Added substr function (#1152)

* Added substr function

* Added comment for substr function and fixed minor issues

* Updated return type to (string, error) for substr

* Updated unit test logic

* Added Fatalf instead of Errorf

internal/template/funcmap.go

@@ -36,6 +36,7 @@ var funcMap = map[string]interface{}{
 	"regexReplaceAll":   regexReplaceAll,
 	"makeSlice":         makeSlice,
 	"schemaDescription": schemaDescription,
+	"substr":            substr,
 }
 
 // invalidIDRE defines the invalid characters not allowed in terraform resource names.
@@ -156,3 +157,19 @@ func schemaDescription(s string) string {
 
 	return fmt.Sprintf(`"%s"`, s)
 }
+
+// substr returns a substring that starts at index 'start'
+// and spans 'length' characters (or until the end of the string,
+// whichever comes first).
+func substr(s string, start int, length int) (string, error) {
+	if start < 0 || start >= len(s) {
+		return "", fmt.Errorf("start index parameter has a invalid value: %d", start)
+	}
+	if length < 0 {
+		return "", fmt.Errorf("length parameter has a invalid value: %d", length)
+	}
+	if start+length > len(s) {
+		length = len(s) - start
+	}
+	return s[start : start+length], nil
+}

internal/template/template_test.go

@@ -298,3 +298,37 @@ func TestMakeSlice(t *testing.T) {
 		}
 	}
 }
+
+func TestSubstr(t *testing.T) {
+	tests := []struct {
+		input         string
+		start, length int
+		want          string
+		wantErr       bool
+	}{
+		{"sample string", -20, 0, "", true},
+		{"sample string", 3, -3, "", true},
+		{"sample string", 100, 4, "", true},
+		{"sample string", 0, 40, "sample string", false},
+		{"sample string", 0, 5, "sampl", false},
+		{"sample string", 5, 4, "e st", false},
+		{"sample string", 3, 0, "", false},
+	}
+	for _, tc := range tests {
+		got, err := substr(tc.input, tc.start, tc.length)
+
+		if err != nil && !tc.wantErr {
+			t.Fatalf("received error for valid input:\"%s\", start:%d, "+
+				"length:%d, error received:\n%v", tc.input, tc.start, tc.length, err)
+		}
+		if err == nil && tc.wantErr {
+			t.Fatalf("did not receive error for invalid input: \"%s\", start:%d, "+
+				"length:%d", tc.input, tc.start, tc.length)
+		}
+
+		if diff := cmp.Diff(tc.want, got); diff != "" {
+			t.Errorf("substr results differ for input:\"%s\", start:%d, "+
+				"length:%d and diff(-want +got):\n%v", tc.input, tc.start, tc.length, diff)
+		}
+	}
+}