Function luaH_getn

3rd/lua-5.4.3/src/ltable.c:912–957

** Try to find a boundary in table 't'. (A 'boundary' is an integer index ** such that t[i] is present and t[i+1] is absent, or 0 if t[1] is absent ** and 'maxinteger' if t[maxinteger] is present.) ** (In the next explanation, we use Lua indices, that is, with base 1. ** The code itself uses base 0 when indexing the array part of the table.) ** The code starts with 'limit = t->alimit', a position

** therefore cannot be used as a new limit.)
*/
lua_Unsigned luaH_getn (Table *t) {
  unsigned int limit = t->alimit;
  if (limit > 0 && isempty(&t->array[limit - 1])) {  /* (1)? */
    /* there must be a boundary before 'limit' */
    if (limit >= 2 && !isempty(&t->array[limit - 2])) {
      /* 'limit - 1' is a boundary; can it be a new limit? */
      if (ispow2realasize(t) && !ispow2(limit - 1)) {
        t->alimit = limit - 1;
        setnorealasize(t);  /* now 'alimit' is not the real size */
      }
      return limit - 1;
    }
    else {  /* must search for a boundary in [0, limit] */
      unsigned int boundary = binsearch(t->array, 0, limit);
      /* can this boundary represent the real size of the array? */
      if (ispow2realasize(t) && boundary > luaH_realasize(t) / 2) {
        t->alimit = boundary;  /* use it as the new limit */
        setnorealasize(t);
      }
      return boundary;
    }
  }
  /* 'limit' is zero or present in table */
  if (!limitequalsasize(t)) {  /* (2)? */
    /* 'limit' > 0 and array has more elements after 'limit' */
    if (isempty(&t->array[limit]))  /* 'limit + 1' is empty? */
      return limit;  /* this is the boundary */
    /* else, try last element in the array */
    limit = luaH_realasize(t);
    if (isempty(&t->array[limit - 1])) {  /* empty? */
      /* there must be a boundary in the array after old limit,
         and it must be a valid new limit */
      unsigned int boundary = binsearch(t->array, t->alimit, limit);
      t->alimit = boundary;
      return boundary;
    }
    /* else, new limit is present in the table; check the hash part */
  }
  /* (3) 'limit' is the last element and either is zero or present in table */
  lua_assert(limit == luaH_realasize(t) &&
             (limit == 0 || !isempty(&t->array[limit - 1])));
  if (isdummy(t) || isempty(luaH_getint(t, cast(lua_Integer, limit + 1))))
    return limit;  /* 'limit + 1' is absent */
  else  /* 'limit + 1' is also present */
    return hash_search(t, limit);
}