minimize.cc

// -*- coding: utf-8 -*-
// Copyright (C) 2010, 2011, 2012, 2013, 2014, 2015 Laboratoire de
// Recherche et Développement de l'Epita (LRDE).
//
// This file is part of Spot, a model checking library.
//
// Spot is free software; you can redistribute it and/or modify it
// under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 3 of the License, or
// (at your option) any later version.
//
// Spot is distributed in the hope that it will be useful, but WITHOUT
// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
// or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
// License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.


//#define TRACE

#ifdef TRACE
#  define trace std::cerr
#else
#  define trace while (0) std::cerr
#endif

#include <queue>
#include <deque>
#include <set>
#include <list>
#include <vector>
#include <sstream>
#include "minimize.hh"
#include "ltlast/allnodes.hh"
#include "misc/hash.hh"
#include "misc/bddlt.hh"
#include "tgbaalgos/product.hh"
#include "tgbaalgos/powerset.hh"
#include "tgbaalgos/gtec/gtec.hh"
#include "tgbaalgos/safety.hh"
#include "tgbaalgos/sccfilter.hh"
#include "tgbaalgos/sccinfo.hh"
#include "tgbaalgos/ltl2tgba_fm.hh"
#include "tgbaalgos/bfssteps.hh"
#include "tgbaalgos/isdet.hh"
#include "tgbaalgos/dtgbacomp.hh"

namespace spot
{
  // FIXME: do we really want to use unordered_set instead of set here?
  // This calls for benchmarking.
  typedef std::unordered_set<const state*,
			     state_ptr_hash, state_ptr_equal> hash_set;
  typedef std::unordered_map<const state*, unsigned,
			     state_ptr_hash, state_ptr_equal> hash_map;

  namespace
  {
    static std::ostream&
    dump_hash_set(const hash_set* hs,
		  const const_tgba_ptr& aut,
		  std::ostream& out)
    {
      out << '{';
      const char* sep = "";
      for (hash_set::const_iterator i = hs->begin(); i != hs->end(); ++i)
	{
	  out << sep << aut->format_state(*i);
	  sep = ", ";
	}
      out << '}';
      return out;
    }

    static std::string
    format_hash_set(const hash_set* hs, const_tgba_ptr aut)
    {
      std::ostringstream s;
      dump_hash_set(hs, aut, s);
      return s.str();
    }
  }

  // Find all states of an automaton.
  void build_state_set(const const_tgba_ptr& a, hash_set* seen)
  {
    std::queue<const state*> tovisit;
    // Perform breadth-first traversal.
    const state* init = a->get_init_state();
    tovisit.push(init);
    seen->insert(init);
    while (!tovisit.empty())
      {
	const state* src = tovisit.front();
	tovisit.pop();

	for (auto sit: a->succ(src))
	  {
	    const state* dst = sit->current_state();
	    // Is it a new state ?
	    if (seen->find(dst) == seen->end())
	      {
		// Register the successor for later processing.
		tovisit.push(dst);
		seen->insert(dst);
	      }
	    else
	      dst->destroy();
	  }
      }
  }

  // From the base automaton and the list of sets, build the minimal
  // resulting automaton
  tgba_digraph_ptr build_result(const const_tgba_ptr& a,
				std::list<hash_set*>& sets,
				hash_set* final)
  {
    auto dict = a->get_dict();
    auto res = make_tgba_digraph(dict);
    res->copy_ap_of(a);
    res->prop_state_based_acc();

    // For each set, create a state in the resulting automaton.
    // For a state s, state_num[s] is the number of the state in the minimal
    // automaton.
    hash_map state_num;
    std::list<hash_set*>::iterator sit;
    for (sit = sets.begin(); sit != sets.end(); ++sit)
      {
	hash_set::iterator hit;
	hash_set* h = *sit;
	unsigned num = res->new_state();
	for (hit = h->begin(); hit != h->end(); ++hit)
	  state_num[*hit] = num;
      }

