lib/scanner, vendor: Fix previous commit

Can't do what I did, as the rolling function is not the same as the
non-rolling one. Instead this uses an improved version of the rolling
adler32 to accomplish the same thing. (PR filed on upstream, so should
be able to use that directly in the future.)

lib/scanner/blocks_test.go

@@ -8,9 +8,13 @@ package scanner
 
 import (
 	"bytes"
+	"crypto/rand"
 	"fmt"
+	origAdler32 "hash/adler32"
 	"testing"
+	"testing/quick"
 
+	rollingAdler32 "github.com/chmduquesne/rollinghash/adler32"
 	"github.com/syncthing/syncthing/lib/protocol"
 )
 
@@ -160,3 +164,66 @@ func TestDiffEmpty(t *testing.T) {
 		}
 	}
 }
+
+func TestAdler32Variants(t *testing.T) {
+	// Verify that the two adler32 functions give matching results for a few
+	// different blocks of data.
+
+	hf1 := origAdler32.New()
+	hf2 := rollingAdler32.New()
+
+	checkFn := func(data []byte) bool {
+		hf1.Write(data)
+		sum1 := hf1.Sum32()
+
+		hf2.Write(data)
+		sum2 := hf2.Sum32()
+
+		hf1.Reset()
+		hf2.Reset()
+
+		return sum1 == sum2
+	}
+
+	// protocol block sized data
+	data := make([]byte, protocol.BlockSize)
+	for i := 0; i < 5; i++ {
+		rand.Read(data)
+		if !checkFn(data) {
+			t.Errorf("Hash mismatch on block sized data")
+		}
+	}
+
+	// random small blocks
+	if err := quick.Check(checkFn, nil); err != nil {
+		t.Error(err)
+	}
+
+	// rolling should have the same result as the individual blocks
+	// themselves. Which is not the same as the original non-rollind adler32
+	// blocks.
+
+	windowSize := 128
+
+	hf2.Reset()
+
+	hf3 := rollingAdler32.New()
+	hf3.Write(data[:windowSize])
+
+	for i := windowSize; i < len(data); i++ {
+		if i%windowSize == 0 {
+			// let the reference function catch up
+			hf2.Write(data[i-windowSize : i])
+
+			// verify that they are in sync with the rolling function
+			sum2 := hf2.Sum32()
+			sum3 := hf3.Sum32()
+			t.Logf("At i=%d, sum2=%08x, sum3=%08x", i, sum2, sum3)
+			if sum2 != sum3 {
+				t.Errorf("Mismatch after roll; i=%d, sum2=%08x, sum3=%08x", i, sum2, sum3)
+				break
+			}
+		}
+		hf3.Roll(data[i])
+	}
+}