// -*- coding: utf-8 -*-
// Copyright (C) 2008, 2010, 2011, 2013, 2014, 2015 Laboratoire de
// recherche et développement de l'Epita (LRDE).
// Copyright (C) 2004, 2005 Laboratoire d'Informatique de Paris 6
// (LIP6), département Systèmes Répartis Coopératifs (SRC), Université
// Pierre et Marie Curie.
//
// 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

#include <iostream>
#ifdef TRACE
#define trace std::cerr
#else
#define trace while (0) std::cerr
#endif

#include <cassert>
#include <utility>
#include <vector>
#include "twa/twa.hh"
#include "misc/hash.hh"
#include "emptiness.hh"
#include "emptiness_stats.hh"
#include "gv04.hh"
#include "bfssteps.hh"

namespace spot
{
  namespace
  {
    struct stack_entry
    {
      const state* s;		  // State stored in stack entry.
      twa_succ_iterator* lasttr; // Last transition explored from this state.
      int lowlink;		  // Lowlink value if this entry.
      int pre;			  // DFS predecessor.
      int acc;			  // Accepting state link.
    };

    struct gv04: public emptiness_check, public ec_statistics
    {
      // Map of visited states.
      typedef std::unordered_map<const state*, size_t,
				 state_ptr_hash, state_ptr_equal> hash_type;
      hash_type h;

      // Stack of visited states on the path.
      typedef std::vector<stack_entry> stack_type;
      stack_type stack;

      int top;			// Top of SCC stack.
      int dftop;		// Top of DFS stack.
      bool violation;		// Whether an accepting run was found.

      gv04(const const_twa_ptr& a, option_map o)
	: emptiness_check(a, o)
      {
	assert(a->num_sets() <= 1);
      }

      ~gv04()
      {
	for (stack_type::iterator i = stack.begin(); i != stack.end(); ++i)
	  a_->release_iter(i->lasttr);
	hash_type::const_iterator s = h.begin();
	while (s != h.end())
	  {
	    // Advance the iterator before deleting the "key" pointer.
	    const state* ptr = s->first;
	    ++s;
	    ptr->destroy();
	  }
      }

      virtual emptiness_check_result_ptr
      check()
      {
	top = dftop = -1;
	violation = false;
	push(a_->get_init_state(), false);

	while (!violation && dftop >= 0)
	  {
	    trace << "Main iteration (top = " << top
		  << ", dftop = " << dftop
		  << ", s = " << a_->format_state(stack[dftop].s)
		  << ')' << std::endl;

	    twa_succ_iterator* iter = stack[dftop].lasttr;
	    bool cont;
	    if (!iter)
	      {
		iter = stack[dftop].lasttr = a_->succ_iter(stack[dftop].s);
		cont = iter->first();
	      }
	    else
	      {
		cont = iter->next();
	      }

	    if (!cont)
	      {
		trace << " No more successors" << std::endl;
		pop();
	      }
	    else
	      {
		const state* s_prime = iter->current_state();
		bool acc =
		  a_->acc().accepting(iter->current_acceptance_conditions());
		inc_transitions();

		trace << " Next successor: s_prime = "
		      << a_->format_state(s_prime)
		      << (acc ? " (with accepting link)" : "");

		hash_type::const_iterator i = h.find(s_prime);

		if (i == h.end())
		  {
		    trace << " is a new state." << std::endl;
		    push(s_prime, acc);
		  }
		else
		  {
		    if (i->second < stack.size()
			&& stack[i->second].s->compare(s_prime) == 0)
		      {
			// s_prime has a clone on stack
			trace << " is on stack." << std::endl;
			// This is an addition to GV04 to support TBA.
			violation |= acc;
			lowlinkupdate(dftop, i->second);
		      }
		    else
		      {
			trace << " has been seen, but is no longer on stack."
			      << std::endl;
		      }

		    s_prime->destroy();
		  }
	      }
	    set_states(h.size());
	  }
	if (violation)
	  return std::make_shared<result>(*this);
	return nullptr;
      }

      void
      push(const state* s, bool accepting)
      {
	trace << "  push(s = " << a_->format_state(s)
	      << ", accepting = " << accepting << ")\n";

	h[s] = ++top;

	stack_entry ss = { s, nullptr, top, dftop, 0 };

	if (accepting)
	  ss.acc = top - 1;	// This differs from GV04 to support TBA.
	else if (dftop >= 0)
	  ss.acc = stack[dftop].acc;
	else
	  ss.acc = -1;

	trace << "    s.lowlink = " << top << std::endl
	      << "    s.acc = " << ss.acc << std::endl;

	stack.push_back(ss);
	dftop = top;
	inc_depth();
      }

      void
      pop()
      {
	trace << "  pop()\n";

	int p = stack[dftop].pre;
	if (p >= 0)
	  lowlinkupdate(p, dftop);
	if (stack[dftop].lowlink == dftop)
	  {
	    assert(static_cast<unsigned int>(top + 1) == stack.size());
	    for (int i = top; i >= dftop; --i)
	      {
		a_->release_iter(stack[i].lasttr);
		stack.pop_back();
		dec_depth();
	      }
	    top = dftop - 1;
	  }
	dftop = p;
      }