    // For each transition in the initial automaton, add the corresponding
    // transition in res.

    if (!final->empty())
      res->set_single_acceptance_set();

    for (sit = sets.begin(); sit != sets.end(); ++sit)
      {
	hash_set::iterator hit;
	hash_set* h = *sit;

	// Pick one state.
	const state* src = *h->begin();
	unsigned src_num = state_num[src];
	bool accepting = (final->find(src) != final->end());

	// Connect it to all destinations.
	for (auto succit: a->succ(src))
	  {
	    const state* dst = succit->current_state();
	    hash_map::const_iterator i = state_num.find(dst);
	    dst->destroy();
	    if (i == state_num.end()) // Ignore useless destinations.
	      continue;
	    res->new_acc_transition(src_num, i->second,
				    succit->current_condition(), accepting);
	  }
      }
    res->merge_transitions();
    if (res->num_states() > 0)
      {
	const state* init_state = a->get_init_state();
	unsigned init_num = state_num[init_state];
	init_state->destroy();
	res->set_init_state(init_num);
      }
    return res;
  }


  namespace
  {

    struct wdba_search_acc_loop : public bfs_steps
    {
      wdba_search_acc_loop(const const_tgba_ptr& det_a,
			   unsigned scc_n, scc_info& sm,
			   power_map& pm, const state* dest)
	: bfs_steps(det_a), scc_n(scc_n), sm(sm), pm(pm), dest(dest)
      {
	seen(dest);
      }

      virtual const state*
      filter(const state* s)
      {
	s = seen(s);
	if (sm.scc_of(std::static_pointer_cast<const tgba_digraph>(a_)
		      ->state_number(s)) != scc_n)
	  return 0;
	return s;
      }

      virtual bool
      match(tgba_run::step&, const state* to)
      {
	return to == dest;
      }

      unsigned scc_n;
      scc_info& sm;
      power_map& pm;
      const state* dest;
      state_unicity_table seen;
    };


    bool
    wdba_scc_is_accepting(const const_tgba_digraph_ptr& det_a, unsigned scc_n,
			  const const_tgba_digraph_ptr& orig_a, scc_info& sm,
			  power_map& pm)
    {

      // Get some state from the SCC #n.
      const state* start = det_a->state_from_number(sm.one_state_of(scc_n));

      // Find a loop around START in SCC #n.
      wdba_search_acc_loop wsal(det_a, scc_n, sm, pm, start);
      tgba_run::steps loop;
      const state* reached = wsal.search(start, loop);
      assert(reached == start);
      (void)reached;

      // Build an automaton representing this loop.
      auto loop_a = make_tgba_digraph(det_a->get_dict());
      tgba_run::steps::const_iterator i;
      int loop_size = loop.size();
      loop_a->new_states(loop_size);
      int n;
      for (n = 1, i = loop.begin(); n < loop_size; ++n, ++i)
	{
	  loop_a->new_transition(n - 1, n, i->label);
	  i->s->destroy();
	}
      assert(i != loop.end());
      loop_a->new_transition(n - 1, 0, i->label);
      i->s->destroy();
      assert(++i == loop.end());

      loop_a->set_init_state(0U);

      // Check if the loop is accepting in the original automaton.
      bool accepting = false;

      // Iterate on each original state corresponding to start.
      const power_map::power_state& ps =
	pm.states_of(det_a->state_number(start));
      for (auto& it: ps)
	{
	  // Construct a product between LOOP_A and ORIG_A starting in
	  // *IT.  FIXME: This could be sped up a lot!
	  if (!product(loop_a, orig_a, 0U,
		       orig_a->state_number(it))->is_empty())
	    {
	      accepting = true;
	      break;
	    }
	}

      return accepting;
    }

  }

  tgba_digraph_ptr minimize_dfa(const const_tgba_digraph_ptr& det_a,
				hash_set* final, hash_set* non_final)
  {
    typedef std::list<hash_set*> partition_t;
    partition_t cur_run;
    partition_t next_run;

