From 08ddb4233ba89f0461758ee4039013312628dcba Mon Sep 17 00:00:00 2001
From: Hajime Hoshi <hajimehoshi@gmail.com>
Date: Wed, 12 Jan 2022 01:41:06 +0900
Subject: [PATCH] internal/shaderir/glsl: bug fix: % was not available on old
 GLSLs

Use a new utility function modInt instead.

Closes #1951
---
 internal/shader/shader_test.go | 14 +++++++++++---
 internal/shaderir/glsl/glsl.go | 22 ++++++++++++++++++++--
 2 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/internal/shader/shader_test.go b/internal/shader/shader_test.go
index 07c0d8f87..5527fd8f9 100644
--- a/internal/shader/shader_test.go
+++ b/internal/shader/shader_test.go
@@ -29,7 +29,15 @@ import (
 	"github.com/hajimehoshi/ebiten/v2/internal/shaderir/metal"
 )
 
-func glslNormalize(str string) string {
+func glslVertexNormalize(str string) string {
+	p := glsl.VertexPrelude(glsl.GLSLVersionDefault)
+	if strings.HasPrefix(str, p) {
+		str = str[len(p):]
+	}
+	return strings.TrimSpace(str)
+}
+
+func glslFragmentNormalize(str string) string {
 	p := glsl.FragmentPrelude(glsl.GLSLVersionDefault)
 	if strings.HasPrefix(str, p) {
 		str = str[len(p):]
@@ -158,11 +166,11 @@ func TestCompile(t *testing.T) {
 
 			// GLSL
 			vs, fs := glsl.Compile(s, glsl.GLSLVersionDefault)
-			if got, want := glslNormalize(vs), glslNormalize(string(tc.VS)); got != want {
+			if got, want := glslVertexNormalize(vs), glslVertexNormalize(string(tc.VS)); got != want {
 				compare(t, "GLSL Vertex", got, want)
 			}
 			if tc.FS != nil {
-				if got, want := glslNormalize(fs), glslNormalize(string(tc.FS)); got != want {
+				if got, want := glslFragmentNormalize(fs), glslFragmentNormalize(string(tc.FS)); got != want {
 					compare(t, "GLSL Fragment", got, want)
 				}
 			}
diff --git a/internal/shaderir/glsl/glsl.go b/internal/shaderir/glsl/glsl.go
index 11b32d186..43da99803 100644
--- a/internal/shaderir/glsl/glsl.go
+++ b/internal/shaderir/glsl/glsl.go
@@ -33,8 +33,18 @@ const (
 	GLSLVersionES300
 )
 
+// utilFunctions is GLSL utility functions for old GLSL versions.
+const utilFunctions = `int modInt(int x, int y) {
+	return x - y*(x/y);
+}`
+
 func VertexPrelude(version GLSLVersion) string {
-	if version == GLSLVersionES300 {
+	switch version {
+	case GLSLVersionDefault:
+		return utilFunctions
+	case GLSLVersionES100:
+		return utilFunctions
+	case GLSLVersionES300:
 		return `#version 300 es`
 	}
 	return ""
@@ -48,13 +58,17 @@ func FragmentPrelude(version GLSLVersion) string {
 	case GLSLVersionES300:
 		prefix = `#version 300 es` + "\n\n"
 	}
-	return prefix + `#if defined(GL_ES)
+	prelude := prefix + `#if defined(GL_ES)
 precision highp float;
 #else
 #define lowp
 #define mediump
 #define highp
 #endif`
+	if version == GLSLVersionDefault || version == GLSLVersionES100 {
+		prelude += "\n\n" + utilFunctions
+	}
+	return prelude
 }
 
 type compileContext struct {
@@ -496,6 +510,10 @@ func (c *compileContext) glslBlock(p *shaderir.Program, topBlock, block *shaderi
 			}
 			return fmt.Sprintf("%s(%s)", op, glslExpr(&e.Exprs[0]))
 		case shaderir.Binary:
+			if e.Op == shaderir.ModOp && (c.version == GLSLVersionDefault || c.version == GLSLVersionES100) {
+				// '%' is not defined.
+				return fmt.Sprintf("modInt((%s), (%s))", glslExpr(&e.Exprs[0]), glslExpr(&e.Exprs[1]))
+			}
 			return fmt.Sprintf("(%s) %s (%s)", glslExpr(&e.Exprs[0]), e.Op, glslExpr(&e.Exprs[1]))
 		case shaderir.Selection:
 			return fmt.Sprintf("(%s) ? (%s) : (%s)", glslExpr(&e.Exprs[0]), glslExpr(&e.Exprs[1]), glslExpr(&e.Exprs[2]))