/* exactly-one */

static int fd_exactly_one_propagate(fd_constraint this, fd_int revise)
{
  int value;
  int changed = 0, candidates = 0, which = -1;
  int i;

  // only revise if one other variable has the requested value
  for (i = 0; i < this->nvariables; ++i)
    if (VAR(this, i) != revise &&
	fd_var_single(VAR(this, i), &value) &&
	value == this->constants[0])
      {
	changed |= _fd_var_del_val(value, revise);

	candidates++;

	break;
      }

  if (changed && fd_domain_empty(revise))
    return FD_NOSOLUTION;

  if (changed)
    _fd_revise_connected(this, revise); // XXX: NULL or this?

#if 01
  // if any (other) variable is set to the required value, stop
  // (if more than one variable is set to the value, it will be
  // detected in another propagation step, which is (hopefully)
  // already scheduled)
  if (candidates)
    return FD_OK;

  // XXX: if value only belongs to one variable (revise?) domain, the
  //      variable could be set to it
  candidates = 0;
  for (i = 0; i < this->nvariables; ++i)
    if (_fd_var_contains_val(VAR(this, i), this->constants[0]))
      {
	if (++candidates > 1)
	    // more than one variable has the value in its domain
	  break;

	which = i;
      }

  // if there is no variable whose domain contains the requested value,
  // no solution can be reached
  if (candidates == 0)
    return FD_NOSOLUTION;

  // if the value only belongs to one variable's domain, that variable
  // can be set to it
  if (candidates == 1 && !fd_var_single(VAR(this, which), NULL))
    if (VAR(this, which) == revise)
      {
	_fd_var_set_value(revise, this->constants[0]);
	if (!changed)
	  _fd_revise_connected(NULL, revise);
      }
    else
      {
	_fd_var_set_value(VAR(this, which), this->constants[0]);
	_fd_revise_connected(NULL, VAR(this, which));
      }
#endif

  return FD_OK;
}

fd_constraint fd_exactly_one(fd_int *variables, int nvariables, int value)
{
  fd_constraint c = _fd_constraint_new(nvariables, 1);
  int i;

  if (c)
    {
      for (i = 0; i < nvariables; ++i)
	c->variables[i] = FD_INT2C_VAR(variables[i]);
      c->constants[0] = value;
#ifdef CONSTRAINT_CLASS
      c->kind = FD_CONSTR_EXACTLY_ONE;
#else /* CONSTRAINT_CLASS */
      c->propagator = fd_exactly_one_propagate;
#endif /* CONSTRAINT_CLASS */

      for (i = 0; i < nvariables; ++i)
	_fd_var_add_constraint(VAR(c, i), c);

      _fd_add_constraint(c);
    }

  return c;
}