unpack hash part of table
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- Subject: unpack hash part of table
- From: 云风 Cloud Wu <cloudwu@...>
- Date: 2022年3月29日 16:29:09 +0800
I need to read all of the data in a lua table (no array part) in C. I
know I can use `lua_next` to iterate the table :
int unpack_table_with_next(lua_State *L, int n) {
int idx = lua_gettop(L);
int s;
lua_settop(L, idx + n * 2);
lua_pushnil(L);
s = 0;
while (lua_next(L, idx) != 0) {
lua_replace(L, idx + s * 2 + 2);
lua_copy(L, -1, idx + s * 2 + 1);
++s;
}
lua_settop(L, idx + s * 2);
return s;
}
This function can unpack the first n key-value pairs into the stack
and returns the number of actually unpacked.
I found if I can access the internal data structure of lua table, it
can run much faster.
int unpack_table(lua_State *L, int n) {
int idx = lua_gettop(L);
int s;
lua_settop(L, idx + n * 2);
const Table * t = (const Table *)lua_topointer(L, idx);
int sz = sizenode(t);
s = 0;
StkId r = L->top - n * 2;
for (i=0;i<sz && s<n;i++) {
if (!isempty(gval(gnode(t, i)))) {
Node *n = gnode(t, i);
getnodekey(L, s2v(r++), n);
setobj2s(L, r++, gval(n));
++s;
}
}
lua_settop(L, idx + s * 2);
return s;
}
It's 7x~8x faster.
I wish a similar C API can be introduced into a future version of lua.
--
http://blog.codingnow.com