lib/model, lib/weakhash: Hash using adler32, add heuristic in puller

Adler32 is much faster, and the heuristic avoid the obvious cases where it
will not help.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3872

vendor/github.com/chmduquesne/rollinghash/rabinkarp32/rabinkarp32.go

@@ -0,0 +1,89 @@
+// Package rollinghash/rabinkarp32 implements a particular case of
+// rabin-karp where the modulus is 0xffffffff (32 bits of '1')
+
+package rabinkarp32
+
+import rollinghash "github.com/chmduquesne/rollinghash"
+
+// The size of a rabinkarp32 checksum.
+const Size = 4
+
+// digest represents the partial evaluation of a checksum.
+type digest struct {
+	a       uint32
+	h       uint32
+	aPowerN uint32
+
+	// window is treated like a circular buffer, where the oldest element
+	// is indicated by d.oldest
+	window []byte
+	oldest int
+}
+
+// Reset resets the Hash to its initial state.
+func (d *digest) Reset() {
+	d.h = 0
+	d.aPowerN = 1
+	d.window = nil
+	d.oldest = 0
+}
+
+func NewFromInt(a uint32) rollinghash.Hash32 {
+	return &digest{a: a, h: 0, aPowerN: 1, window: nil, oldest: 0}
+}
+
+func New() rollinghash.Hash32 {
+	return NewFromInt(65521) // largest prime fitting in 16 bits
+}
+
+// Size returns the number of bytes Sum will return.
+func (d *digest) Size() int { return Size }
+
+// BlockSize returns the hash's underlying block size.
+// The Write method must be able to accept any amount
+// of data, but it may operate more efficiently if all
+// writes are a multiple of the block size.
+func (d *digest) BlockSize() int { return 1 }
+
+// Write (via the embedded io.Writer interface) adds more data to the
+// running hash. It never returns an error.
+func (d *digest) Write(data []byte) (int, error) {
+	// Copy the window
+	d.window = make([]byte, len(data))
+	copy(d.window, data)
+	for _, c := range d.window {
+		d.h *= d.a
+		d.h += uint32(c)
+		d.aPowerN *= d.a
+	}
+	return len(d.window), nil
+}
+
+func (d *digest) Sum32() uint32 {
+	return d.h
+}
+
+func (d *digest) Sum(b []byte) []byte {
+	v := d.Sum32()
+	return append(b, byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
+}
+
+// Roll updates the checksum of the window from the leaving byte and the
+// entering byte.
+func (d *digest) Roll(c byte) {
+	if len(d.window) == 0 {
+		d.window = make([]byte, 1)
+		d.window[0] = c
+	}
+	// extract the entering/leaving bytes and update the circular buffer.
+	enter := uint32(c)
+	leave := uint32(d.window[d.oldest])
+	d.window[d.oldest] = c
+	l := len(d.window)
+	d.oldest += 1
+	if d.oldest >= l {
+		d.oldest = 0
+	}
+
+	d.h = d.h*d.a + enter - leave*d.aPowerN
+}