add the hasCommonSubstring and add a test case

score.go

@@ -71,6 +71,9 @@ func splitSsdeep(hash string) (int, string, string, error) {
 }
 
 func scoreDistance(h1, h2 string, _ int) int {
+	if !hasCommonSubstring(h1, h2) {
+		return 0
+	}
 	d := distance(h1, h2)
 	d = (d * spamSumLength) / (len(h1) + len(h2))
 	d = (100 * d) / spamSumLength
@@ -83,3 +86,36 @@ func scoreDistance(h1, h2 string, _ int) int {
 	*/
 	return d
 }
+func hasCommonSubstring(s1, s2 string) bool {
+	i := 0
+	j := 0
+	s1Len := len(s1)
+	s2Len := len(s2)
+	hashes := make([]uint32, (spamSumLength - (rollingWindow - 1)))
+	if s1Len < rollingWindow || s2Len < rollingWindow {
+		return false
+	}
+	state := &rollingState{}
+	for i = 0; i < rollingWindow-1; i++ {
+		state.rollHash(s1[i])
+	}
+	for i = rollingWindow - 1; i < s1Len; i++ {
+		state.rollHash(s1[i])
+		hashes[i-(rollingWindow-1)] = state.rollSum()
+	}
+	s1Len -= (rollingWindow - 1)
+	state.rollReset()
+	for j = 0; j < rollingWindow-1; j++ {
+		state.rollHash(s2[j])
+	}
+	for j = 0; j < s2Len-(rollingWindow-1); j++ {
+		state.rollHash(s2[j+(rollingWindow-1)])
+		var h = state.rollSum()
+		for i = 0; i < s1Len; i++ {
+			if hashes[i] == h && s1[i:i+rollingWindow] == s2[j:j+rollingWindow] {
+				return true
+			}
+		}
+	}
+	return false
+}

score_test.go

@@ -14,6 +14,10 @@ var h3 = "196608:pDSC8olnoL1v/uawvbQD7XlZUFYzYyMb615NktYHF7dREN/JNnQrmhnUPI+/n2Y
 
 var h4 = "196608:7DSC8olnoL1v/uawvbQD7XlZUFYzYyMb615NktYHF7dREN/JNnQrmhnUPI+/n2Y7:3DHoJXv7XOq7Mb2TwYHXREN/3QrmktPt"
 
+var h5 = "24:YDVLfsT1ds/1H9Wpgq7n4XMijV6h4Z3QCw4qat:YD51H9CiMuV6uACwVat"
+
+var h6 = "24:YDVLfyvDj+C+opg8DV0Mdle6hPZ3QCw4qat:YDMvDj+C+kBOM+6HACwVat"
+
 func assertDistanceEqual(t *testing.T, expected, actual int) {
 	if expected != actual {
 		t.Fatalf("Distance mismatch: %d (expected)\n"+
@@ -39,6 +43,12 @@ func TestHashDistance2(t *testing.T) {
 	assertDistanceEqual(t, 97, d)
 }
 
+func TestHashDistance3(t *testing.T) {
+	d, err := Distance(h5, h6)
+	require.NoError(t, err)
+	assertDistanceEqual(t, 54, d)
+}
+
 func TestEmptyHash1(t *testing.T) {
 	d, err := Distance("", h2)
 	require.Error(t, err)

ssdeep.go

@@ -51,6 +51,15 @@ type rollingState struct {
 func (rs *rollingState) rollSum() uint32 {
 	return rs.h1 + rs.h2 + rs.h3
 }
+func (rs *rollingState) rollReset() {
+	rs.h1 = 0
+	rs.h2 = 0
+	rs.h3 = 0
+	rs.n = 0
+	for i := 0; i < len(rs.window); i++ {
+		rs.window[i] = 0
+	}
+}
 
 type ssdeepState struct {
 	rollingState rollingState