summaryrefslogtreecommitdiff
path: root/src/p_maputl.c
blob: 1566263a7f08077e22f4e330cdd37afa6efdacef (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
// Emacs style mode select   -*- C++ -*- 
//-----------------------------------------------------------------------------
//
// $Id: p_maputl.c 19 2005-07-23 19:17:11Z fraggle $
//
// Copyright(C) 1993-1996 Id Software, Inc.
// Copyright(C) 2005 Simon Howard
//
// This program 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 2
// of the License, or (at your option) any later version.
//
// This program 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, write to the Free Software
// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
// 02111-1307, USA.
//
// $Log$
// Revision 1.3  2005/07/23 19:17:11  fraggle
// Use ANSI-standard limit constants.  Remove LINUX define.
//
// Revision 1.2  2005/07/23 16:44:56  fraggle
// Update copyright to GNU GPL
//
// Revision 1.1.1.1  2005/07/23 16:20:03  fraggle
// Initial import
//
//
// DESCRIPTION:
//	Movement/collision utility functions,
//	as used by function in p_map.c. 
//	BLOCKMAP Iterator functions,
//	and some PIT_* functions to use for iteration.
//
//-----------------------------------------------------------------------------

static const char
rcsid[] = "$Id: p_maputl.c 19 2005-07-23 19:17:11Z fraggle $";


#include <stdlib.h>


#include "m_bbox.h"

#include "doomdef.h"
#include "p_local.h"


// State.
#include "r_state.h"

//
// P_AproxDistance
// Gives an estimation of distance (not exact)
//

fixed_t
P_AproxDistance
( fixed_t	dx,
  fixed_t	dy )
{
    dx = abs(dx);
    dy = abs(dy);
    if (dx < dy)
	return dx+dy-(dx>>1);
    return dx+dy-(dy>>1);
}


//
// P_PointOnLineSide
// Returns 0 or 1
//
int
P_PointOnLineSide
( fixed_t	x,
  fixed_t	y,
  line_t*	line )
{
    fixed_t	dx;
    fixed_t	dy;
    fixed_t	left;
    fixed_t	right;
	
    if (!line->dx)
    {
	if (x <= line->v1->x)
	    return line->dy > 0;
	
	return line->dy < 0;
    }
    if (!line->dy)
    {
	if (y <= line->v1->y)
	    return line->dx < 0;
	
	return line->dx > 0;
    }
	
    dx = (x - line->v1->x);
    dy = (y - line->v1->y);
	
    left = FixedMul ( line->dy>>FRACBITS , dx );
    right = FixedMul ( dy , line->dx>>FRACBITS );
	
    if (right < left)
	return 0;		// front side
    return 1;			// back side
}



//
// P_BoxOnLineSide
// Considers the line to be infinite
// Returns side 0 or 1, -1 if box crosses the line.
//
int
P_BoxOnLineSide
( fixed_t*	tmbox,
  line_t*	ld )
{
    int		p1;
    int		p2;
	
    switch (ld->slopetype)
    {
      case ST_HORIZONTAL:
	p1 = tmbox[BOXTOP] > ld->v1->y;
	p2 = tmbox[BOXBOTTOM] > ld->v1->y;
	if (ld->dx < 0)
	{
	    p1 ^= 1;
	    p2 ^= 1;
	}
	break;
	
      case ST_VERTICAL:
	p1 = tmbox[BOXRIGHT] < ld->v1->x;
	p2 = tmbox[BOXLEFT] < ld->v1->x;
	if (ld->dy < 0)
	{
	    p1 ^= 1;
	    p2 ^= 1;
	}
	break;
	
      case ST_POSITIVE:
	p1 = P_PointOnLineSide (tmbox[BOXLEFT], tmbox[BOXTOP], ld);
	p2 = P_PointOnLineSide (tmbox[BOXRIGHT], tmbox[BOXBOTTOM], ld);
	break;
	
      case ST_NEGATIVE:
	p1 = P_PointOnLineSide (tmbox[BOXRIGHT], tmbox[BOXTOP], ld);
	p2 = P_PointOnLineSide (tmbox[BOXLEFT], tmbox[BOXBOTTOM], ld);
	break;
    }

    if (p1 == p2)
	return p1;
    return -1;
}


//
// P_PointOnDivlineSide
// Returns 0 or 1.
//
int
P_PointOnDivlineSide
( fixed_t	x,
  fixed_t	y,
  divline_t*	line )
{
    fixed_t	dx;
    fixed_t	dy;
    fixed_t	left;
    fixed_t	right;
	
    if (!line->dx)
    {
	if (x <= line->x)
	    return line->dy > 0;
	
	return line->dy < 0;
    }
    if (!line->dy)
    {
	if (y <= line->y)
	    return line->dx < 0;

	return line->dx > 0;
    }
	
    dx = (x - line->x);
    dy = (y - line->y);
	
