Vector Library Sample

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Description

This is a very simple example of wrapping some vector code. The function lua_vec3libopen() uses luaL_openl() to add a global function vector3(), a vector constructor. A tag is created so that the tag methods gettable() and settable() can be used to access elements in the vector. See the test code for usage.

[!] VersionNotice: The below code pertains to an older Lua version, Lua 4. Certain features used like tag methods (settagmethod) are no longer present in Lua 5 but have been replaced with metamethods.

Caveat: The code below uses a static variable TAG. This can be avoided, and should. I should have used the registry but I was lazy. I could also have used TAG local to lua_vec3libopen() and used it as an upvalue to vec3new(). I was also lazy in my treatment of field names: I only look at the first letter. --lhf

This code is for Lua 4.0. I've updated it for Lua 5.0 [here]. --lhf

C code

/*
* vec3lib.c
* add 3d vectors to Lua
* this code is in the public domain
*/

#include "lua.h"
#include "lauxlib.h"

LUALIB_API int lua_vec3libopen (lua_State *L);

typedef struct { double a[3]; } Vector3;

static int TAG; /* lazy! */

static int vec3new(lua_State *L) {
  Vector3* v;
  lua_newuserdata(L,sizeof(Vector3));
  lua_settag(L,TAG);
  v=lua_touserdata(L,-1);
  if (lua_gettop(L)>1) {
    v->a[0]=luaL_check_number(L,1);
    v->a[1]=luaL_check_number(L,2);
    v->a[2]=luaL_check_number(L,3);
  }
  else
    v->a[0] = v->a[1] = v->a[2] = 0;
  return 1;
}

static int vec3get(lua_State *L) {
  Vector3* v=lua_touserdata(L,1);
  const char* i=luaL_check_string(L,2);
  switch (*i) { /* lazy! */
    case 'x': case 'r': lua_pushnumber(L,v->a[0]); break;
    case 'y': case 'g': lua_pushnumber(L,v->a[1]); break;
    case 'z': case 'b': lua_pushnumber(L,v->a[2]); break;
    default: lua_pushnumber(L,0.0); break;
  }
  return 1;
}

static int vec3set(lua_State *L) {
  Vector3* v=lua_touserdata(L,1);
  const char* i=luaL_check_string(L,2);
  double t=luaL_check_number(L,3);
  switch (*i) { /* lazy! */
    case 'x': case 'r': v->a[0]=t; break;
    case 'y': case 'g': v->a[1]=t; break;
    case 'z': case 'b': v->a[2]=t; break;
    default: break;
  }
  return 1;
}

static const struct luaL_reg vec3lib[] = {
  {"vector3", vec3new},
};

int lua_vec3libopen (lua_State *L) {
  luaL_openl(L,vec3lib);
  TAG=lua_newtag(L);
  lua_pushcfunction(L,vec3get); lua_settagmethod(L,TAG,"gettable");
  lua_pushcfunction(L,vec3set); lua_settagmethod(L,TAG,"settable");
  return 0;
}

Test Code

The following code can be used to test it:

-- a = vector3()
a = vector3(10,20,30)
print(a,tag(a))

print(a.x,a.y,a.z)
print(a.t,a[1])

a.x,a.y,a.z=100,200,300
print(a.x,a.y,a.z)

-- try also a.r, a.g, a.b, if you want to use vectors to store colors...