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

lib/weakhash/benchmark_test.go

@@ -9,9 +9,12 @@ package weakhash
 import (
 	"os"
 	"testing"
+
+	"github.com/chmduquesne/rollinghash/adler32"
 )
 
 const testFile = "../model/testdata/~syncthing~file.tmp"
+const size = 128 << 10
 
 func BenchmarkFind1MFile(b *testing.B) {
 	b.ReportAllocs()
@@ -21,10 +24,38 @@ func BenchmarkFind1MFile(b *testing.B) {
 		if err != nil {
 			b.Fatal(err)
 		}
-		_, err = Find(fd, []uint32{0, 1, 2}, 128<<10)
+		_, err = Find(fd, []uint32{0, 1, 2}, size)
 		if err != nil {
 			b.Fatal(err)
 		}
 		fd.Close()
 	}
 }
+
+func BenchmarkWeakHashAdler32(b *testing.B) {
+	data := make([]byte, size)
+	hf := adler32.New()
+
+	for i := 0; i < b.N; i++ {
+		hf.Write(data)
+	}
+
+	_ = hf.Sum32()
+	b.SetBytes(size)
+}
+
+func BenchmarkWeakHashAdler32Roll(b *testing.B) {
+	data := make([]byte, size)
+	hf := adler32.New()
+	hf.Write(data)
+
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		for i := 0; i <= size; i++ {
+			hf.Roll('a')
+		}
+	}
+
+	b.SetBytes(size)
+}