MINOR: lua: Add concat class

This patch adds the Concat class. This class provides a fast
way for the string concatenation.

doc/lua-api/index.rst

@@ -533,6 +533,14 @@ Core class
 
   :returns: A :ref:`socket_class` object.
 
+.. js:function:: core.concat()
+
+  **context**: body, init, task, action, sample-fetch, converter
+
+  This function retruns a new concat object.
+
+  :returns: A :ref:`concat_class` object.
+
 .. js:function:: core.done(data)
 
   **context**: body, init, task, action, sample-fetch, converter
@@ -553,6 +561,76 @@ Core class
   Give back the hand at the HAProxy scheduler. It is used when the LUA
   processing consumes a lot of processing time.
 
+.. _concat_class:
+
+Concat class
+============
+
+.. js:class:: Concat
+
+  This class provides a fast way for string concatenation. The way using native
+  Lua concatenation like the code below is slow for some reasons.
+
+.. code-block:: lua
+
+  str = "string1"
+  str = str .. ", string2"
+  str = str .. ", string3"
+..
+
+  For each concatenation, Lua:
+  * allocate memory for the result,
+  * catenate the two string copying the strings in the new memory bloc,
+  * free the old memory block containing the string whoch is no longer used.
+  This process does many memory move, allocation and free. In addition, the
+  memory is not really freed, it is just mark mark as unsused and wait for the
+  garbage collector.
+
+  The Concat class provide an alternative way for catenating strings. It uses
+  the internal Lua mechanism (it does not allocate memory), but it doesn't copy
+  the data more than once.
+
+  On my computer, the following loops spends 0.2s for the Concat method and
+  18.5s for the pure Lua implementation. So, the Concat class is about 1000x
+  faster than the embedded solution.
+
+.. code-block:: lua
+
+  for j = 1, 100 do
+    c = core.concat()
+    for i = 1, 20000 do
+      c:add("#####")
+    end
+  end
+..
+
+.. code-block:: lua
+
+  for j = 1, 100 do
+    c = ""
+    for i = 1, 20000 do
+      c = c .. "#####"
+    end
+  end
+..
+
+.. js:function:: Concat.add(concat, string)
+
+  This function adds a string to the current concatenated string.
+
+  :param class_concat concat: A :ref:`concat_class` which contains the currently
+    builded string.
+  :param string string: A new string to concatenate to the current builded
+    string.
+
+.. js:function:: Concat.dump(concat)
+
+  This function returns the concanated string.
+
+  :param class_concat concat: A :ref:`concat_class` which contains the currently
+    builded string.
+  :returns: the concatenated string
+
 .. _fetches_class:
 
 Fetches class

src/hlua_fcn.c

@@ -9,6 +9,9 @@
 
 #include <common/time.h>
 
+/* Contains the class reference of the concat object. */
+static int class_concat_ref;
+
 /* Return an object of the expected type, or throws an error. */
 void *hlua_checkudata(lua_State *L, int ud, int class_ref)
 {
@@ -118,12 +121,97 @@ static void hlua_array_add_fcn(lua_State *L, const char *name,
 	lua_rawset(L, -3);
 }
 
+static luaL_Buffer *hlua_check_concat(lua_State *L, int ud)
+{
+	return (luaL_Buffer *)(hlua_checkudata(L, ud, class_concat_ref));
+}
+
+static int hlua_concat_add(lua_State *L)
+{
+	luaL_Buffer *b;
+	const char *str;
+	size_t l;
+
+	/* First arg must be a concat object. */
+	b = hlua_check_concat(L, 1);
+
+	/* Second arg must be a string. */
+	str = luaL_checklstring(L, 2, &l);
+
+	luaL_addlstring(b, str, l);
+	return 0;
+}
+
+static int hlua_concat_dump(lua_State *L)
+{
+	luaL_Buffer *b;
+
+	/* First arg must be a concat object. */
+	b = hlua_check_concat(L, 1);
+
+	/* Push the soncatenated strng in the stack. */
+	luaL_pushresult(b);
+	return 1;
+}
+
+int hlua_concat_new(lua_State *L)
+{
+	luaL_Buffer *b;
+
+	lua_newtable(L);
+	b = lua_newuserdata(L, sizeof(luaL_Buffer));
+	lua_rawseti(L, -2, 0);
+
+	lua_rawgeti(L, LUA_REGISTRYINDEX, class_concat_ref);
+	lua_setmetatable(L, -2);
+
+	luaL_buffinit(L, b);
+	return 1;
+}
+
+static int concat_tostring(lua_State *L)
+{
+	const void *ptr = lua_topointer(L, 1);
+	lua_pushfstring(L, "Concat object: %p", ptr);
+	return 1;
+}
+
+static int hlua_concat_init(lua_State *L)
+{
+	/* Creates the buffered concat object. */
+	lua_newtable(L);
+
+	lua_pushstring(L, "__tostring");
+	lua_pushcclosure(L, concat_tostring, 0);
+	lua_settable(L, -3);
+
+	lua_pushstring(L, "__index"); /* Creates the index entry. */
+	lua_newtable(L); /* The "__index" content. */
+
+	lua_pushstring(L, "add");
+	lua_pushcclosure(L, hlua_concat_add, 0);
+	lua_settable(L, -3);
+
+	lua_pushstring(L, "dump");
+	lua_pushcclosure(L, hlua_concat_dump, 0);
+	lua_settable(L, -3);
+
+	lua_settable(L, -3); /* Sets the __index entry. */
+	class_concat_ref = luaL_ref(L, LUA_REGISTRYINDEX);
+
+	return 1;
+}
+
 int hlua_fcn_reg_core_fcn(lua_State *L)
 {
+	if (!hlua_concat_init(L))
+		return 0;
+
 	hlua_array_add_fcn(L, "now", hlua_now);
 	hlua_array_add_fcn(L, "http_date", hlua_http_date);
 	hlua_array_add_fcn(L, "imf_date", hlua_imf_date);
 	hlua_array_add_fcn(L, "rfc850_date", hlua_rfc850_date);
 	hlua_array_add_fcn(L, "asctime_date", hlua_asctime_date);
+	hlua_array_add_fcn(L, "concat", hlua_concat_new);
 	return 5;
 }