Direct access vector in lua from C

luavector.c

#include <lua.h>
#include <lauxlib.h>
#include <ltable.h>
#include <lstate.h>
#include <lobject.h>
#include <stdio.h>
#include <string.h>

#define MAGIC_VECTOR 1.23456789

struct vector_offset {
	int x;
	int y;
	int z;
	int magic;
};

#define X(t,vo) fltvalue(gval(gnode(t,(vo)->x)))
#define Y(t,vo) fltvalue(gval(gnode(t,(vo)->y)))
#define Z(t,vo) fltvalue(gval(gnode(t,(vo)->z)))

static int
find_key(const Table *t, const char * key) {
	int i;
	for (i=0;i<sizenode(t);i++) {
		const TValue *v = gkey(gnode(t,i));
		if (strcmp(svalue(v), key) == 0) {
			return i;
		}
	}
	return -1;
}

static void
vector_init(lua_State *L, struct vector_offset *vo) {
	lua_createtable(L, 0, 4);
	lua_pushnumber(L, 0);
	lua_setfield(L, -2, "x");
	lua_pushnumber(L, 0);
	lua_setfield(L, -2, "y");
	lua_pushnumber(L, 0);
	lua_setfield(L, -2, "z");
	lua_pushnumber(L, MAGIC_VECTOR);
	lua_setfield(L, -2, "__vector3");
	const Table * t = lua_topointer(L, -1);
	vo->x = find_key(t, "x");
	vo->y = find_key(t, "y");
	vo->z = find_key(t, "z");
	vo->magic = find_key(t, "__vector3");
}

static lua_Number
getfield(lua_State *L, int index, const char *key) {
	lua_getfield(L, index, key);
	lua_Number v = lua_tonumber(L, -1);
	lua_pop(L, 1);
	return v;
}

Table *
vector_get(lua_State *L, int index, struct vector_offset *vo) {
	luaL_checktype(L, index, LUA_TTABLE);
	Table *t = (Table *)lua_topointer(L, index);
	const TValue *magic = gval(gnode(t,vo->magic));
	if (!ttisfloat(magic) || fltvalue(magic) != MAGIC_VECTOR) {
		// convert to vector struct
		index = lua_absindex(L, index);
		lua_Number x = getfield(L, index, "x");
		lua_Number y = getfield(L, index, "y");
		lua_Number z = getfield(L, index, "z");
		// clear table
		lua_pushnil(L);
		while (lua_next(L, index) != 0) {
			lua_pop(L, 1);
			lua_pushvalue(L, -1);
			lua_pushnil(L);
			lua_rawset(L, index);
		}
		// copy values
		luaH_resize(L, t, 0, 4);
		lua_pushnumber(L, x);
		lua_setfield(L, index, "x");
		lua_pushnumber(L, y);
		lua_setfield(L, index, "y");
		lua_pushnumber(L, z);
		lua_setfield(L, index, "z");
		lua_pushnumber(L, MAGIC_VECTOR);
		lua_setfield(L, index, "__vector3");
	}
	return t;
}

int
main() {
	lua_State *L = luaL_newstate();
	struct vector_offset vo;
	vector_init(L, &vo);
	printf("offset: x=%d y=%d z=%d magic=%d\n",
		vo.x,vo.y,vo.z,vo.magic);
	lua_newtable(L);
	Table *t = vector_get(L, -1, &vo);
	X(t,&vo) = 1;
	Y(t,&vo) = 2;
	Z(t,&vo) = 3;
	printf("x=%f,%f\n",X(t,&vo), getfield(L, -1, "x"));
	printf("y=%f,%f\n",Y(t,&vo), getfield(L, -1, "y"));
	printf("z=%f,%f\n",Z(t,&vo), getfield(L, -1, "z"));
	return 0;
}