gtk2/gdk/gdkregion-generic.c

/* $TOG: Region.c /main/31 1998/02/06 17:50:22 kaleb $ */
/************************************************************************

Copyright 1987, 1988, 1998  The Open Group

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Copyright 1987, 1988 by Digital Equipment Corporation, Maynard, Massachusetts.

                        All Rights Reserved

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************************************************************************/
/* $XFree86: xc/lib/X11/Region.c,v 1.5 1999/05/09 10:50:01 dawes Exp $ */
/*
 * The functions in this file implement the Region abstraction, similar to one
 * used in the X11 sample server. A Region is simply an area, as the name
 * implies, and is implemented as a "y-x-banded" array of rectangles. To
 * explain: Each Region is made up of a certain number of rectangles sorted
 * by y coordinate first, and then by x coordinate.
 *
 * Furthermore, the rectangles are banded such that every rectangle with a
 * given upper-left y coordinate (y1) will have the same lower-right y
 * coordinate (y2) and vice versa. If a rectangle has scanlines in a band, it
 * will span the entire vertical distance of the band. This means that some
 * areas that could be merged into a taller rectangle will be represented as
 * several shorter rectangles to account for shorter rectangles to its left
 * or right but within its "vertical scope".
 *
 * An added constraint on the rectangles is that they must cover as much
 * horizontal area as possible. E.g. no two rectangles in a band are allowed
 * to touch.
 *
 * Whenever possible, bands will be merged together to cover a greater vertical
 * distance (and thus reduce the number of rectangles). Two bands can be merged
 * only if the bottom of one touches the top of the other and they have
 * rectangles in the same places (of the same width, of course). This maintains
 * the y-x-banding that's so nice to have...
 */

#include <config.h>
#include <stdlib.h>
#include <string.h>
#include <gdkregion.h>
#include "gdkregion-generic.h"
#include "gdkalias.h"

typedef void (*overlapFunc) (GdkRegion    *pReg,
			     GdkRegionBox *r1,
			     GdkRegionBox *r1End,
			     GdkRegionBox *r2,
			     GdkRegionBox *r2End,
			     gint          y1,
			     gint          y2);
typedef void (*nonOverlapFunc) (GdkRegion    *pReg,
				GdkRegionBox *r,
				GdkRegionBox *rEnd,
				gint          y1,
				gint          y2);

static void miRegionCopy (GdkRegion      *dstrgn,
			  GdkRegion      *rgn);
static void miRegionOp   (GdkRegion      *newReg,
			  GdkRegion      *reg1,
			  GdkRegion      *reg2,
			  overlapFunc     overlapFn,
			  nonOverlapFunc  nonOverlap1Fn,
			  nonOverlapFunc  nonOverlap2Fn);

/**
 * gdk_region_new:
 *
 * Creates a new empty #GdkRegion.
 *
 * Returns: a new empty #GdkRegion
 */
GdkRegion *
gdk_region_new ()
{
  GdkRegion *temp;

  temp = g_slice_new (GdkRegion);

  temp->numRects = 0;
  temp->rects = &temp->extents;
  temp->extents.x1 = 0;
  temp->extents.y1 = 0;
  temp->extents.x2 = 0;
  temp->extents.y2 = 0;
  temp->size = 1;
  
  return temp;
}

/**
 * gdk_region_rectangle:
 * @rectangle: a #GdkRectangle
 * 
 * Creates a new region containing the area @rectangle.
 * 
 * Return value: a new region
 **/
GdkRegion *
gdk_region_rectangle (GdkRectangle *rectangle)
{
  GdkRegion *temp;

  g_return_val_if_fail (rectangle != NULL, NULL);

  if (rectangle->width <= 0 || rectangle->height <= 0)
    return gdk_region_new();

  temp = g_slice_new (GdkRegion);

  temp->numRects = 1;
  temp->rects = &temp->extents;
  temp->extents.x1 = rectangle->x;
  temp->extents.y1 = rectangle->y;
  temp->extents.x2 = rectangle->x + rectangle->width;
  temp->extents.y2 = rectangle->y + rectangle->height;
  temp->size = 1;
  
