Ensure the substr from is always compatible with the size of string

jsonlogic_test.go

@@ -669,3 +669,31 @@ func TestIssue51_example2(t *testing.T) {
 	expected := `false`
 	assert.JSONEq(t, expected, result.String())
 }
+
+func TestIssue52_example1(t *testing.T) {
+	data := strings.NewReader(`{}`)
+	logic := strings.NewReader(`{"substr": ["jsonlogic", -10]}`)
+
+	var result bytes.Buffer
+	err := Apply(logic, data, &result)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	expected := `"jsonlogic"`
+	assert.JSONEq(t, expected, result.String())
+}
+
+func TestIssue52_example2(t *testing.T) {
+	data := strings.NewReader(`{}`)
+	logic := strings.NewReader(`{"substr": ["jsonlogic", 10]}`)
+
+	var result bytes.Buffer
+	err := Apply(logic, data, &result)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	expected := `"jsonlogic"`
+	assert.JSONEq(t, expected, result.String())
+}

strings.go

@@ -17,6 +17,11 @@ func substr(values interface{}) interface{} {
 		from = length + from
 	}
 
+	if from < 0 || from > length {
+		// case from is still negative, we must stop right now and return the original string
+		return string(runes)
+	}
+
 	if len(parsed) == 3 {
 		length = int(toNumber(parsed[2]))
 	}