    // The list of equivalent states.
    partition_t done;

    hash_map state_set_map;

    // Size of det_a
    unsigned size = final->size() + non_final->size();
    // Use bdd variables to number sets.  set_num is the first variable
    // available.
    unsigned set_num =
      det_a->get_dict()->register_anonymous_variables(size, det_a);

    std::set<int> free_var;
    for (unsigned i = set_num; i < set_num + size; ++i)
      free_var.insert(i);
    std::map<int, int> used_var;

    hash_set* final_copy;

    if (!final->empty())
      {
	unsigned s = final->size();
	used_var[set_num] = s;
	free_var.erase(set_num);
	if (s > 1)
	  cur_run.push_back(final);
	else
	  done.push_back(final);
	for (hash_set::const_iterator i = final->begin();
	     i != final->end(); ++i)
	  state_set_map[*i] = set_num;

	final_copy = new hash_set(*final);
      }
    else
      {
	final_copy = final;
      }

    if (!non_final->empty())
      {
	unsigned s = non_final->size();
	unsigned num = set_num + 1;
	used_var[num] = s;
	free_var.erase(num);
	if (s > 1)
	  cur_run.push_back(non_final);
	else
	  done.push_back(non_final);
	for (hash_set::const_iterator i = non_final->begin();
	     i != non_final->end(); ++i)
	  state_set_map[*i] = num;
      }
    else
      {
	delete non_final;
      }

    // A bdd_states_map is a list of formulae (in a BDD form) associated with a
    // destination set of states.
    typedef std::map<bdd, hash_set*, bdd_less_than> bdd_states_map;

    bool did_split = true;

    while (did_split)
      {
	did_split = false;
	while (!cur_run.empty())
	  {
	    // Get a set to process.
	    hash_set* cur = cur_run.front();
	    cur_run.pop_front();

	    trace << "processing " << format_hash_set(cur, det_a) << std::endl;

	    hash_set::iterator hi;
	    bdd_states_map bdd_map;
	    for (hi = cur->begin(); hi != cur->end(); ++hi)
	      {
		const state* src = *hi;
		bdd f = bddfalse;
		for (auto si: det_a->succ(src))
		  {
		    const state* dst = si->current_state();
		    hash_map::const_iterator i = state_set_map.find(dst);
		    dst->destroy();
		    if (i == state_set_map.end())
		      // The destination state is not in our
		      // partition.  This can happen if the initial
		      // FINAL and NON_FINAL supplied to the algorithm
		      // do not cover the whole automaton (because we
		      // want to ignore some useless states).  Simply
		      // ignore these states here.
		      continue;
		    f |= (bdd_ithvar(i->second) & si->current_condition());
		  }

		// Have we already seen this formula ?
		bdd_states_map::iterator bsi = bdd_map.find(f);
		if (bsi == bdd_map.end())
		  {
		    // No, create a new set.
		    hash_set* new_set = new hash_set;
		    new_set->insert(src);
		    bdd_map[f] = new_set;
		  }
		else
		  {
		    // Yes, add the current state to the set.
		    bsi->second->insert(src);
		  }
	      }