      void
      lowlinkupdate(int f, int t)
      {
	trace << "  lowlinkupdate(f = " << f << ", t = " << t
	      << ")\n    t.lowlink = " << stack[t].lowlink
	      << "\n    f.lowlink = " << stack[f].lowlink
	      << "\n    f.acc = " << stack[f].acc << '\n';
	int stack_t_lowlink = stack[t].lowlink;
	if (stack_t_lowlink <= stack[f].lowlink)
	  {
	    if (stack_t_lowlink <= stack[f].acc)
	      violation = true;
	    stack[f].lowlink = stack_t_lowlink;
	    trace << "    f.lowlink updated to "
		  << stack[f].lowlink << '\n';
	  }
      }

      virtual std::ostream&
      print_stats(std::ostream& os) const
      {
	os << h.size() << " unique states visited\n";
	os << transitions() << " transitions explored\n";
	os << max_depth() << " items max on stack\n";
	return os;
      }

      struct result:
	public emptiness_check_result,
	public acss_statistics
      {
	gv04& data;

	result(gv04& data)
	  : emptiness_check_result(data.automaton(), data.options()),
	    data(data)
	{
	}

	void
	update_lowlinks()
	{
	  // Transitively update the lowlinks, so we can use them in
	  // to check SCC inclusion
	  for (int i = 0; i <= data.top; ++i)
	    {
	      int l = data.stack[i].lowlink;
	      if (l < i)
		{
		  int ll = data.stack[i].lowlink = data.stack[l].lowlink;
		  for (int j = i - 1; data.stack[j].lowlink != ll; --j)
		    data.stack[j].lowlink = ll;
		}
	    }
	}

	virtual unsigned
	acss_states() const
	{
	  // Gross!
	  const_cast<result*>(this)->update_lowlinks();

	  int scc = data.stack[data.dftop].lowlink;
	  int j = data.dftop;
	  int s = 0;
	  while (j >= 0 && data.stack[j].lowlink == scc)
	    {
	      --j;
	      ++s;
	    }
	  assert(s > 0);
	  return s;
	}

	virtual twa_run_ptr
	accepting_run()
	{
	  auto res = std::make_shared<twa_run>();

	  update_lowlinks();
#ifdef TRACE
	  for (int i = 0; i <= data.top; ++i)
	    {
	      trace << "state " << i << " ("
		    << data.a_->format_state(data.stack[i].s)
		    << ") has lowlink = " << data.stack[i].lowlink << std::endl;
	    }
#endif

	  // We will use the root of the last SCC as the start of the
	  // cycle.
	  int scc_root = data.stack[data.dftop].lowlink;
	  assert(scc_root >= 0);

	  // Construct the prefix by unwinding the DFS stack before
	  // scc_root.
	  int father = data.stack[scc_root].pre;
	  while (father >= 0)
	    {
	      twa_run::step st =
		{
		  data.stack[father].s->clone(),
		  data.stack[father].lasttr->current_condition(),
		  data.stack[father].lasttr->current_acceptance_conditions()
		};
	      res->prefix.push_front(st);
	      father = data.stack[father].pre;
	    }

	  // Construct the cycle in two phases.  A first BFS finds the
	  // shortest path from scc_root to an accepting transition.
	  // A second BFS then search a path back to scc_root.  If
	  // there is no acceptance conditions we just use the second
	  // BFS to find a cycle around scc_root.

	  struct first_bfs: bfs_steps
	  {
	    const gv04& data;
	    int scc_root;
	    result* r;

	    first_bfs(result* r, int scc_root)
	      : bfs_steps(r->data.automaton()), data(r->data),
		scc_root(scc_root), r(r)
	    {
	    }

	    virtual const state*
	    filter(const state* s)
	    {
	      // Do not escape the SCC
	      hash_type::const_iterator j = data.h.find(s);
	      if (// This state was never visited so far.
		  j == data.h.end()
		  // Or it was discarded
		  || j->second >= data.stack.size()
		  // Or it was discarded (but its stack slot reused)
		  || data.stack[j->second].s->compare(s)
		  // Or it is still on the stack but not in the SCC
		  || data.stack[j->second].lowlink < scc_root)
		{
		  s->destroy();
		  return nullptr;
		}
	      r->inc_ars_cycle_states();
	      s->destroy();
	      return j->first;
	    }

	    virtual bool
	    match(twa_run::step& step, const state*)
	    {
	      return step.acc != 0U;
	    }
	  };

	  struct second_bfs: first_bfs
	  {
	    const state* target;
	    second_bfs(result* r, int scc_root, const state* target)
	      : first_bfs(r, scc_root), target(target)
	    {
	    }

	    virtual bool
	    match(twa_run::step&, const state* s)
	    {
	      return s == target;
	    }
	  };

	  const state* bfs_start = data.stack[scc_root].s;
	  const state* bfs_end = bfs_start;
	  if (a_->num_sets() > 0)
	    {
	      first_bfs b1(this, scc_root);
	      bfs_start = b1.search(bfs_start, res->cycle);
	      assert(bfs_start);
	    }
	  if (bfs_start != bfs_end || res->cycle.empty())
	    {
	      second_bfs b2(this, scc_root, bfs_end);
	      bfs_start = b2.search(bfs_start, res->cycle);
	      assert(bfs_start == bfs_end);
	    }

	  assert(res->cycle.begin() != res->cycle.end());
	  return res;
	}
      };


    };

  } // anonymous

  emptiness_check_ptr
  explicit_gv04_check(const const_twa_ptr& a, option_map o)
  {
    return std::make_shared<gv04>(a, o);
  }
}