Re: A useful useless fact about Lua keywords

[Francisco Olarte <folarte@...>]

On Wed, Sep 15, 2021 at 6:52 PM Flyer31 Test <flyer31@googlemail.com> wrote:
> Oh, very helpful and impressive info is this, thank you very much ... .
>
> (this was my main objection against this string.sub, that extensive
> use would somehow start many many super-small allocations on the
> heap... I would not have expected that Lua has some means to avoid
> this - very ingenious indeed then ... In this case I really could

Check that fact first, at least in 5.4.3 string.sub is:

static int str_sub (lua_State *L) {
  size_t l;
  const char *s = luaL_checklstring(L, 1, &l);
  size_t start = posrelatI(luaL_checkinteger(L, 2), l);
  size_t end = getendpos(L, 3, -1, l);
  if (start <= end)
    lua_pushlstring(L, s + start - 1, (end - start) + 1);
  else lua_pushliteral(L, "");
  return 1;
}

So it seems content is in fact not shared and this:

Lua 5.4.3  Copyright (C) 1994-2021 Lua.org, PUC-Rio
> collectgarbage('stop')
0
> collectgarbage('count')
21.3662109375
> s="0123456789abcdefghijklmnopqrstuvwxyz"
> for i=1,10 do for j=1,26-i do t=string.sub(s,j,i) end end
> collectgarbage('count')
28.2568359375
> for i=1,26 do for j=1,27-i do t=string.sub(s,j,i) end end
> collectgarbage('count')
34.75
> collectgarbage()
0
> collectgarbage('count')
23.2294921875

Points to memory being consumed for more than a pointer ( 1st loop
executes about 250 times generating strings of about 5.5 bytes
averages, uses 7k, which woould be about 300 bytes/str if content was
shared )

On your ministring, making your own string.charat is trivial in C, you
could probably make what you need in about a page ( 66 lines ) of C
code.

Francisco Olarte.