  return temp;
}

/**
 * gdk_region_copy:
 * @region: a #GdkRegion
 * 
 * Copies @region, creating an identical new region.
 * 
 * Return value: a new region identical to @region
 **/
GdkRegion *
gdk_region_copy (GdkRegion *region)
{
  GdkRegion *temp;

  g_return_val_if_fail (region != NULL, NULL);

  temp = gdk_region_new ();

  miRegionCopy (temp, region);

  return temp;
}

/**
 * gdk_region_get_clipbox:
 * @region: a #GdkRegion
 * @rectangle: return location for the clipbox
 *
 * Obtains the smallest rectangle which includes the entire #GdkRegion.
 *
 */
void
gdk_region_get_clipbox (GdkRegion    *region, 
			GdkRectangle *rectangle)
{
  g_return_if_fail (region != NULL);
  g_return_if_fail (rectangle != NULL);
  
  rectangle->x = region->extents.x1;
  rectangle->y = region->extents.y1;
  rectangle->width = region->extents.x2 - region->extents.x1;
  rectangle->height = region->extents.y2 - region->extents.y1;
}


/**
 * gdk_region_get_rectangles:
 * @region: a #GdkRegion
 * @rectangles: return location for an array of rectangles
 * @n_rectangles: length of returned array
 *
 * Obtains the area covered by the region as a list of rectangles.
 * The array returned in @rectangles must be freed with g_free().
 **/
void
gdk_region_get_rectangles (GdkRegion     *region,
                           GdkRectangle **rectangles,
                           gint          *n_rectangles)
{
  gint i;
  
  g_return_if_fail (region != NULL);
  g_return_if_fail (rectangles != NULL);
  g_return_if_fail (n_rectangles != NULL);
  
  *n_rectangles = region->numRects;
  *rectangles = g_new (GdkRectangle, region->numRects);

  for (i = 0; i < region->numRects; i++)
    {
      GdkRegionBox rect;
      rect = region->rects[i];
      (*rectangles)[i].x = rect.x1;
      (*rectangles)[i].y = rect.y1;
      (*rectangles)[i].width = rect.x2 - rect.x1;
      (*rectangles)[i].height = rect.y2 - rect.y1;
    }
}

/**
 * gdk_region_union_with_rect:
 * @region: a #GdkRegion.
 * @rect: a #GdkRectangle.
 * 
 * Sets the area of @region to the union of the areas of @region and
 * @rect. The resulting area is the set of pixels contained in
 * either @region or @rect.
 **/
void
gdk_region_union_with_rect (GdkRegion    *region,
			    GdkRectangle *rect)
{
  GdkRegion tmp_region;

  g_return_if_fail (region != NULL);
  g_return_if_fail (rect != NULL);

  if (rect->width <= 0 || rect->height <= 0)
    return;
    
  tmp_region.rects = &tmp_region.extents;
  tmp_region.numRects = 1;
  tmp_region.extents.x1 = rect->x;
  tmp_region.extents.y1 = rect->y;
  tmp_region.extents.x2 = rect->x + rect->width;
  tmp_region.extents.y2 = rect->y + rect->height;
  tmp_region.size = 1;

  gdk_region_union (region, &tmp_region);
}

/*-
 *-----------------------------------------------------------------------
 * miSetExtents --
 *	Reset the extents of a region to what they should be. Called by
 *	miSubtract and miIntersect b/c they can't figure it out along the
 *	way or do so easily, as miUnion can.
 *
 * Results:
 *	None.
 *
 * Side Effects:
 *	The region's 'extents' structure is overwritten.
 *
 *-----------------------------------------------------------------------
 */
static void
miSetExtents (GdkRegion *pReg)
{
  GdkRegionBox *pBox, *pBoxEnd, *pExtents;

  if (pReg->numRects == 0)
    {
      pReg->extents.x1 = 0;
      pReg->extents.y1 = 0;
      pReg->extents.x2 = 0;
      pReg->extents.y2 = 0;
      return;
    }
  
  pExtents = &pReg->extents;
  pBox = pReg->rects;
  pBoxEnd = &pBox[pReg->numRects - 1];

