// Copyright 2022 The Ebitengine Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//go:build !darwin && !js && !nintendosdk && !windows

package gl

// #cgo LDFLAGS: -ldl
//
// #include <dlfcn.h>
// #include <stdlib.h>
//
// static const char* libGLName;
// static const char* libGLESName;
//
// static void setLibGLName(const char* name) {
//   libGLName = name;
// }
//
// static void setLibGLESName(const char* name) {
//   libGLESName = name;
// }
//
// static void* libGL() {
//   static void* so;
//   if (!so) {
//     so = dlopen(libGLName, RTLD_LAZY | RTLD_GLOBAL);
//   }
//   return so;
// }
//
// static void* libGLES() {
//   static void* so;
//   if (!so) {
//     so = dlopen(libGLESName, RTLD_LAZY | RTLD_GLOBAL);
//   }
//   return so;
// }
//
// static void* getProcAddressGL(const char* name) {
//   static void*(*glXGetProcAddress)(const char*);
//   if (!glXGetProcAddress) {
//     glXGetProcAddress = dlsym(libGL(), "glXGetProcAddress");
//     if (!glXGetProcAddress) {
//       glXGetProcAddress = dlsym(libGL(), "glXGetProcAddressARB");
//     }
//   }
//   return glXGetProcAddress(name);
// }
//
// static void* getProcAddressGLES(const char* name) {
//   return dlsym(libGLES(), name);
// }
import "C"

import (
	"fmt"
	"os"
	"path/filepath"
	"runtime"
	"sort"
	"strings"
	"unsafe"
)

func findLib(libraryPaths []string, libName string) (string, error) {
	// Look for a library file. In some environments like Steam, a library with the exactly same name might not exist (#2523).
	// For example, libGL.so.1 might exist instead of libGL.so.
	for _, dir := range libraryPaths {
		libs, err := listLibs(dir, libName)
		if err != nil {
			return "", err
		}
		if len(libs) == 0 {
			continue
		}

		// The file names are sorted in the alphabetical order. Use the first item.
		// TODO: What is the best version to use?
		return filepath.Join(dir, libs[0]), nil
	}

	// LD_LIBRARY_PATH might be empty. Use the original name.
	return libName, nil
}

func listLibs(dir string, prefix string) ([]string, error) {
	ents, err := os.ReadDir(dir)
	if err != nil {
		return nil, err
	}

	var files []string
	for _, ent := range ents {
		if ent.IsDir() {
			continue
		}
		if ent.Name() == prefix {
			files = append(files, ent.Name())
			continue
		}
		if strings.HasPrefix(ent.Name(), prefix+".") {
			files = append(files, ent.Name())
			continue
		}
	}

	sort.Strings(files)
	return files, nil
}

func (c *defaultContext) init() error {
	var preferES bool
	if runtime.GOOS == "android" {
		preferES = true
	}
	if !preferES {
		for _, t := range strings.Split(os.Getenv("EBITENGINE_OPENGL"), ",") {
			switch strings.TrimSpace(t) {
			case "es":
				preferES = true
				break
			}
		}
	}

	libraryPaths := filepath.SplitList(os.Getenv("LD_LIBRARY_PATH"))

	// Try OpenGL first. OpenGL is preferrable as this doesn't cause context losts.
	if !preferES {
		libGLName, err := findLib(libraryPaths, "libGL.so")
		if err != nil {
			return err
		}

		// This string is never released.
		C.setLibGLName(C.CString(libGLName))
		if C.libGL() != nil {
			return nil
		}
	}

	// Try OpenGL ES.
	libGLESName, err := findLib(libraryPaths, "libGLESv2.so")
	if err != nil {
		return err
	}

	// This string is never released.
	C.setLibGLESName(C.CString(libGLESName))
	if C.libGLES() != nil {
		c.isES = true
		return nil
	}

	return fmt.Errorf("gl: failed to load libGL.so and libGLESv2.so")
}

func (c *defaultContext) getProcAddress(name string) unsafe.Pointer {
	if c.isES {
		return getProcAddressGLES(name)
	}
	return getProcAddressGL(name)
}

func getProcAddressGL(name string) unsafe.Pointer {
	cname := C.CString(name)
	defer C.free(unsafe.Pointer(cname))
	return C.getProcAddressGL(cname)
}

func getProcAddressGLES(name string) unsafe.Pointer {
	name = strings.TrimSuffix(name, "EXT")
	cname := C.CString(name)
	defer C.free(unsafe.Pointer(cname))
	return C.getProcAddressGLES(cname)
}