From fa82d26b64a329051537c909b4f4b9b540653e3e Mon Sep 17 00:00:00 2001
From: Yutaka Ichibangase <yichiban@gmail.com>
Date: Sat, 19 Mar 2022 14:02:22 +0900
Subject: [PATCH] add evaluable functor: atan2

---
 engine/number.go      | 38 ++++++++++++++++++++++++++++----
 engine/number_test.go | 50 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 84 insertions(+), 4 deletions(-)

diff --git a/engine/number.go b/engine/number.go
index 4babfe6..a29da4c 100644
--- a/engine/number.go
+++ b/engine/number.go
@@ -45,10 +45,11 @@ var DefaultEvaluableFunctors = EvaluableFunctors{
 		`/\`: BitwiseAnd,
 		`\/`: BitwiseOr,
 
-		`div`: IntFloorDiv,
-		`max`: Max,
-		`min`: Min,
-		`^`:   IntegerPower,
+		`div`:   IntFloorDiv,
+		`max`:   Max,
+		`min`:   Min,
+		`^`:     IntegerPower,
+		`atan2`: Atan2,
 	},
 }
 
@@ -960,6 +961,35 @@ func Acos(x Number) (Number, error) {
 	return Float(math.Acos(vx)), nil
 }
 
+// Atan2 returns the arc tangent of y/x.
+func Atan2(y, x Number) (Number, error) {
+	var vy float64
+	switch y := y.(type) {
+	case Integer:
+		vy = float64(y)
+	case Float:
+		vy = float64(y)
+	default:
+		return nil, ErrUndefined
+	}
+
+	var vx float64
+	switch x := x.(type) {
+	case Integer:
+		vx = float64(x)
+	case Float:
+		vx = float64(x)
+	default:
+		return nil, ErrUndefined
+	}
+
+	if vx == 0 && vy == 0 {
+		return nil, ErrUndefined
+	}
+
+	return Float(math.Atan2(vy, vx)), nil
+}
+
 // Comparison
 
 func eqF(x, y Float) bool {
diff --git a/engine/number_test.go b/engine/number_test.go
index 90ed79b..5f5f6d4 100644
--- a/engine/number_test.go
+++ b/engine/number_test.go
@@ -1662,6 +1662,56 @@ func TestAcos(t *testing.T) {
 	})
 }
 
+func TestAtan2(t *testing.T) {
+	t.Run("integer", func(t *testing.T) {
+		t.Run("integer", func(t *testing.T) {
+			r, err := Atan2(Integer(0), Integer(1))
+			assert.NoError(t, err)
+			assert.Equal(t, Float(0), r)
+		})
+
+		t.Run("float", func(t *testing.T) {
+			r, err := Atan2(Integer(0), Float(1))
+			assert.NoError(t, err)
+			assert.Equal(t, Float(0), r)
+		})
+
+		t.Run("not a number", func(t *testing.T) {
+			_, err := Atan2(Integer(0), mockNumber{})
+			assert.Equal(t, ErrUndefined, err)
+		})
+	})
+
+	t.Run("float", func(t *testing.T) {
+		t.Run("integer", func(t *testing.T) {
+			r, err := Atan2(Float(0), Integer(1))
+			assert.NoError(t, err)
+			assert.Equal(t, Float(0), r)
+		})
+
+		t.Run("float", func(t *testing.T) {
+			r, err := Atan2(Float(0), Float(1))
+			assert.NoError(t, err)
+			assert.Equal(t, Float(0), r)
+		})
+
+		t.Run("not a number", func(t *testing.T) {
+			_, err := Atan2(Float(0), mockNumber{})
+			assert.Equal(t, ErrUndefined, err)
+		})
+	})
+
+	t.Run("not a number", func(t *testing.T) {
+		_, err := Atan2(mockNumber{}, Integer(1))
+		assert.Equal(t, ErrUndefined, err)
+	})
+
+	t.Run("x and y both equal to 0", func(t *testing.T) {
+		_, err := Atan2(Integer(0), Integer(0))
+		assert.Equal(t, ErrUndefined, err)
+	})
+}
+
 type mockNumber struct {
 	mock.Mock
 }