cloud-sat/kissat-inc/src/xors.c

#include "eliminate.h"
#include "gates.h"
#include "inline.h"
#include "sort.h"
#include "terminate.h"
#include "xors.h"

static void
mark_literals (kissat * solver, value * marks,
	       unsigned size, unsigned *lits, unsigned signs)
{
  assert (size < 32);
  for (unsigned i = 0, bit = 1; i < size; i++, bit <<= 1)
    {
      unsigned sign = ((bit & signs) != 0);
      const unsigned lit = lits[i] ^ sign;
      assert (!marks[lit]);
      marks[lit] = 1;
      LOG ("marked %s", LOGLIT (lit));
    }
#ifndef LOGGING
  (void) solver;
#endif
}

static void
unmark_literals (kissat * solver, value * marks,
		 unsigned size, unsigned *lits, unsigned signs)
{
  assert (size < 32);
  for (unsigned i = 0, bit = 1; i < size; i++, bit <<= 1)
    {
      unsigned sign = ((bit & signs) != 0);
      const unsigned lit = lits[i] ^ sign;
      assert (marks[lit]);
      marks[lit] = 0;
      LOG ("unmarked %s", LOGLIT (lit));
    }
#ifndef LOGGING
  (void) solver;
#endif
}

static unsigned
copy_literals (kissat * solver, unsigned lit,
	       const value * values, unsigned *lits, clause * c)
{
  const unsigned *end = c->lits + c->size;
  unsigned *q = lits;
#ifndef NDEBUG
  bool found_lit = false;
#endif
  for (const unsigned *p = c->lits; p != end; p++)
    {
      const unsigned other = *p;
      if (other == lit)
	{
#ifndef NDEBUG
	  assert (!found_lit);
	  assert (!values[other]);
	  found_lit = true;
#endif
	}
      else
	{
	  const value value = values[other];
	  assert (value <= 0);
	  if (value < 0)
	    {
	      LOG ("skipping falsified %s", LOGLIT (other));
	      continue;
	    }
	  LOG ("copying %s", LOGLIT (other));
	  *q++ = other;
	}
    }
  assert (found_lit);
  *q++ = lit;
  const unsigned size = q - lits;
  LOGLITS (size, lits, "copied", size);
#ifndef LOGGING
  (void) solver;
#endif
  return size;
}

static bool
odd_parity (unsigned size, unsigned signs)
{
  bool res = false;
  for (unsigned i = 0; i < size; i++)
    if (signs & (1u << i))
      res = !res;
  return res;
}

static unsigned
next_marking (kissat * solver, value * marks,
	      unsigned size, unsigned *lits, unsigned prev)
{
  LOG ("prev signs %s", FORMAT_SIGNS (size, prev));
  assert (2 < size);
  assert (size < 32);
  const unsigned limit = (1u << size);

  assert (prev < limit);

  unsigned next;

  for (next = prev + 1; odd_parity (size, next); next++)
    ;

  LOG ("next signs %s", FORMAT_SIGNS (size, next));

  for (unsigned i = 0, bit = 1; i < size; i++, bit <<= 1)
    {
      const unsigned lit = lits[i];
      const unsigned not_lit = NOT (lit);
      if (!(prev & bit) && (next & bit))
	{
	  assert (marks[lit]);
	  assert (!marks[not_lit]);
	  marks[lit] = 0;
	  LOG ("unmarked %s", LOGLIT (lit));
	  marks[not_lit] = 1;
	  LOG ("marked %s", LOGLIT (not_lit));
	}
      else if ((prev & bit) && !(next & bit))
	{
	  assert (marks[not_lit]);
	  assert (!marks[lit]);
	  marks[not_lit] = 0;
	  LOG ("unmarked %s", LOGLIT (not_lit));
	  marks[lit] = 1;
	  LOG ("marked %s", LOGLIT (lit));
	}
    }
#ifndef LOGGING
  (void) solver;
#endif
  return next == limit ? 0 : next;
}

static bool
match_lits_ref (kissat * solver, const value * marks, const value * values,
		unsigned size, reference ref)
{
  clause *c = kissat_dereference_clause (solver, ref);
  unsigned found = 0;
  for (all_literals_in_clause (lit, c))
    {
      const value value = values[lit];
      if (value > 0)
	{
	  kissat_eliminate_clause (solver, c, INVALID_LIT);
	  return false;
	}
      if (value < 0)
	continue;
      if (!marks[lit])
	return false;
      found++;
    }
  assert (found <= size);
  if (found < size)
    solver->resolve_gate = true;
  return true;
}

static bool
match_lits_watch (kissat * solver,
		  const value * marks, const value * values,
		  unsigned size, watch watch)
{
  if (watch.type.binary)
    {
      const unsigned other = watch.binary.lit;
      if (!marks[other])
	return false;
      assert (size > 2);
      solver->resolve_gate = true;
      return true;
    }
  else
    {
      const reference ref = watch.large.ref;
      return match_lits_ref (solver, marks, values, size, ref);
    }
}

static watch *
find_lits_watch (kissat * solver, watch * begin, watch * end,
		 const value * marks, const value * values,
		 unsigned size, uint64_t * steps)
{
  assert (begin <= end);
  for (watch * p = begin; p != end; p++)
    {
      *steps += 1;
      if (match_lits_watch (solver, marks, values, size, *p))
	return p;
    }
  return 0;
}

