using System;
using System.Runtime.InteropServices;

namespace CallingLua
{
    public static class LuaBridge
    {
        [DllImport("lua53.dll", EntryPoint = "luaL_newstate", CallingConvention = CallingConvention.Cdecl)]
        public static extern IntPtr LuaL_NewState();

        [DllImport("lua53.dll", EntryPoint = "luaL_openlibs", CallingConvention = CallingConvention.Cdecl)]
        public static extern void LuaL_OpenLibs(IntPtr luaState);

        [DllImport("lua53.dll", EntryPoint = "luaL_loadfilex", CallingConvention = CallingConvention.Cdecl)]
        public static extern int LuaL_LoadFileX(IntPtr luaState, IntPtr filename, IntPtr mode);

        [DllImport("lua53.dll", EntryPoint = "lua_tolstring", CallingConvention = CallingConvention.Cdecl)]
        public static extern IntPtr Lua_ToLString(IntPtr luaState, int index, IntPtr size);

        [DllImport("lua53.dll", EntryPoint = "lua_settop", CallingConvention = CallingConvention.Cdecl)]
        public static extern void Lua_SetTop(IntPtr luaState, int n);

        [DllImport("lua53.dll", EntryPoint = "lua_pcallk", CallingConvention = CallingConvention.Cdecl)]
        public static extern int Lua_PCallK(IntPtr luaState, int nArgs, int nRet, int errIndex, int boh, IntPtr boh2);

        public static void Lua_Pop(IntPtr luaState, int n)
        {
            Lua_SetTop(luaState, -n - 1);
        }

        public static int Lua_PCall(IntPtr luaState, int nArgs, int nRet, int errIndex)
        {
            return Lua_PCallK(luaState, nArgs, nRet, errIndex, 0, IntPtr.Zero);
        }
    }

    public class Lua
    {
        private IntPtr luaState;
        public Lua()
        {
            luaState = LuaBridge.LuaL_NewState();
            if (luaState == IntPtr.Zero)
                throw new Exception("Unable to initialize Lua VM");

            LuaBridge.LuaL_OpenLibs(luaState);
        }

        private string GetError()
        {
            IntPtr cStr = LuaBridge.Lua_ToLString(luaState, -1, IntPtr.Zero);
            string errorMessage = Marshal.PtrToStringAnsi(cStr);
            LuaBridge.Lua_Pop(luaState, 1);
            return errorMessage;
        }

        public void LoadFile(string filename)
        {
            IntPtr cFilename = Marshal.StringToHGlobalAnsi(filename);
            int ret = LuaBridge.LuaL_LoadFileX(luaState, cFilename, IntPtr.Zero);
            Marshal.FreeHGlobal(cFilename);
            if (ret != 0)
            {
                throw new Exception(GetError());
            }
        }

        public void Run(int nArgs = 0, int nRet = 0, int errIndex = 0)
        {
            int ret = LuaBridge.Lua_PCall(luaState, nArgs, nRet, errIndex);
            if (ret != 0)
            {
                throw new Exception(GetError());
            }
        }
    }
}