    // try to quickly decide by looking at sign bits
    if ( (line->dy ^ line->dx ^ dx ^ dy)&0x80000000 )
    {
	if ( (line->dy ^ dx) & 0x80000000 )
	    return 1;		// (left is negative)
	return 0;
    }
	
    left = FixedMul ( line->dy>>8, dx>>8 );
    right = FixedMul ( dy>>8 , line->dx>>8 );
	
    if (right < left)
	return 0;		// front side
    return 1;			// back side
}



//
// P_MakeDivline
//
void
P_MakeDivline
( line_t*	li,
  divline_t*	dl )
{
    dl->x = li->v1->x;
    dl->y = li->v1->y;
    dl->dx = li->dx;
    dl->dy = li->dy;
}



//
// P_InterceptVector
// Returns the fractional intercept point
// along the first divline.
// This is only called by the addthings
// and addlines traversers.
//
fixed_t
P_InterceptVector
( divline_t*	v2,
  divline_t*	v1 )
{
#if 1
    fixed_t	frac;
    fixed_t	num;
    fixed_t	den;
	
    den = FixedMul (v1->dy>>8,v2->dx) - FixedMul(v1->dx>>8,v2->dy);

    if (den == 0)
	return 0;
    //	I_Error ("P_InterceptVector: parallel");
    
    num =
	FixedMul ( (v1->x - v2->x)>>8 ,v1->dy )
	+FixedMul ( (v2->y - v1->y)>>8, v1->dx );

    frac = FixedDiv (num , den);

    return frac;
#else	// UNUSED, float debug.
    float	frac;
    float	num;
    float	den;
    float	v1x;
    float	v1y;
    float	v1dx;
    float	v1dy;
    float	v2x;
    float	v2y;
    float	v2dx;
    float	v2dy;

    v1x = (float)v1->x/FRACUNIT;
    v1y = (float)v1->y/FRACUNIT;
    v1dx = (float)v1->dx/FRACUNIT;
    v1dy = (float)v1->dy/FRACUNIT;
    v2x = (float)v2->x/FRACUNIT;
    v2y = (float)v2->y/FRACUNIT;
    v2dx = (float)v2->dx/FRACUNIT;
    v2dy = (float)v2->dy/FRACUNIT;
	
    den = v1dy*v2dx - v1dx*v2dy;

    if (den == 0)
	return 0;	// parallel
    
    num = (v1x - v2x)*v1dy + (v2y - v1y)*v1dx;
    frac = num / den;

    return frac*FRACUNIT;
#endif
}


//
// P_LineOpening
// Sets opentop and openbottom to the window
// through a two sided line.
// OPTIMIZE: keep this precalculated
//
fixed_t opentop;
fixed_t openbottom;
fixed_t openrange;
fixed_t	lowfloor;


void P_LineOpening (line_t* linedef)
{
    sector_t*	front;
    sector_t*	back;
	
    if (linedef->sidenum[1] == -1)
    {
	// single sided line
	openrange = 0;
	return;
    }
	 
    front = linedef->frontsector;
    back = linedef->backsector;
	
    if (front->ceilingheight < back->ceilingheight)
	opentop = front->ceilingheight;
    else
	opentop = back->ceilingheight;

    if (front->floorheight > back->floorheight)
    {
	openbottom = front->floorheight;
	lowfloor = back->floorheight;
    }
    else
    {
	openbottom = back->floorheight;
	lowfloor = front->floorheight;
    }
	
    openrange = opentop - openbottom;
}


//
// THING POSITION SETTING
//


//
// P_UnsetThingPosition
// Unlinks a thing from block map and sectors.
// On each position change, BLOCKMAP and other
// lookups maintaining lists ot things inside
// these structures need to be updated.
//
void P_UnsetThingPosition (mobj_t* thing)
{
    int		blockx;
    int		blocky;

    if ( ! (thing->flags & MF_NOSECTOR) )
    {
	// inert things don't need to be in blockmap?
	// unlink from subsector
	if (thing->snext)
	    thing->snext->sprev = thing->sprev;

	if (thing->sprev)
	    thing->sprev->snext = thing->snext;
	else
	    thing->subsector->sector->thinglist = thing->snext;
    }
	
    if ( ! (thing->flags & MF_NOBLOCKMAP) )
    {
	// inert things don't need to be in blockmap
	// unlink from block map
	if (thing->bnext)
	    thing->bnext->bprev = thing->bprev;
	
	if (thing->bprev)
	    thing->bprev->bnext = thing->bnext;
	else
	{
	    blockx = (thing->x - bmaporgx)>>MAPBLOCKSHIFT;
	    blocky = (thing->y - bmaporgy)>>MAPBLOCKSHIFT;

	    if (blockx>=0 && blockx < bmapwidth
		&& blocky>=0 && blocky <bmapheight)
	    {
		blocklinks[blocky*bmapwidth+blockx] = thing->bnext;
	    }
	}
    }
}


