Add test for modulo bias

Successfully detects bias fixed by PR #9.

uniuri_test.go

@@ -83,3 +83,20 @@ func TestNewLenCharsMaxLength(t *testing.T) {
 	chars := make([]byte, 257)
 	NewLenChars(32, chars)
 }
+
+func TestBias(t *testing.T) {
+	chars := []byte("abcdefghijklmnopqrstuvwxyz")
+	slen := 100000
+	s := NewLenChars(slen, chars)
+	counts := make(map[rune]int)
+	for _, b := range s {
+		counts[b]++
+	}
+	avg := float64(slen) / float64(len(chars))
+	for k, n := range counts {
+		diff := float64(n) / avg
+		if diff < 0.95 || diff > 1.05 {
+			t.Errorf("Bias on '%c': expected average %f, got %d", k, avg, n)
+		}
+	}
+}