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/scanner/blocks.go

@@ -11,9 +11,9 @@ import (
 	"fmt"
 	"io"
 
+	"github.com/chmduquesne/rollinghash/adler32"
 	"github.com/syncthing/syncthing/lib/protocol"
 	"github.com/syncthing/syncthing/lib/sha256"
-	"github.com/syncthing/syncthing/lib/weakhash"
 )
 
 var SHA256OfNothing = []uint8{0xe3, 0xb0, 0xc4, 0x42, 0x98, 0xfc, 0x1c, 0x14, 0x9a, 0xfb, 0xf4, 0xc8, 0x99, 0x6f, 0xb9, 0x24, 0x27, 0xae, 0x41, 0xe4, 0x64, 0x9b, 0x93, 0x4c, 0xa4, 0x95, 0x99, 0x1b, 0x78, 0x52, 0xb8, 0x55}
@@ -26,7 +26,8 @@ type Counter interface {
 func Blocks(r io.Reader, blocksize int, sizehint int64, counter Counter) ([]protocol.BlockInfo, error) {
 	hf := sha256.New()
 	hashLength := hf.Size()
-	whf := weakhash.NewHash(blocksize)
+	whf := adler32.New()
+	mhf := io.MultiWriter(hf, whf)
 
 	var blocks []protocol.BlockInfo
 	var hashes, thisHash []byte
@@ -46,7 +47,7 @@ func Blocks(r io.Reader, blocksize int, sizehint int64, counter Counter) ([]prot
 	var offset int64
 	for {
 		lr := io.LimitReader(r, int64(blocksize))
-		n, err := io.CopyBuffer(hf, io.TeeReader(lr, whf), buf)
+		n, err := io.CopyBuffer(mhf, lr, buf)
 		if err != nil {
 			return nil, err
 		}