Alternative export method of arrays from C to Lua

The patch is mainly to export client_array_t object to Lua,
but can be used to export any ..._array_t object.

The idea: export to Lua not a table, but userdata with
metamethods to get/set/define length of ..._array_t object
directly.

Now when I get clients field from tag object C code
creates full copy of client_array_t structure into Lua table.
It takes traversing a whole array of data.

I did it in other way: userdata is exported, with __index,
__newindex, and __len meta-methods defined, and Lua
script gains direct access to client_array_t C-array:
it can get client object, get length of array and assign
client objects to some index in C-array.

Pros:
No overhead of creation a copy of C-structure into Lua-table:
if I want just to test a number of clients for a tag, I don't need
a whole loop to build table, I just want to read clients->len field,
and I do so via __len meta-method.
Also if I want to get some client from tags.clients, I don't need
to create ALL clients Lua-objects, I just get client_t C-struct
and create Lua-object from it. Just in place.
So Lua-loop enuming all tag.clients is not 2 loops internally
(first create copy of tag.clients into Lua-table, then enum this table),
but only one, and if I break out of loop in the middle, I create
only some client Lua-objects, not all of them from tag.clients.

Contras:
As far as clients field is not a table, I cant use pairs/ipairs
and other table functions for it.
But it can be implemented in other way:
for k,c pairs(tag.clients) => for k = 1, #tag.clients,
table.insert(tag.clients, client) => tag.clients[#tag.clients+1] = client

etc.

One more Pro now:
As far as tag.clients in current implementation returns copy of data
table.insert doesn't do what's expected: it doesn't really add client
into tag.clients "array".
With my implementation client is added as expected, as we work with
client_array_t structure directly.

Signed-off-by: Julien Danjou <julien@danjou.info>

client.h

@@ -51,6 +51,7 @@ int luaA_client_newindex(lua_State *);
 int luaA_client_userdata_new(lua_State *, client_t *);
 
 DO_SLIST(client_t, client, client_unref)
+DO_LUA_EXPORT_ARRAY(client, "client_array", client_t, client_array_t, luaA_client_userdata_new)
 
 #endif
 // vim: filetype=c:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:encoding=utf-8:textwidth=80

lib/awful.lua.in

@@ -1052,9 +1052,10 @@ function widget.taglist.label.all(t, args)
     if sel and sel.tags[t] then
         background = "resize=\"true\" image=\"@AWESOME_ICON_PATH@/taglist/squarefw.png\""
     elseif bg_urgent and fg_urgent then
-        for k, c in pairs(t.clients) do
+        local cli = t.clients
+        for k = 1, #cli do
             background = "resize=\"true\" image=\"@AWESOME_ICON_PATH@/taglist/squarew.png\""
-            if c.urgent then
+            if cli[k].urgent then
                 bg_color = bg_urgent
                 fg_color = fg_urgent
                 break

lua.h

@@ -259,5 +259,66 @@ luaA_generic_pairs(lua_State *L)
     return 3;
 }
 
+#define DO_LUA_EXPORT_ARRAY(pfx, typename, type, atype, ctor)               \
+    static inline int                                                       \
+    luaA_##pfx##_array_index(lua_State *L)                                  \
+    {                                                                       \
+        atype *value = lua_touserdata(L, 1);                                \
+        type **obj;                                                         \
+        int idx;                                                            \
+                                                                            \
+        if ((obj = lua_touserdata(L, 2)))                                   \
+        {                                                                   \
+            for (idx = 0; idx < value->len; idx++)                          \
+                if (value->tab[idx] == *obj)                                \
+                    return ctor(L, value->tab[idx]);                        \
+            return 0;                                                       \
+        }                                                                   \
+                                                                            \
+        idx = luaL_checknumber(L, 2) - 1;                                   \
+        if (idx < 0 || idx >= value->len)                                   \
+            return 0;                                                       \
+                                                                            \
+        return ctor(L, value->tab[idx]);                                    \
+    }                                                                       \
+    static inline int                                                       \
+    luaA_##pfx##_array_newindex(lua_State *L)                               \
+    {                                                                       \
+        atype *value = lua_touserdata(L, 1);                                \
+        int idx = luaL_checknumber(L, 2);                                   \
+        type **elem = luaL_checkudata(L, 3, #pfx);                          \
+        pfx##_array_splice(value, idx - 1, 1, elem, 1);                     \
+        return 0;                                                           \
+    }                                                                       \
+    static inline int                                                       \
+    luaA_##pfx##_array_len(lua_State *L)                                    \
+    {                                                                       \
+        atype *value = lua_touserdata(L, 1);                                \
+        lua_pushnumber(L, value->len);                                      \
+        return 1;                                                           \
+    }                                                                       \
+    static inline int                                                       \
+    luaA_##pfx##_array_tostring(lua_State *L)                               \
+    {                                                                       \
+        atype *value = lua_touserdata(L, 1);                                \
+        lua_pushfstring(L, "["typename" udata(%p)]", value);                \
+        return 1;                                                           \
+    }                                                                       \
+    static inline void                                                      \
+    luaA_##pfx##_array_export(lua_State *L, atype *arr)                     \
+    {                                                                       \
+        lua_pushlightuserdata(L, arr);                                      \
+        lua_newtable(L);                                                    \
+        lua_pushcfunction(L, luaA_##pfx##_array_index);                     \
+        lua_setfield(L, -2, "__index");                                     \
+        lua_pushcfunction(L, luaA_##pfx##_array_tostring);                  \
+        lua_setfield(L, -2, "__tostring");                                  \
+        lua_pushcfunction(L, luaA_##pfx##_array_newindex);                  \
+        lua_setfield(L, -2, "__newindex");                                  \
+        lua_pushcfunction(L, luaA_##pfx##_array_len);                       \
+        lua_setfield(L, -2, "__len");                                       \
+        lua_setmetatable(L, -2);                                            \
+    }
+
 #endif
 // vim: filetype=c:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:encoding=utf-8:textwidth=80

tag.c

@@ -349,8 +349,6 @@ luaA_tag_index(lua_State *L)
     size_t len;
     tag_t **tag = luaA_checkudata(L, 1, "tag");
     const char *attr;
-    client_array_t *clients;
-    int i;
 
     if(luaA_usemetatable(L, 1, 2))
         return 1;
@@ -383,14 +381,7 @@ luaA_tag_index(lua_State *L)
         lua_pushnumber(L, (*tag)->ncol);
         break;
       case A_TK_CLIENTS:
-        clients = &(*tag)->clients;
+        luaA_client_array_export(L, &(*tag)->clients);
-        luaA_otable_new(L);
-        for(i = 0; i < clients->len; i++)
-        {
-            luaA_client_userdata_new(L, clients->tab[i]);
-            luaA_client_userdata_new(L, clients->tab[i]);
-            lua_rawset(L, -3);
-        }
         break;
       default:
         return 0;