    /*
     * Since pBox is the first rectangle in the region, it must have the
     * smallest y1 and since pBoxEnd is the last rectangle in the region,
     * it must have the largest y2, because of banding. Initialize x1 and
     * x2 from  pBox and pBoxEnd, resp., as good things to initialize them
     * to...
     */
  pExtents->x1 = pBox->x1;
  pExtents->y1 = pBox->y1;
  pExtents->x2 = pBoxEnd->x2;
  pExtents->y2 = pBoxEnd->y2;

  g_assert(pExtents->y1 < pExtents->y2);
  while (pBox <= pBoxEnd)
    {
      if (pBox->x1 < pExtents->x1)
	{
	  pExtents->x1 = pBox->x1;
	}
      if (pBox->x2 > pExtents->x2)
	{
	  pExtents->x2 = pBox->x2;
	}
      pBox++;
    }
  g_assert(pExtents->x1 < pExtents->x2);
}

/**
 * gdk_region_destroy:
 * @region: a #GdkRegion
 *
 * Destroys a #GdkRegion.
 */
void
gdk_region_destroy (GdkRegion *region)
{
  g_return_if_fail (region != NULL);

  if (region->rects != &region->extents)
    g_free (region->rects);
  g_slice_free (GdkRegion, region);
}


/**
 * gdk_region_offset:
 * @region: a #GdkRegion
 * @dx: the distance to move the region horizontally
 * @dy: the distance to move the region vertically
 *
 * Moves a region the specified distance.
 */
void
gdk_region_offset (GdkRegion *region,
		   gint       x,
		   gint       y)
{
  int nbox;
  GdkRegionBox *pbox;

  g_return_if_fail (region != NULL);

  pbox = region->rects;
  nbox = region->numRects;

  while(nbox--)
    {
      pbox->x1 += x;
      pbox->x2 += x;
      pbox->y1 += y;
      pbox->y2 += y;
      pbox++;
    }
  if (region->rects != &region->extents)
    {
      region->extents.x1 += x;
      region->extents.x2 += x;
      region->extents.y1 += y;
      region->extents.y2 += y;
    }
}

/* 
   Utility procedure Compress:
   Replace r by the region r', where 
     p in r' iff (Quantifer m <= dx) (p + m in r), and
     Quantifier is Exists if grow is TRUE, For all if grow is FALSE, and
     (x,y) + m = (x+m,y) if xdir is TRUE; (x,y+m) if xdir is FALSE.

   Thus, if xdir is TRUE and grow is FALSE, r is replaced by the region
   of all points p such that p and the next dx points on the same
   horizontal scan line are all in r.  We do this using by noting
   that p is the head of a run of length 2^i + k iff p is the head
   of a run of length 2^i and p+2^i is the head of a run of length
   k. Thus, the loop invariant: s contains the region corresponding
   to the runs of length shift.  r contains the region corresponding
   to the runs of length 1 + dxo & (shift-1), where dxo is the original
   value of dx.  dx = dxo & ~(shift-1).  As parameters, s and t are
   scratch regions, so that we don't have to allocate them on every
   call.
*/

#define ZOpRegion(a,b) if (grow) gdk_region_union (a, b); \
			 else gdk_region_intersect (a,b)
#define ZShiftRegion(a,b) if (xdir) gdk_region_offset (a,b,0); \
			  else gdk_region_offset (a,0,b)

static void
Compress(GdkRegion *r,
	 GdkRegion *s,
	 GdkRegion *t,
	 guint      dx,
	 int        xdir,
	 int        grow)
{
  guint shift = 1;

  miRegionCopy (s, r);
  while (dx)
    {
      if (dx & shift)
	{
	  ZShiftRegion(r, -(int)shift);
	  ZOpRegion(r, s);
	  dx -= shift;
	  if (!dx) break;
        }
      miRegionCopy (t, s);
      ZShiftRegion(s, -(int)shift);
      ZOpRegion(s, t);
      shift <<= 1;
    }
}