//
// P_SetThingPosition
// Links a thing into both a block and a subsector
// based on it's x y.
// Sets thing->subsector properly
//
void
P_SetThingPosition (mobj_t* thing)
{
    subsector_t*	ss;
    sector_t*		sec;
    int			blockx;
    int			blocky;
    mobj_t**		link;

    
    // link into subsector
    ss = R_PointInSubsector (thing->x,thing->y);
    thing->subsector = ss;
    
    if ( ! (thing->flags & MF_NOSECTOR) )
    {
	// invisible things don't go into the sector links
	sec = ss->sector;
	
	thing->sprev = NULL;
	thing->snext = sec->thinglist;

	if (sec->thinglist)
	    sec->thinglist->sprev = thing;

	sec->thinglist = thing;
    }

    
    // link into blockmap
    if ( ! (thing->flags & MF_NOBLOCKMAP) )
    {
	// inert things don't need to be in blockmap		
	blockx = (thing->x - bmaporgx)>>MAPBLOCKSHIFT;
	blocky = (thing->y - bmaporgy)>>MAPBLOCKSHIFT;

	if (blockx>=0
	    && blockx < bmapwidth
	    && blocky>=0
	    && blocky < bmapheight)
	{
	    link = &blocklinks[blocky*bmapwidth+blockx];
	    thing->bprev = NULL;
	    thing->bnext = *link;
	    if (*link)
		(*link)->bprev = thing;

	    *link = thing;
	}
	else
	{
	    // thing is off the map
	    thing->bnext = thing->bprev = NULL;
	}
    }
}



//
// BLOCK MAP ITERATORS
// For each line/thing in the given mapblock,
// call the passed PIT_* function.
// If the function returns false,
// exit with false without checking anything else.
//


//
// P_BlockLinesIterator
// The validcount flags are used to avoid checking lines
// that are marked in multiple mapblocks,
// so increment validcount before the first call
// to P_BlockLinesIterator, then make one or more calls
// to it.
//
boolean
P_BlockLinesIterator
( int			x,
  int			y,
  boolean(*func)(line_t*) )
{
    int			offset;
    short*		list;
    line_t*		ld;
	
    if (x<0
	|| y<0
	|| x>=bmapwidth
	|| y>=bmapheight)
    {
	return true;
    }
    
    offset = y*bmapwidth+x;
	
    offset = *(blockmap+offset);

    for ( list = blockmaplump+offset ; *list != -1 ; list++)
    {
	ld = &lines[*list];

	if (ld->validcount == validcount)
	    continue; 	// line has already been checked

	ld->validcount = validcount;
		
	if ( !func(ld) )
	    return false;
    }
    return true;	// everything was checked
}


//
// P_BlockThingsIterator
//
boolean
P_BlockThingsIterator
( int			x,
  int			y,
  boolean(*func)(mobj_t*) )
{
    mobj_t*		mobj;
	
    if ( x<0
	 || y<0
	 || x>=bmapwidth
	 || y>=bmapheight)
    {
	return true;
    }
    

    for (mobj = blocklinks[y*bmapwidth+x] ;
	 mobj ;
	 mobj = mobj->bnext)
    {
	if (!func( mobj ) )
	    return false;
    }
    return true;
}



//
// INTERCEPT ROUTINES
//
intercept_t	intercepts[INT_MAXERCEPTS];
intercept_t*	intercept_p;

divline_t 	trace;
boolean 	earlyout;
int		ptflags;

//
// PIT_AddLineIntercepts.
// Looks for lines in the given block
// that intercept the given trace
// to add to the intercepts list.
//
// A line is crossed if its endpoints
// are on opposite sides of the trace.
// Returns true if earlyout and a solid line hit.
//
boolean
PIT_AddLineIntercepts (line_t* ld)
{
    int			s1;
    int			s2;
    fixed_t		frac;
    divline_t		dl;
	
