internal/shaderir/metal: bug fix: define the 'mod' function correctly

Closes #2006

internal/shaderir/metal/metal.go

@@ -52,7 +52,12 @@ const Prelude = `#include <metal_stdlib>
 
 using namespace metal;
 
-constexpr sampler texture_sampler{filter::nearest};`
+constexpr sampler texture_sampler{filter::nearest};
+
+template<typename T>
+T mod(T x, T y) {
+	return x - y * floor(x/y);
+}`
 
 func Compile(p *shaderir.Program, vertex, fragment string) (shader string) {
 	c := &compileContext{

internal/shaderir/metal/type.go

@@ -103,7 +103,8 @@ func builtinFuncString(f shaderir.BuiltinFunc) string {
 	case shaderir.Inversesqrt:
 		return "rsqrt"
 	case shaderir.Mod:
-		return "fmod"
+		// mod is a special function that Metal doesn't originally has (#2006).
+		return "mod"
 	case shaderir.Texture2DF:
 		return "?(texture2D)"
 	}

shader_test.go

@@ -1677,3 +1677,35 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 		}
 	}
 }
+
+// Issue #2006
+func TestShaderFuncMod(t *testing.T) {
+	const w, h = 16, 16
+
+	dst := ebiten.NewImage(w, h)
+	s, err := ebiten.NewShader([]byte(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	r := mod(-0.25, 1.0)
+	return vec4(r, 0, 0, 1)
+}
+`))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	dst.DrawRectShader(w/2, h/2, s, nil)
+
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			got := dst.At(i, j).(color.RGBA)
+			var want color.RGBA
+			if i < w/2 && j < h/2 {
+				want = color.RGBA{0xc0, 0, 0, 0xff}
+			}
+			if !sameColors(got, want, 2) {
+				t.Errorf("dst.At(%d, %d): got: %v, want: %v", i, j, got, want)
+			}
+		}
+	}
+}