#undef ZOpRegion
#undef ZShiftRegion
#undef ZCopyRegion

/**
 * gdk_region_shrink:
 * @region: a #GdkRegion
 * @dx: the number of pixels to shrink the region horizontally
 * @dy: the number of pixels to shrink the region vertically
 *
 * Resizes a region by the specified amount.
 * Positive values shrink the region. Negative values expand it.
 */
void
gdk_region_shrink (GdkRegion *region,
		   int        dx,
		   int        dy)
{
  GdkRegion *s, *t;
  int grow;

  g_return_if_fail (region != NULL);

  if (!dx && !dy)
    return;

  s = gdk_region_new ();
  t = gdk_region_new ();

  grow = (dx < 0);
  if (grow)
    dx = -dx;
  if (dx)
     Compress(region, s, t, (unsigned) 2*dx, TRUE, grow);
     
  grow = (dy < 0);
  if (grow)
    dy = -dy;
  if (dy)
     Compress(region, s, t, (unsigned) 2*dy, FALSE, grow);
  
  gdk_region_offset (region, dx, dy);
  gdk_region_destroy (s);
  gdk_region_destroy (t);
}


/*======================================================================
 *	    Region Intersection
 *====================================================================*/
/*-
 *-----------------------------------------------------------------------
 * miIntersectO --
 *	Handle an overlapping band for miIntersect.
 *
 * Results:
 *	None.
 *
 * Side Effects:
 *	Rectangles may be added to the region.
 *
 *-----------------------------------------------------------------------
 */
/* static void*/
static void
miIntersectO (GdkRegion    *pReg,
	      GdkRegionBox *r1,
	      GdkRegionBox *r1End,
	      GdkRegionBox *r2,
	      GdkRegionBox *r2End,
	      gint          y1,
	      gint          y2)
{
  int  	x1;
  int  	x2;
  GdkRegionBox *pNextRect;

  pNextRect = &pReg->rects[pReg->numRects];

  while ((r1 != r1End) && (r2 != r2End))
    {
      x1 = MAX (r1->x1,r2->x1);
      x2 = MIN (r1->x2,r2->x2);

      /*
       * If there's any overlap between the two rectangles, add that
       * overlap to the new region.
       * There's no need to check for subsumption because the only way
       * such a need could arise is if some region has two rectangles
       * right next to each other. Since that should never happen...
       */
      if (x1 < x2)
	{
	  g_assert (y1<y2);

	  MEMCHECK (pReg, pNextRect, pReg->rects);
	  pNextRect->x1 = x1;
	  pNextRect->y1 = y1;
	  pNextRect->x2 = x2;
	  pNextRect->y2 = y2;
	  pReg->numRects += 1;
	  pNextRect++;
	  g_assert (pReg->numRects <= pReg->size);
	}

      /*
       * Need to advance the pointers. Shift the one that extends
       * to the right the least, since the other still has a chance to
       * overlap with that region's next rectangle, if you see what I mean.
       */
      if (r1->x2 < r2->x2)
	{
	  r1++;
	}
      else if (r2->x2 < r1->x2)
	{
	  r2++;
	}
      else
	{
	  r1++;
	  r2++;
	}
    }
}

/**
 * gdk_region_intersect:
 * @source1: a #GdkRegion
 * @source2: another #GdkRegion
 *
 * Sets the area of @source1 to the intersection of the areas of @source1
 * and @source2. The resulting area is the set of pixels contained in
 * both @source1 and @source2.
 **/
void
gdk_region_intersect (GdkRegion *source1,
		      GdkRegion *source2)
{
  g_return_if_fail (source1 != NULL);
  g_return_if_fail (source2 != NULL);
  
  /* check for trivial reject */
  if ((!(source1->numRects)) || (!(source2->numRects))  ||
      (!EXTENTCHECK(&source1->extents, &source2->extents)))
    source1->numRects = 0;
  else
    miRegionOp (source1, source1, source2, 
    		miIntersectO, (nonOverlapFunc) NULL, (nonOverlapFunc) NULL);
    