    // avoid precision problems with two routines
    if ( trace.dx > FRACUNIT*16
	 || trace.dy > FRACUNIT*16
	 || trace.dx < -FRACUNIT*16
	 || trace.dy < -FRACUNIT*16)
    {
	s1 = P_PointOnDivlineSide (ld->v1->x, ld->v1->y, &trace);
	s2 = P_PointOnDivlineSide (ld->v2->x, ld->v2->y, &trace);
    }
    else
    {
	s1 = P_PointOnLineSide (trace.x, trace.y, ld);
	s2 = P_PointOnLineSide (trace.x+trace.dx, trace.y+trace.dy, ld);
    }
    
    if (s1 == s2)
	return true;	// line isn't crossed
    
    // hit the line
    P_MakeDivline (ld, &dl);
    frac = P_InterceptVector (&trace, &dl);

    if (frac < 0)
	return true;	// behind source
	
    // try to early out the check
    if (earlyout
	&& frac < FRACUNIT
	&& !ld->backsector)
    {
	return false;	// stop checking
    }
    
	
    intercept_p->frac = frac;
    intercept_p->isaline = true;
    intercept_p->d.line = ld;
    intercept_p++;

    return true;	// continue
}



//
// PIT_AddThingIntercepts
//
boolean PIT_AddThingIntercepts (mobj_t* thing)
{
    fixed_t		x1;
    fixed_t		y1;
    fixed_t		x2;
    fixed_t		y2;
    
    int			s1;
    int			s2;
    
    boolean		tracepositive;

    divline_t		dl;
    
    fixed_t		frac;
	
    tracepositive = (trace.dx ^ trace.dy)>0;
		
    // check a corner to corner crossection for hit
    if (tracepositive)
    {
	x1 = thing->x - thing->radius;
	y1 = thing->y + thing->radius;
		
	x2 = thing->x + thing->radius;
	y2 = thing->y - thing->radius;			
    }
    else
    {
	x1 = thing->x - thing->radius;
	y1 = thing->y - thing->radius;
		
	x2 = thing->x + thing->radius;
	y2 = thing->y + thing->radius;			
    }
    
    s1 = P_PointOnDivlineSide (x1, y1, &trace);
    s2 = P_PointOnDivlineSide (x2, y2, &trace);

    if (s1 == s2)
	return true;		// line isn't crossed
	
    dl.x = x1;
    dl.y = y1;
    dl.dx = x2-x1;
    dl.dy = y2-y1;
    
    frac = P_InterceptVector (&trace, &dl);

    if (frac < 0)
	return true;		// behind source

    intercept_p->frac = frac;
    intercept_p->isaline = false;
    intercept_p->d.thing = thing;
    intercept_p++;

    return true;		// keep going
}


//
// P_TraverseIntercepts
// Returns true if the traverser function returns true
// for all lines.
// 
boolean
P_TraverseIntercepts
( traverser_t	func,
  fixed_t	maxfrac )
{
    int			count;
    fixed_t		dist;
    intercept_t*	scan;
    intercept_t*	in;
	
    count = intercept_p - intercepts;
    
    in = 0;			// shut up compiler warning
	
    while (count--)
    {
	dist = INT_MAX;
	for (scan = intercepts ; scan<intercept_p ; scan++)
	{
	    if (scan->frac < dist)
	    {
		dist = scan->frac;
		in = scan;
	    }
	}
	
	if (dist > maxfrac)
	    return true;	// checked everything in range		

#if 0  // UNUSED
    {
	// don't check these yet, there may be others inserted
	in = scan = intercepts;
	for ( scan = intercepts ; scan<intercept_p ; scan++)
	    if (scan->frac > maxfrac)
		*in++ = *scan;
	intercept_p = in;
	return false;
    }
#endif

        if ( !func (in) )
	    return false;	// don't bother going farther

	in->frac = INT_MAX;
    }
	
    return true;		// everything was traversed
}




//
// P_PathTraverse
// Traces a line from x1,y1 to x2,y2,
// calling the traverser function for each.
// Returns true if the traverser function returns true
// for all lines.
//
boolean
P_PathTraverse
( fixed_t		x1,
  fixed_t		y1,
  fixed_t		x2,
  fixed_t		y2,
  int			flags,
  boolean (*trav) (intercept_t *))
{
    fixed_t	xt1;
    fixed_t	yt1;
    fixed_t	xt2;
    fixed_t	yt2;
    
    fixed_t	xstep;
    fixed_t	ystep;
    
    fixed_t	partial;
    
    fixed_t	xintercept;
    fixed_t	yintercept;
    
    int		mapx;
    int		mapy;
    
    int		mapxstep;
    int		mapystep;

    int		count;
		
    earlyout = flags & PT_EARLYOUT;
		
    validcount++;
    intercept_p = intercepts;
	
    if ( ((x1-bmaporgx)&(MAPBLOCKSIZE-1)) == 0)
	x1 += FRACUNIT;	// don't side exactly on a line
    
    if ( ((y1-bmaporgy)&(MAPBLOCKSIZE-1)) == 0)
	y1 += FRACUNIT;	// don't side exactly on a line

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

    x1 -= bmaporgx;
    y1 -= bmaporgy;
    xt1 = x1>>MAPBLOCKSHIFT;
    yt1 = y1>>MAPBLOCKSHIFT;

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

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

    yintercept = (y1>>MAPBTOFRAC) + FixedMul (partial, ystep);

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