	    bdd_states_map::iterator bsi = bdd_map.begin();
	    if (bdd_map.size() == 1)
	      {
		// The set was not split.
		trace << "set " << format_hash_set(bsi->second, det_a)
		      << " was not split" << std::endl;
		next_run.push_back(bsi->second);
	      }
	    else
	      {
		did_split = true;
		for (; bsi != bdd_map.end(); ++bsi)
		  {
		    hash_set* set = bsi->second;
		    // Free the number associated to these states.
		    unsigned num = state_set_map[*set->begin()];
		    assert(used_var.find(num) != used_var.end());
		    unsigned left = (used_var[num] -= set->size());
		    // Make sure LEFT does not become negative (hence bigger
		    // than SIZE when read as unsigned)
		    assert(left < size);
		    if (left == 0)
		      {
			used_var.erase(num);
			free_var.insert(num);
		      }
		    // Pick a free number
		    assert(!free_var.empty());
		    num = *free_var.begin();
		    free_var.erase(free_var.begin());
		    used_var[num] = set->size();
		    for (hash_set::iterator hit = set->begin();
			 hit != set->end(); ++hit)
		      state_set_map[*hit] = num;
		    // Trivial sets can't be splitted any further.
		    if (set->size() == 1)
		      {
			trace << "set " << format_hash_set(set, det_a)
			      << " is minimal" << std::endl;
			done.push_back(set);
		      }
		    else
		      {
			trace << "set " << format_hash_set(set, det_a)
			      << " should be processed further" << std::endl;
			next_run.push_back(set);
		      }
		  }
	      }
	    delete cur;
	  }
	if (did_split)
	  trace << "splitting did occur during this pass." << std::endl;
	else
	  trace << "splitting did not occur during this pass." << std::endl;
	std::swap(cur_run, next_run);
      }

    done.splice(done.end(), cur_run);

#ifdef TRACE
    trace << "Final partition: ";
    for (partition_t::const_iterator i = done.begin(); i != done.end(); ++i)
      trace << format_hash_set(*i, det_a) << ' ';
    trace << std::endl;
#endif

    // Build the result.
    auto res = build_result(det_a, done, final_copy);

    // Free all the allocated memory.
    delete final_copy;
    hash_map::iterator hit;
    for (hit = state_set_map.begin(); hit != state_set_map.end();)
      {
	hash_map::iterator old = hit++;
	old->first->destroy();
      }
    std::list<hash_set*>::iterator it;
    for (it = done.begin(); it != done.end(); ++it)
      delete *it;

    return res;
  }


  tgba_digraph_ptr minimize_monitor(const const_tgba_digraph_ptr& a)
  {
    hash_set* final = new hash_set;
    hash_set* non_final = new hash_set;
    tgba_digraph_ptr det_a;

    {
      power_map pm;
      det_a = tgba_powerset(a, pm);
    }

    // non_final contain all states.
    // final is empty: there is no acceptance condition
    build_state_set(det_a, non_final);
    auto res = minimize_dfa(det_a, final, non_final);
    res->prop_deterministic();
    res->prop_inherently_weak();
    res->prop_state_based_acc();
    return res;
  }

  tgba_digraph_ptr minimize_wdba(const const_tgba_digraph_ptr& a)
  {
    hash_set* final = new hash_set;
    hash_set* non_final = new hash_set;

    tgba_digraph_ptr det_a;

    {
      power_map pm;
      det_a = tgba_powerset(a, pm);

      // For each SCC of the deterministic automaton, determine if it
      // is accepting or not.

      // This corresponds to the algorithm in Fig. 1 of "Efficient
      // minimization of deterministic weak omega-automata" written by
      // Christof Löding and published in Information Processing
      // Letters 79 (2001) pp 105--109.

      // We also keep track of whether an SCC is useless
      // (i.e., it is not the start of any accepting word).

      scc_info sm(det_a);
      unsigned scc_count = sm.scc_count();
      // SCC that have been marked as useless.
      std::vector<bool> useless(scc_count);
      // The "color".  Even number correspond to
      // accepting SCCs.
      std::vector<unsigned> d(scc_count);

      // An even number larger than scc_count.
      unsigned k = (scc_count | 1) + 1;

      // SCC are numbered in topological order
      // (but in the reverse order as Löding's)
      for (unsigned m = 0; m < scc_count; ++m)
	{
	  bool is_useless = true;
	  bool transient = sm.is_trivial(m);
	  auto& succ = sm.succ(m);

	  if (transient && succ.empty())
	    {
	      // A trivial SCC without successor is useless.
	      useless[m] = true;
	      d[m] = k - 1;
	      continue;
	    }