  /*
   * Can't alter source1's extents before miRegionOp depends on the
   * extents of the regions being unchanged. Besides, this way there's
   * no checking against rectangles that will be nuked due to
   * coalescing, so we have to examine fewer rectangles.
   */
  miSetExtents(source1);
}

static void
miRegionCopy (GdkRegion *dstrgn, 
	      GdkRegion *rgn)
{
  if (dstrgn != rgn) /*  don't want to copy to itself */
    {  
      if (dstrgn->size < rgn->numRects)
        {
	  if (dstrgn->rects != &dstrgn->extents)
	    g_free (dstrgn->rects);

	  dstrgn->rects = g_new (GdkRegionBox, rgn->numRects);
	  dstrgn->size = rgn->numRects;
	}

      dstrgn->numRects = rgn->numRects;
      dstrgn->extents = rgn->extents;

      memcpy (dstrgn->rects, rgn->rects, rgn->numRects * sizeof (GdkRegionBox));
    }
}


/*======================================================================
 *	    Generic Region Operator
 *====================================================================*/

/*-
 *-----------------------------------------------------------------------
 * miCoalesce --
 *	Attempt to merge the boxes in the current band with those in the
 *	previous one. Used only by miRegionOp.
 *
 * Results:
 *	The new index for the previous band.
 *
 * Side Effects:
 *	If coalescing takes place:
 *	    - rectangles in the previous band will have their y2 fields
 *	      altered.
 *	    - pReg->numRects will be decreased.
 *
 *-----------------------------------------------------------------------
 */
/* static int*/
static int
miCoalesce (GdkRegion *pReg,         /* Region to coalesce */
	    gint       prevStart,    /* Index of start of previous band */
	    gint       curStart)     /* Index of start of current band */
{
  GdkRegionBox *pPrevBox;   	/* Current box in previous band */
  GdkRegionBox *pCurBox;    	/* Current box in current band */
  GdkRegionBox *pRegEnd;    	/* End of region */
  int	    	curNumRects;	/* Number of rectangles in current
				 * band */
  int	    	prevNumRects;	/* Number of rectangles in previous
				 * band */
  int	    	bandY1;	    	/* Y1 coordinate for current band */

  pRegEnd = &pReg->rects[pReg->numRects];

  pPrevBox = &pReg->rects[prevStart];
  prevNumRects = curStart - prevStart;

    /*
     * Figure out how many rectangles are in the current band. Have to do
     * this because multiple bands could have been added in miRegionOp
     * at the end when one region has been exhausted.
     */
  pCurBox = &pReg->rects[curStart];
  bandY1 = pCurBox->y1;
  for (curNumRects = 0;
       (pCurBox != pRegEnd) && (pCurBox->y1 == bandY1);
       curNumRects++)
    {
      pCurBox++;
    }
    
  if (pCurBox != pRegEnd)
    {
      /*
       * If more than one band was added, we have to find the start
       * of the last band added so the next coalescing job can start
       * at the right place... (given when multiple bands are added,
       * this may be pointless -- see above).
       */
      pRegEnd--;
      while (pRegEnd[-1].y1 == pRegEnd->y1)
	{
	  pRegEnd--;
	}
      curStart = pRegEnd - pReg->rects;
      pRegEnd = pReg->rects + pReg->numRects;
    }
	
  if ((curNumRects == prevNumRects) && (curNumRects != 0)) {
    pCurBox -= curNumRects;
    /*
     * The bands may only be coalesced if the bottom of the previous
     * matches the top scanline of the current.
     */
    if (pPrevBox->y2 == pCurBox->y1)
      {
	/*
	 * Make sure the bands have boxes in the same places. This
	 * assumes that boxes have been added in such a way that they
	 * cover the most area possible. I.e. two boxes in a band must
	 * have some horizontal space between them.
	 */
	do
	  {
	    if ((pPrevBox->x1 != pCurBox->x1) ||
		(pPrevBox->x2 != pCurBox->x2))
	      {
		/*
		 * The bands don't line up so they can't be coalesced.
		 */
		return (curStart);
	      }
	    pPrevBox++;
	    pCurBox++;
	    prevNumRects -= 1;
	  } while (prevNumRects != 0);

	pReg->numRects -= curNumRects;
	pCurBox -= curNumRects;
	pPrevBox -= curNumRects;

	/*
	 * The bands may be merged, so set the bottom y of each box
	 * in the previous band to that of the corresponding box in
	 * the current band.
	 */
	do
	  {
	    pPrevBox->y2 = pCurBox->y2;
	    pPrevBox++;
	    pCurBox++;
	    curNumRects -= 1;
	  }
	while (curNumRects != 0);

