fix distribution

pw.go

@@ -24,7 +24,8 @@ const AllChars = UpperChars + LowerChars + SpecialChars + NumberChars
 type Option func() string
 
 func randomFromChars(length int, chars string) string {
-	max := len(chars) - 1
+	// rand.Int returns value in [0, max)
+	max := len(chars)
 	p := ""
 
 	for i := 0; i < length; i++ {

pw_test.go

@@ -15,6 +15,16 @@ func contains(chars string, password string) bool {
 	return false
 }
 
+// containsAll checks if password contains all chars in the charset
+func containsAll(password, charset string) bool {
+	for _, char := range strings.Split(charset, "") {
+		if !strings.Contains(password, char) {
+			return false
+		}
+	}
+	return true
+}
+
 func TestDefaultNewPassword(t *testing.T) {
 	l := 100
 	p := NewPassword(l)
@@ -139,3 +149,11 @@ func TestSpecialCharOnlyPassword(t *testing.T) {
 		t.Errorf("should not contain upper characters")
 	}
 }
+
+func TestDistribution(t *testing.T) {
+	const charset = "12"
+	pass := randomFromChars(100, charset)
+	if !containsAll(pass, charset) {
+		t.Errorf("should contain all the chars from charset %q\n", charset)
+	}
+}