#define LESS_POINTER(P,Q) \
  ((P) < (Q))

static void
sort_watch_pointers (kissat * solver, patches * patches)
{
  SORT_STACK (watch *, *patches, LESS_POINTER);
}

bool
kissat_find_xor_gate (kissat * solver, unsigned lit, unsigned negative)
{
  if (!GET_OPTION (xors))
    return false;

  const unsigned size_limit = solver->bounds.xork.clause_size;
  if (size_limit < 3)
    return false;
  assert (size_limit < 32);

  watches *watches0 = &WATCHES (lit);
  watch *begin0 = BEGIN_WATCHES (*watches0);
  watch *end0 = END_WATCHES (*watches0);
  if (begin0 == end0)
    return false;

  uint64_t large_clauses0 = 0;
  for (watch * p = begin0; p != end0; p++)
    if (!p->type.binary && large_clauses0++)
      break;
  if (large_clauses0 < 2)
    return false;

  const unsigned not_lit = NOT (lit);
  watches *watches1 = &WATCHES (not_lit);
  watch *begin1 = BEGIN_WATCHES (*watches1);
  watch *end1 = END_WATCHES (*watches1);

  uint64_t large_clauses1 = 0;
  for (watch * p = begin1; p != end1; p++)
    if (!p->type.binary && large_clauses1++)
      break;
  if (large_clauses1 < 2)
    return false;

  unsigned lits[size_limit];

  const value *values = solver->values;
  value *marks = solver->marks;

  const unsigned steps_limit = solver->bounds.eliminate.occurrences;

  uint64_t steps = 0;

  for (watch * p = begin0; p != end0; p++)
    {
      if (p->type.binary)
	continue;

      if (steps > steps_limit)
	break;

      if (TERMINATED (26))
	break;

      steps++;
      clause *c = kissat_dereference_clause (solver, p->large.ref);
      if (c->size > size_limit)
	continue;

      unsigned size = copy_literals (solver, lit, values, lits, c);

      assert (size <= 32);
      if (size < 3)
	continue;

      solver->resolve_gate = false;

      assert (EMPTY_STACK (solver->xorted[0]));
      assert (EMPTY_STACK (solver->xorted[1]));
      PUSH_STACK (solver->xorted[0], p);

      unsigned signs = 0;
      mark_literals (solver, marks, size, lits, signs);

      while (steps <= steps_limit &&
	     (signs = next_marking (solver, marks, size, lits, signs)))
	{
	  if (marks[lit])
	    {
	      watch *q = find_lits_watch (solver, begin0, end0,
					  marks, values, size, &steps);
	      if (!q)
		{
		  LOGLITS (size, lits,
			   "could not match signs %s of copied",
			   FORMAT_SIGNS (size, signs));
		  break;
		}

	      LOGWATCH (lit, *q, "literal %s XOR", LOGLIT (lit));
	      PUSH_STACK (solver->xorted[0], q);
	    }
	  else
	    {
	      watch *q = find_lits_watch (solver, begin1, end1,
					  marks, values, size, &steps);
	      if (!q)
		break;

	      LOGWATCH (not_lit, *q, "found %s literal XOR",
			LOGLIT (not_lit));
	      PUSH_STACK (solver->xorted[1], q);
	    }
	}

      unmark_literals (solver, marks, size, lits, signs);

      unsigned nsort[2] = {
	SIZE_STACK (solver->xorted[0]), SIZE_STACK (solver->xorted[1])
      };

      if (nsort[0] + nsort[1] == (1u << (size - 1)))
	{
	  assert (nsort[0] == (1u << (size - 2)));
	  assert (nsort[1] == (1u << (size - 2)));

	  sort_watch_pointers (solver, &solver->xorted[0]);

	  const watch *prev = 0;
	  for (unsigned i = 0; i < nsort[0]; i++)
	    {
	      const watch *p0 = PEEK_STACK (solver->xorted[0], i);
	      const watch w0 = *p0;
	      if (p0 == prev)
		LOGWATCH (lit, w0, "dropping repeated");
	      else
		{
		  LOGWATCH (lit, w0, "%s %s XOR",
			    FORMAT_ORDINAL (i + 1), LOGLIT (lit));
		  PUSH_STACK (solver->gates[negative], w0);
		}
	      prev = p0;
	    }

	  sort_watch_pointers (solver, &solver->xorted[1]);
	  prev = 0;
	  for (unsigned i = 0; i < nsort[1]; i++)
	    {
	      const watch *p1 = PEEK_STACK (solver->xorted[1], i);
	      const watch w1 = *p1;
	      if (p1 == prev)
		LOGWATCH (not_lit, w1, "dropping repeated");
	      else
		{
		  LOGWATCH (not_lit, w1, "%s %s XOR",
			    FORMAT_ORDINAL (i + 1), LOGLIT (not_lit));
		  PUSH_STACK (solver->gates[!negative], w1);
		}
	      prev = p1;
	    }

	  assert (!EMPTY_STACK (solver->gates[0]));
#ifdef LOGGING
	  assert (size > 1);
	  assert (lits[size - 1] == lit);
	  lits[0] = NOT (lits[0]);
	  LOGXOR (lit, size - 1, lits, "found");
	  lits[0] = NOT (lits[0]);
#endif
	}

      CLEAR_STACK (solver->xorted[0]);
      CLEAR_STACK (solver->xorted[1]);

      if (EMPTY_STACK (solver->gates[0]))
	continue;

      solver->gate_eliminated = GATE_ELIMINATED (xors);

      return true;
    }

  assert (EMPTY_STACK (solver->xorted[0]));
  assert (EMPTY_STACK (solver->xorted[1]));

  return false;
}