	  // Compute the minimum color l of the successors.
	  // Also SCCs are useless if all their successor are
	  // useless.
	  unsigned l = k;
	  for (auto& j: succ)
	    {
	      is_useless &= useless[j.dst];
	      unsigned dj = d[j.dst];
	      if (dj < l)
		l = dj;
	    }

	  if (transient)
	    {
	      d[m] = l;
	    }
	  else
	    {
	      // Regular SCCs are accepting if any of their loop
	      // corresponds to an accepted word in the original
	      // automaton.
	      if (wdba_scc_is_accepting(det_a, m, a, sm, pm))
		{
		  is_useless = false;
		  d[m] = l & ~1; // largest even number inferior or equal
		}
	      else
		{
		  d[m] = (l - 1) | 1; // largest odd number inferior or equal
		}
	    }

	  useless[m] = is_useless;

	  if (!is_useless)
	    {
	      hash_set* dest_set = (d[m] & 1) ? non_final : final;
	      for (auto s: sm.states_of(m))
		dest_set->insert(det_a->state_from_number(s));
	    }
	}
    }

    auto res = minimize_dfa(det_a, final, non_final);
    res->prop_deterministic();
    res->prop_inherently_weak();
    return res;
  }

  tgba_digraph_ptr
  minimize_obligation(const const_tgba_digraph_ptr& aut_f,
		      const ltl::formula* f,
		      const_tgba_digraph_ptr aut_neg_f,
		      bool reject_bigger)
  {
    auto min_aut_f = minimize_wdba(aut_f);

    if (reject_bigger)
      {
	// Abort if min_aut_f has more states than aut_f.
	unsigned orig_states = aut_f->num_states();
	if (orig_states < min_aut_f->num_states())
	  return std::const_pointer_cast<tgba_digraph>(aut_f);
      }

    // If the input automaton was already weak and deterministic, the
    // output is necessary correct.
    if (aut_f->is_inherently_weak() && aut_f->is_deterministic())
      return min_aut_f;

    // if f is a syntactic obligation formula, the WDBA minimization
    // must be correct.
    if (f && f->is_syntactic_obligation())
      return min_aut_f;

    // If aut_f is a guarantee automaton, the WDBA minimization must be
    // correct.
    if (is_guarantee_automaton(aut_f))
      return min_aut_f;

    // Build negation automaton if not supplied.
    if (!aut_neg_f)
      {
	if (f)
	  {
	    // If we know the formula, simply build the automaton for
	    // its negation.
	    const ltl::formula* neg_f =
	      ltl::unop::instance(ltl::unop::Not, f->clone());
	    aut_neg_f = ltl_to_tgba_fm(neg_f, aut_f->get_dict());
	    neg_f->destroy();
	    // Remove useless SCCs.
	    aut_neg_f = scc_filter(aut_neg_f, true);
	  }
	else if (is_deterministic(aut_f))
	  {
	    // If the automaton is deterministic, complementing is
	    // easy.
	    aut_neg_f = dtgba_complement(aut_f);
	  }
	else
	  {
	    // Otherwise, we cannot check if the minimization is safe.
	    return nullptr;
	  }
      }

    // If the negation is a guarantee automaton, then the
    // minimization is correct.
    if (is_guarantee_automaton(aut_neg_f))
      {
	return min_aut_f;
      }

    bool ok = false;

    if (product(min_aut_f, aut_neg_f)->is_empty())
      {
	// Complement the minimized WDBA.
	assert(min_aut_f->is_inherently_weak());
	auto neg_min_aut_f = dtgba_complement(min_aut_f);
	if (product(aut_f, neg_min_aut_f)->is_empty())
	  // Finally, we are now sure that it was safe
	  // to minimize the automaton.
	  ok = true;
      }

    if (ok)
      return min_aut_f;
    return std::const_pointer_cast<tgba_digraph>(aut_f);
  }
}