Fixup PrevLabel and add a test

labels.go

@@ -146,13 +146,16 @@ func NextLabel(s string, offset int) (i int, end bool) {
 
 // PrevLabel returns the index of the label when starting from the right and
 // jumping n labels to the left.
-// The bool start is true when the start of the string has been reached.
+// The bool start is true when the start of the string has been overshot.
 func PrevLabel(s string, n int) (i int, start bool) {
+	if n == 0 {
+		return len(s), false
+	}
 	lab := Split(s)
 	if lab == nil {
 		return 0, true
 	}
-	if len(lab) < n {
+	if n > len(lab) {
 		return 0, true
 	}
 	return lab[len(lab)-n], false

labels_test.go

@@ -94,6 +94,35 @@ func TestSplit2(t *testing.T) {
 	}
 }
 
+func TestPrevLabel(t *testing.T) {
+	type prev struct {
+		string
+		int
+	}
+	prever := map[prev]int{
+		prev{"www.miek.nl.", 0}: 12,
+		prev{"www.miek.nl.", 1}: 9,
+		prev{"www.miek.nl.", 2}: 4,
+
+		prev{"www.miek.nl", 0}: 11,
+		prev{"www.miek.nl", 1}: 9,
+		prev{"www.miek.nl", 2}: 4,
+
+		prev{"www.miek.nl.", 5}: 0,
+		prev{"www.miek.nl", 5}:  0,
+
+		prev{"www.miek.nl.", 3}: 0,
+		prev{"www.miek.nl", 3}:  0,
+	}
+	for s, i := range prever {
+		x, ok := PrevLabel(s.string, s.int)
+		if i != x {
+			t.Logf("Label should be %d, got %d, %t: preving %d, %s\n", i, x, ok, s.int, s.string)
+			t.Fail()
+		}
+	}
+}
+
 func TestCountLabel(t *testing.T) {
 	splitter := map[string]int{
 		"www.miek.nl.": 3,