add is_even (#2278)

pkg/gcs/eval_fn_test.go

@@ -259,3 +259,43 @@ func TestNestedActions(t *testing.T) {
 		t.Error(eval.Err())
 	}
 }
+
+func TestIsEven(t *testing.T) {
+	prog := `is_even(1);`
+	p := ast.New(prog)
+	_, gcsl, err := p.Parse()
+	if err != nil {
+		t.Fatal(err)
+	}
+	fmt.Println("program:")
+	fmt.Println(gcsl.String())
+	eval, _ := NewEvaluator(gcsl, nil)
+	eval.Log = log.Default()
+	resultChan := make(chan Obj)
+	go func() {
+		res, err := eval.Run()
+		fmt.Printf("done with result: %v, err: %v\n", res, err)
+		resultChan <- res
+	}()
+	for {
+		eval.Continue()
+		a, err := eval.NextAction()
+		if a == nil {
+			break
+		}
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+	result := <-resultChan
+	if result.Typ() != typNum {
+		t.Errorf("expecting type to return num, got %v", typStrings[result.Typ()])
+	}
+	if eval.Err() != nil {
+		t.Error(eval.Err())
+	}
+	val := result.(*number)
+	if val.ival != 0 {
+		t.Errorf("expecting result to be 0, got %v", val.ival)
+	}
+}

pkg/gcs/sysfunc.go

@@ -43,6 +43,7 @@ func (e *Eval) initSysFuncs(env *Env) {
 	e.addSysFunc("cos", e.cos, env)
 	e.addSysFunc("asin", e.asin, env)
 	e.addSysFunc("acos", e.acos, env)
+	e.addSysFunc("is_even", e.isEven, env)
 
 	// events
 	e.addSysFunc("set_on_tick", e.setOnTick, env)
@@ -495,6 +496,36 @@ func (e *Eval) pickUpCrystallize(c *ast.CallExpr, env *Env) (Obj, error) {
 	}, nil
 }
 
+func (e *Eval) isEven(c *ast.CallExpr, env *Env) (Obj, error) {
+	// is_even(number goes in here)
+	if len(c.Args) != 1 {
+		return nil, fmt.Errorf("invalid number of params for is_even, expected 1 got %v", len(c.Args))
+	}
+
+	// should eval to a number
+	val, err := e.evalExpr(c.Args[0], env)
+	if err != nil {
+		return nil, err
+	}
+
+	n, ok := val.(*number)
+	if !ok {
+		return nil, fmt.Errorf("is_even argument should evaluate to a number, got %v", val.Inspect())
+	}
+
+	// if float, floor it
+	var v int64
+	v = n.ival
+	if n.isFloat {
+		v = int64(n.fval)
+	}
+
+	if v%2 == 0 {
+		return bton(true), nil
+	}
+	return bton(false), nil
+}
+
 func (e *Eval) sin(c *ast.CallExpr, env *Env) (Obj, error) {
 	// sin(number goes in here)
 	if len(c.Args) != 1 {

ui/packages/docs/docs/reference/system_functions.md

@@ -360,6 +360,18 @@ pick_up_crystallize(element);
 `element` must be a string or an expression that evaluates to a string.
 :::
 
+## is_even
+
+```
+is_even(arg);
+```
+
+`is_even` evaluates if a given number is even or not. If a number is a floating point, the number if floored first. 
+
+:::danger
+`arg` must be a number or an expression that evaluates to a number.
+:::
+
 ## sin
 
 ```