	/*
	 * If only one band was added to the region, we have to backup
	 * curStart to the start of the previous band.
	 *
	 * If more than one band was added to the region, copy the
	 * other bands down. The assumption here is that the other bands
	 * came from the same region as the current one and no further
	 * coalescing can be done on them since it's all been done
	 * already... curStart is already in the right place.
	 */
	if (pCurBox == pRegEnd)
	  {
	    curStart = prevStart;
	  }
	else
	  {
	    do
	      {
		*pPrevBox++ = *pCurBox++;
	      }
	    while (pCurBox != pRegEnd);
	  }
	    
      }
  }
  return curStart;
}

/*-
 *-----------------------------------------------------------------------
 * miRegionOp --
 *	Apply an operation to two regions. Called by miUnion, miInverse,
 *	miSubtract, miIntersect...
 *
 * Results:
 *	None.
 *
 * Side Effects:
 *	The new region is overwritten.
 *
 * Notes:
 *	The idea behind this function is to view the two regions as sets.
 *	Together they cover a rectangle of area that this function divides
 *	into horizontal bands where points are covered only by one region
 *	or by both. For the first case, the nonOverlapFunc is called with
 *	each the band and the band's upper and lower extents. For the
 *	second, the overlapFunc is called to process the entire band. It
 *	is responsible for clipping the rectangles in the band, though
 *	this function provides the boundaries.
 *	At the end of each band, the new region is coalesced, if possible,
 *	to reduce the number of rectangles in the region.
 *
 *-----------------------------------------------------------------------
 */
/* static void*/
static void
miRegionOp(GdkRegion *newReg,
	   GdkRegion *reg1,
	   GdkRegion *reg2,
	   overlapFunc    overlapFn,   	        /* Function to call for over-
						 * lapping bands */
	   nonOverlapFunc nonOverlap1Fn,	/* Function to call for non-
						 * overlapping bands in region
						 * 1 */
	   nonOverlapFunc nonOverlap2Fn)	/* Function to call for non-
						 * overlapping bands in region
						 * 2 */
{
    GdkRegionBox *r1; 	    	    	/* Pointer into first region */
    GdkRegionBox *r2; 	    	    	/* Pointer into 2d region */
    GdkRegionBox *r1End;    	    	/* End of 1st region */
    GdkRegionBox *r2End;    	    	/* End of 2d region */
    int    	  ybot;	    	    	/* Bottom of intersection */
    int  	  ytop;	    	    	/* Top of intersection */
    GdkRegionBox *oldRects;   	    	/* Old rects for newReg */
    int	    	  prevBand;   	    	/* Index of start of
					 * previous band in newReg */
    int	  	  curBand;    	    	/* Index of start of current
					 * band in newReg */
    GdkRegionBox *r1BandEnd;  	    	/* End of current band in r1 */
    GdkRegionBox *r2BandEnd;  	    	/* End of current band in r2 */
    int     	  top;	    	    	/* Top of non-overlapping
					 * band */
    int     	  bot;	    	    	/* Bottom of non-overlapping
					 * band */
    
    /*
     * Initialization:
     *	set r1, r2, r1End and r2End appropriately, preserve the important
     * parts of the destination region until the end in case it's one of
     * the two source regions, then mark the "new" region empty, allocating
     * another array of rectangles for it to use.
     */
    r1 = reg1->rects;
    r2 = reg2->rects;
    r1End = r1 + reg1->numRects;
    r2End = r2 + reg2->numRects;
    
    oldRects = newReg->rects;
    
    EMPTY_REGION(newReg);

    /*
     * Allocate a reasonable number of rectangles for the new region. The idea
     * is to allocate enough so the individual functions don't need to
     * reallocate and copy the array, which is time consuming, yet we don't
     * have to worry about using too much memory. I hope to be able to
     * nuke the Xrealloc() at the end of this function eventually.
     */
    newReg->size = MAX (reg1->numRects, reg2->numRects) * 2;
    newReg->rects = g_new (GdkRegionBox, newReg->size);
    
