint32 {
 	a, b := d.a, d.b;
-	if a >= mod || b >= mod {
+	if b >= mod {
 		a %= mod;
 		b %= mod;
 	}

src/lib/hash/adler32_test.go

@@ -47,6 +47,7 @@ var golden = []_Adler32Test {
 	_Adler32Test{ 0x39111dd0, "Even if I could be Shakespeare, I think I should still choose to be Faraday. - A. Huxley" },
 	_Adler32Test{ 0x91dd304f, "The fugacity of a constituent in a mixture of gases at a given temperature is proportional to its mole fraction.  Lewis-Randall Rule" },
 	_Adler32Test{ 0x2e5d1316, "How can you write a big system without C++?  -Paul Glick" },
+	_Adler32Test{ 0xd0201df6, "'Invariant assertions' is the most elegant programming technique!  -Tom Szymanski" },
 }
 
 func TestGolden(t *testing.T) {