package main

import (
	"fmt"
	"syscall"
	"unsafe"
)

var (
	modkernel32     = syscall.NewLazyDLL("kernel32.dll")
	procCreateMutex = modkernel32.NewProc("CreateMutexW")
	procCloseHandle = modkernel32.NewProc("CloseHandle")
	user32          = syscall.NewLazyDLL("user32.dll")
	procMessageBox  = user32.NewProc("MessageBoxW")
)

const MB_OK = 0x00000000

const MB_ICONWARNING = 0x00000030

func MessageBox(title, msg string) {
	procMessageBox.Call(
		0,
		uintptr(unsafe.Pointer(syscall.StringToUTF16Ptr(msg))),
		uintptr(unsafe.Pointer(syscall.StringToUTF16Ptr(title))),
		MB_OK|MB_ICONWARNING,
	)
}

var ErrAlreadyExists = fmt.Errorf("程序已在运行")

func createMutex(name string) (uintptr, error) {
	n, _ := syscall.UTF16PtrFromString(name)

	r, _, err := procCreateMutex.Call(
		0,
		0,
		uintptr(unsafe.Pointer(n)),
	)

	if r == 0 {
		return 0, err
	}

	if errno, ok := err.(syscall.Errno); ok && errno == 183 { // ERROR_ALREADY_EXISTS
		return 0, ErrAlreadyExists
	}

	return r, nil
}

func lock() {
	mutex, err := createMutex("back_cover_mutex")
	if err != nil {
		fmt.Println(err)
		return
	}
	defer procCloseHandle.Call(mutex)
}