    /*
     * Initialize ybot and ytop.
     * In the upcoming loop, ybot and ytop serve different functions depending
     * on whether the band being handled is an overlapping or non-overlapping
     * band.
     * 	In the case of a non-overlapping band (only one of the regions
     * has points in the band), ybot is the bottom of the most recent
     * intersection and thus clips the top of the rectangles in that band.
     * ytop is the top of the next intersection between the two regions and
     * serves to clip the bottom of the rectangles in the current band.
     *	For an overlapping band (where the two regions intersect), ytop clips
     * the top of the rectangles of both regions and ybot clips the bottoms.
     */
    if (reg1->extents.y1 < reg2->extents.y1)
      ybot = reg1->extents.y1;
    else
      ybot = reg2->extents.y1;
    
    /*
     * prevBand serves to mark the start of the previous band so rectangles
     * can be coalesced into larger rectangles. qv. miCoalesce, above.
     * In the beginning, there is no previous band, so prevBand == curBand
     * (curBand is set later on, of course, but the first band will always
     * start at index 0). prevBand and curBand must be indices because of
     * the possible expansion, and resultant moving, of the new region's
     * array of rectangles.
     */
    prevBand = 0;
    
    do
      {
	curBand = newReg->numRects;

	/*
	 * This algorithm proceeds one source-band (as opposed to a
	 * destination band, which is determined by where the two regions
	 * intersect) at a time. r1BandEnd and r2BandEnd serve to mark the
	 * rectangle after the last one in the current band for their
	 * respective regions.
	 */
	r1BandEnd = r1;
	while ((r1BandEnd != r1End) && (r1BandEnd->y1 == r1->y1))
	  {
	    r1BandEnd++;
	  }
	
	r2BandEnd = r2;
	while ((r2BandEnd != r2End) && (r2BandEnd->y1 == r2->y1))
	  {
	    r2BandEnd++;
	  }
	
	/*
	 * First handle the band that doesn't intersect, if any.
	 *
	 * Note that attention is restricted to one band in the
	 * non-intersecting region at once, so if a region has n
	 * bands between the current position and the next place it overlaps
	 * the other, this entire loop will be passed through n times.
	 */
	if (r1->y1 < r2->y1)
	  {
	    top = MAX (r1->y1,ybot);
	    bot = MIN (r1->y2,r2->y1);

	    if ((top != bot) && (nonOverlap1Fn != (void (*)())NULL))
	      {
		(* nonOverlap1Fn) (newReg, r1, r1BandEnd, top, bot);
	      }

	    ytop = r2->y1;
	  }
	else if (r2->y1 < r1->y1)
	  {
	    top = MAX (r2->y1,ybot);
	    bot = MIN (r2->y2,r1->y1);

	    if ((top != bot) && (nonOverlap2Fn != (void (*)())NULL))
	      {
		(* nonOverlap2Fn) (newReg, r2, r2BandEnd, top, bot);
	      }

	    ytop = r1->y1;
	  }
	else
	  {
	    ytop = r1->y1;
	  }

	/*
	 * If any rectangles got added to the region, try and coalesce them
	 * with rectangles from the previous band. Note we could just do
	 * this test in miCoalesce, but some machines incur a not
	 * inconsiderable cost for function calls, so...
	 */
	if (newReg->numRects != curBand)
	  {
	    prevBand = miCoalesce (newReg, prevBand, curBand);
	  }

	/*
	 * Now see if we've hit an intersecting band. The two bands only
	 * intersect if ybot > ytop
	 */
	ybot = MIN (r1->y2, r2->y2);
	curBand = newReg->numRects;
	if (ybot > ytop)
	  {
	    (* overlapFn) (newReg, r1, r1BandEnd, r2, r2BandEnd, ytop, ybot);

	  }
	
	if (newReg->numRects != curBand)
	  {
	    prevBand = miCoalesce (newReg, prevBand, curBand);
	  }

	/*
	 * If we've finished with a band (y2 == ybot) we skip forward
	 * in the region to the next band.
	 */
	if (r1->y2 == ybot)
	  {
	    r1 = r1BandEnd;
	  }
	if (r2->y2 == ybot)
	  {
	    r2 = r2BandEnd;
	  }
      } while ((r1 != r1End) && (r2 != r2End));

