Your situation sounds like a good candidate for Lua.
- You need sandboxing: This is easy to do in Lua. You simply initialize the users' environment by overwriting or deleting the
os.execute command, for instance, and there is no way for the user to access that function anymore.
- You want fast: Check out some of the Lua benchmarks against other languages.
- Assumably you need to interoperate with another language. Lua is very easy (IMO) to embed in C or C++, at least. I haven't used LuaInterface, but that's the C# binding.
- Lua has first-order functions, so it should be easy to swap functions on-the-fly.
- Lua supports OOP to some extent with metatables.
- Lua's primary data structure is the table (associative array) which is well-suited to sparse data structures like integrating with a world map.
- Lua has a very regular syntax. There are no funny tricks with semicolons or indentation, so that's one less thing for your users to learn when they are picking up your language -- not to mention, using a well-documented language takes away some of the work you have to do in terms of documenting it yourself.
Also, as @elviejo points out in a comment, Lua is already used as a scripting language in many games. If nothing else, there's certainly some precedent for using Lua in the way you've described. And, as @gmonc mentions, there is a chance that your users have already used Lua in another game.
As far as
how to integrate with Lua: generally, your users should simply need to upload a Lua script file. To grossly oversimplify, you might provide the users with available functions such as
TurnLeft,
TurnRight,
Go, and
Stop. Then, the users would upload a script like
Actions = {} -- empty table, but you might want to provide default functions
function Actions.Cone()
TurnLeft()
end
function Actions.Wall()
Stop()
TurnRight()
TurnRight()
Go()
end
Then server-side, you would might start them off with a Go(). Then, when their car reaches a cone, you call their Actions.Cone() function; a wall leads to the Actions.Wall() function, etc. At this point, you've (hopefully) already sandboxed the Lua environment, so you can simply execute their script without even much regard for error checking -- if their script results in an error, no reason you can't pass the error on directly to the user. And if there aren't any errors, the lua_State in your server's code should contain the final state of their car.
Better example
Here's a standalone C file that takes a Lua script from stdin and runs it like I explained above. The game is that you'll encounter Ground, a Fence, or a Branch, and you have to respectively Run, Jump, or Duck to pass. You input a Lua script via stdin to decide how to react. The source is a little long, but hopefully it's easy to understand (besides the Lua API which takes a while to get used to). This is my original creation over the past 30 minutes, hope it helps:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include "lua.h"
#include "lauxlib.h"
#include "lualib.h"
#define FAIL 0
#define SUCCESS 1
/* Possible states for the player */
enum STATE {
RUNNING,
JUMPING,
DUCKING
};
/* Possible obstacles */
enum OBSTACLE {
GROUND,
FENCE,
BRANCH
};
/* Using global vars here for brevity */
enum STATE playerstate = RUNNING;
enum OBSTACLE currentobstacle = GROUND;
/* Functions to be bound to Lua */
int Duck(lua_State *L)
{
playerstate = DUCKING;
return 0; /* no return values to Lua */
}
int Run(lua_State *L)
{
playerstate = RUNNING;
return 0;
}
int Jump(lua_State *L)
{
playerstate = JUMPING;
return 0;
}
/* Check if player can pass obstacle, offer feedback */
int CanPassObstacle()
{
if ( (playerstate == RUNNING && currentobstacle == GROUND) )
{
printf("Successful run!\n");
return SUCCESS;
}
if (playerstate == JUMPING && currentobstacle == FENCE)
{
printf("Successful jump!\n");
return SUCCESS;
}
if (playerstate == DUCKING && currentobstacle == BRANCH)
{
printf("Successful duck!\n");
return SUCCESS;
}
printf("Wrong move!\n");
return FAIL;
}
/* Pick a random obstacle */
enum OBSTACLE GetNewObstacle()
{
int i = rand() % 3;
if (i == 0) { return GROUND; }
if (i == 1) { return FENCE; }
else { return BRANCH; }
}
/* Execute appropriate function defined in Lua for the next obstacle */
int HandleObstacle(lua_State *L)
{
/* Get the table named Actions */
lua_getglobal(L, "Actions");
if (!lua_istable(L, -1)) {return FAIL;}
currentobstacle = GetNewObstacle();
/* Decide which user function to call */
if (currentobstacle == GROUND)
{
lua_getfield(L, -1, "Ground");
}
else if (currentobstacle == FENCE)
{
lua_getfield(L, -1, "Fence");
}
else if (currentobstacle == BRANCH)
{
lua_getfield(L, -1, "Branch");
}
if (lua_isfunction(L, -1))
{
lua_call(L, 0, 0); /* 0 args, 0 results */
return CanPassObstacle();
}
return FAIL;
}
int main()
{
int i, res;
srand(time(NULL));
lua_State *L = lua_open();
/* Bind the C functions to Lua functions */
lua_pushcfunction(L, &Duck);
lua_setglobal(L, "Duck");
lua_pushcfunction(L, &Run);
lua_setglobal(L, "Run");
lua_pushcfunction(L, &Jump);
lua_setglobal(L, "Jump");
/* execute script from stdin */
res = luaL_dofile(L, NULL);
if (res)
{
printf("Lua script error: %s\n", lua_tostring(L, -1));
return 1;
}
for (i = 0 ; i < 5 ; i++)
{
if (HandleObstacle(L) == FAIL)
{
printf("You failed!\n");
return 0;
}
}
printf("You passed!\n");
return 0;
}
Build the above on GCC with gcc runner.c -o runner -llua5.1 -I/usr/include/lua5.1.
And pretty much the only Lua script that will pass successfully every time is:
Actions = {}
function Actions.Ground() Run() end
function Actions.Fence() Jump() end
function Actions.Branch() Duck() end
which could also be written as
Actions = {}
Actions.Ground = Run
Actions.Fence = Jump
Actions.Branch = Duck
With the good script, you'll see output like:
Successful duck!
Successful run!
Successful jump!
Successful jump!
Successful duck!
You passed!
If the user tries something malicious, the program will simply provide an error:
$ echo "Actions = {} function Actions.Ground() os.execute('rm -rf /') end" | ./runner
PANIC: unprotected error in call to Lua API (stdin:1: attempt to index global 'os' (a nil value))
With an incorrect move script, the user will see that he performed the wrong move:
$ echo "Actions = {} Actions.Ground = Jump; Actions.Fence = Duck; Actions.Branch = Run" | ./runner
Wrong move!
You failed!