dboolean P_PathTraverse(fixed_t x1, fixed_t y1, fixed_t x2, fixed_t y2,
                       int flags, dboolean trav(intercept_t *))
{
  fixed_t xt1, yt1;
  fixed_t xt2, yt2;
  fixed_t xstep, ystep;
  fixed_t partial;
  fixed_t xintercept, yintercept;
  int     mapx, mapy;
  int     mapx1, mapy1;
  int     mapxstep, mapystep;
  int     count;

  validcount++;
  intercept_p = intercepts;

  if (!((x1-bmaporgx)&(MAPBLOCKSIZE-1)))
    x1 += FRACUNIT;     // don't side exactly on a line

  if (!((y1-bmaporgy)&(MAPBLOCKSIZE-1)))
    y1 += FRACUNIT;     // don't side exactly on a line

  trace.x = x1;
  trace.y = y1;
  trace.dx = x2 - x1;
  trace.dy = y2 - y1;

  if (COMPBAD(comperr_blockmap))
  {
    int_64_t _x1, _x2, _y1, _y2;

    _x1 = (int_64_t)x1 - bmaporgx;
    _y1 = (int_64_t)y1 - bmaporgy;
    xt1 = (int)(_x1>>MAPBLOCKSHIFT);
    yt1 = (int)(_y1>>MAPBLOCKSHIFT);

    mapx1 = (int)(_x1>>MAPBTOFRAC);
    mapy1 = (int)(_y1>>MAPBTOFRAC);

    _x2 = (int_64_t)x2 - bmaporgx;
    _y2 = (int_64_t)y2 - bmaporgy;
    xt2 = (int)(_x2>>MAPBLOCKSHIFT);
    yt2 = (int)(_y2>>MAPBLOCKSHIFT);

    x1 -= bmaporgx;
    y1 -= bmaporgy;
    x2 -= bmaporgx;
    y2 -= bmaporgy;
  }
  else
  {
    x1 -= bmaporgx;
    y1 -= bmaporgy;
    xt1 = x1>>MAPBLOCKSHIFT;
    yt1 = y1>>MAPBLOCKSHIFT;

    mapx1 = x1>>MAPBTOFRAC;
    mapy1 = y1>>MAPBTOFRAC;

    x2 -= bmaporgx;
    y2 -= bmaporgy;
    xt2 = x2>>MAPBLOCKSHIFT;
    yt2 = y2>>MAPBLOCKSHIFT;
  }

  if (xt2 > xt1)
    {
      mapxstep = 1;
      partial = FRACUNIT - (mapx1&(FRACUNIT-1));
      ystep = FixedDiv (y2-y1,D_abs(x2-x1));
    }
  else
    if (xt2 < xt1)
      {
        mapxstep = -1;
        partial = mapx1&(FRACUNIT-1);
        ystep = FixedDiv (y2-y1,D_abs(x2-x1));
      }
    else
      {
        mapxstep = 0;
        partial = FRACUNIT;
        ystep = 256*FRACUNIT;
      }

  yintercept = mapy1 + FixedMul(partial, ystep);

  if (yt2 > yt1)
    {
      mapystep = 1;
      partial = FRACUNIT - (mapy1&(FRACUNIT-1));
      xstep = FixedDiv (x2-x1,D_abs(y2-y1));
    }
  else
    if (yt2 < yt1)
      {
        mapystep = -1;
        partial = mapy1&(FRACUNIT-1);
        xstep = FixedDiv (x2-x1,D_abs(y2-y1));
      }
    else
      {
        mapystep = 0;
        partial = FRACUNIT;
        xstep = 256*FRACUNIT;
      }

  xintercept = mapx1 + FixedMul(partial, xstep);

  // Step through map blocks.
  // Count is present to prevent a round off error
  // from skipping the break.

  mapx = xt1;
  mapy = yt1;

  for (count = 0; count < 64; count++)
    {
      if (flags & PT_ADDLINES)
        if (!P_BlockLinesIterator(mapx, mapy,PIT_AddLineIntercepts))
          return false; // early out

      if (flags & PT_ADDTHINGS)
        if (!P_BlockThingsIterator(mapx, mapy,PIT_AddThingIntercepts))
          return false; // early out

      if (mapx == xt2 && mapy == yt2)
        break;

      if ((yintercept >> FRACBITS) == mapy)
        {
          yintercept += ystep;
          mapx += mapxstep;
        }
      else
        if ((xintercept >> FRACBITS) == mapx)
          {
            xintercept += xstep;
            mapy += mapystep;
          }
    }

  // go through the sorted list
  return P_TraverseIntercepts(trav, FRACUNIT);
}