Add time.weekday builtin

Fixes open-policy-agent#789.

Signed-off-by: Stephan Renatus <[email protected]>

ast/builtins.go

@@ -114,6 +114,7 @@ var DefaultBuiltins = [...]*Builtin{
 	ParseDurationNanos,
 	Date,
 	Clock,
+	Weekday,
 
 	// Crypto
 	CryptoX509ParseCertificates,
@@ -860,6 +861,15 @@ var Clock = &Builtin{
 	),
 }
 
+// Weekday returns the day of the week (Monday, Tuesday, ...) for the nanoseconds since epoch.
+var Weekday = &Builtin{
+	Name: "time.weekday",
+	Decl: types.NewFunction(
+		types.Args(types.N),
+		types.S,
+	),
+}
+
 /**
  * Crypto.
  */

docs/book/language-reference.md

@@ -127,6 +127,7 @@ The input `string` is a JSON Web Token encoded with JWS Compact Serialization. J
 | <span class="opa-keep-it-together">``time.parse_duration_ns(duration, output)``</span> | 1 | ``output`` is ``number`` representing the duration ``duration`` in nanoseconds. See the [Go `time` package documentation](https://golang.org/pkg/time/#ParseDuration) for more details on ``duration``. |
 | <span class="opa-keep-it-together">``time.date(ns, [year, month, day])``</span> | 1 | outputs the ``year``, ``month`` (0-12), and ``day`` (0-31) as ``number``s representing the date from the nanoseconds since epoch (``ns``). |
 | <span class="opa-keep-it-together">``time.clock(ns, [hour, minute, second])``</span> | 1 | outputs the ``hour``, ``minute`` (0-59), and ``second`` (0-59) as ``number``s representing the time of day for the nanoseconds since epoch (``ns``). |
+| <span class="opa-keep-it-together">``time.weekday(ns, day)``</span> | 1 | outputs the ``day`` as ``string`` representing the day of the week for the nanoseconds since epoch (``ns``). |
 
 > Multiple calls to the `time.now_ns` built-in function within a single policy
 evaluation query will always return the same value.

topdown/time.go

@@ -120,6 +120,24 @@ func builtinClock(a ast.Value) (ast.Value, error) {
 	return result, nil
 }
 
+func builtinWeekday(a ast.Value) (ast.Value, error) {
+
+	value, err := builtins.NumberOperand(a, 1)
+	if err != nil {
+		return nil, err
+	}
+
+	f := builtins.NumberToFloat(value)
+	i64, acc := f.Int64()
+	if acc != big.Exact {
+		return nil, fmt.Errorf("timestamp too big")
+	}
+
+	t := time.Unix(0, i64).UTC()
+	weekday := t.Weekday().String()
+	return ast.String(weekday), nil
+}
+
 func int64ToJSONNumber(i int64) json.Number {
 	return json.Number(strconv.FormatInt(i, 10))
 }
@@ -131,4 +149,5 @@ func init() {
 	RegisterFunctionalBuiltin1(ast.ParseDurationNanos.Name, builtinParseDurationNanos)
 	RegisterFunctionalBuiltin1(ast.Date.Name, builtinDate)
 	RegisterFunctionalBuiltin1(ast.Clock.Name, builtinClock)
+	RegisterFunctionalBuiltin1(ast.Weekday.Name, builtinWeekday)
 }

topdown/topdown_test.go

@@ -1716,6 +1716,15 @@ func TestTopDownTime(t *testing.T) {
 
 	runTopDownTestCase(t, data, "clock too big", []string{`
 		p = [hour, minute, second] { [hour, minute, second] := time.clock(1582977600*1000*1000*1000*1000) }`}, fmt.Errorf("timestamp too big"))
+
+	for i, day := range []string{"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"} {
+		ts := 1517832000*1000*1000*1000 + i*24*int(time.Hour)
+		runTopDownTestCase(t, data, "weekday", []string{fmt.Sprintf(`p = weekday { weekday := time.weekday(%d)}`, ts)},
+			fmt.Sprintf("%q", day))
+	}
+
+	runTopDownTestCase(t, data, "weekday too big", []string{`
+		p = weekday { weekday := time.weekday(1582977600*1000*1000*1000*1000) }`}, fmt.Errorf("timestamp too big"))
 }
 
 func TestTopDownWalkBuiltin(t *testing.T) {