Function luaV_concat

third-party/lua-5.3.5/src/lvm.c:474–511

** Main operation for concatenation: concat 'total' values in the stack, ** from 'L->top - total' up to 'L->top - 1'. */

** from 'L->top - total' up to 'L->top - 1'.
*/
void luaV_concat (lua_State *L, int total) {
  lua_assert(total >= 2);
  do {
    StkId top = L->top;
    int n = 2;  /* number of elements handled in this pass (at least 2) */
    if (!(ttisstring(top-2) || cvt2str(top-2)) || !tostring(L, top-1))
      luaT_trybinTM(L, top-2, top-1, top-2, TM_CONCAT);
    else if (isemptystr(top - 1))  /* second operand is empty? */
      cast_void(tostring(L, top - 2));  /* result is first operand */
    else if (isemptystr(top - 2)) {  /* first operand is an empty string? */
      setobjs2s(L, top - 2, top - 1);  /* result is second op. */
    }
    else {
      /* at least two non-empty string values; get as many as possible */
      size_t tl = vslen(top - 1);
      TString *ts;
      /* collect total length and number of strings */
      for (n = 1; n < total && tostring(L, top - n - 1); n++) {
        size_t l = vslen(top - n - 1);
        if (l >= (MAX_SIZE/sizeof(char)) - tl)
          luaG_runerror(L, "string length overflow");
        tl += l;
      }
      if (tl <= LUAI_MAXSHORTLEN) {  /* is result a short string? */
        char buff[LUAI_MAXSHORTLEN];
        copy2buff(top, n, buff);  /* copy strings to buffer */
        ts = luaS_newlstr(L, buff, tl);
      }
      else {  /* long string; copy strings directly to final result */
        ts = luaS_createlngstrobj(L, tl);
        copy2buff(top, n, getstr(ts));
      }
      setsvalue2s(L, top - n, ts);  /* create result */
    }
    total -= n-1;  /* got 'n' strings to create 1 new */
    L->top -= n-1;  /* popped 'n' strings and pushed one */
  } while (total > 1);  /* repeat until only 1 result left */
}


/*