    /*
     * Deal with whichever region still has rectangles left.
     */
    curBand = newReg->numRects;
    if (r1 != r1End)
      {
	if (nonOverlap1Fn != (nonOverlapFunc )NULL)
	  {
	    do
	      {
		r1BandEnd = r1;
		while ((r1BandEnd < r1End) && (r1BandEnd->y1 == r1->y1))
		  {
		    r1BandEnd++;
		  }
		(* nonOverlap1Fn) (newReg, r1, r1BandEnd,
				     MAX (r1->y1,ybot), r1->y2);
		r1 = r1BandEnd;
	      } while (r1 != r1End);
	  }
      }
    else if ((r2 != r2End) && (nonOverlap2Fn != (nonOverlapFunc) NULL))
      {
	do
	  {
	    r2BandEnd = r2;
	    while ((r2BandEnd < r2End) && (r2BandEnd->y1 == r2->y1))
	      {
		r2BandEnd++;
	      }
	    (* nonOverlap2Fn) (newReg, r2, r2BandEnd,
			       MAX (r2->y1,ybot), r2->y2);
	    r2 = r2BandEnd;
	  } while (r2 != r2End);
      }

    if (newReg->numRects != curBand)
    {
      (void) miCoalesce (newReg, prevBand, curBand);
    }

    /*
     * A bit of cleanup. To keep regions from growing without bound,
     * we shrink the array of rectangles to match the new number of
     * rectangles in the region. This never goes to 0, however...
     *
     * Only do this stuff if the number of rectangles allocated is more than
     * twice the number of rectangles in the region (a simple optimization...).
     */
    if (newReg->numRects < (newReg->size >> 1))
      {
	if (REGION_NOT_EMPTY (newReg))
	  {
	    newReg->size = newReg->numRects;
	    newReg->rects = g_renew (GdkRegionBox, newReg->rects, newReg->size);
	  }
	else
	  {
	    /*
	     * No point in doing the extra work involved in an Xrealloc if
	     * the region is empty
	     */
	    newReg->size = 1;
	    g_free (newReg->rects);
	    newReg->rects = &newReg->extents;
	  }
      }

    if (oldRects != &newReg->extents)
      g_free (oldRects);
}


/*======================================================================
 *	    Region Union
 *====================================================================*/

/*-
 *-----------------------------------------------------------------------
 * miUnionNonO --
 *	Handle a non-overlapping band for the union operation. Just
 *	Adds the rectangles into the region. Doesn't have to check for
 *	subsumption or anything.
 *
 * Results:
 *	None.
 *
 * Side Effects:
 *	pReg->numRects is incremented and the final rectangles overwritten
 *	with the rectangles we're passed.
 *
 *-----------------------------------------------------------------------
 */
static void
miUnionNonO (GdkRegion    *pReg,
	     GdkRegionBox *r,
	     GdkRegionBox *rEnd,
	     gint          y1,
	     gint          y2)
{
  GdkRegionBox *pNextRect;

  pNextRect = &pReg->rects[pReg->numRects];

  g_assert(y1 < y2);

  while (r != rEnd)
    {
      g_assert(r->x1 < r->x2);
      MEMCHECK(pReg, pNextRect, pReg->rects);
      pNextRect->x1 = r->x1;
      pNextRect->y1 = y1;
      pNextRect->x2 = r->x2;
      pNextRect->y2 = y2;
      pReg->numRects += 1;
      pNextRect++;

      g_assert(pReg->numRects<=pReg->size);
      r++;
    }
}


/*-
 *-----------------------------------------------------------------------
 * miUnionO --
 *	Handle an overlapping band for the union operation. Picks the
 *	left-most rectangle each time and merges it into the region.
 *
 * Results:
 *	None.
 *
 * Side Effects:
 *	Rectangles are overwritten in pReg->rects and pReg->numRects will
 *	be changed.
 *
 *-----------------------------------------------------------------------
 */

/* static void*/
static void
miUnionO (GdkRegion *pReg,
	  GdkRegionBox *r1,
	  GdkRegionBox *r1End,
	  GdkRegionBox *r2,
	  GdkRegionBox *r2End,
	  gint          y1,
	  gint          y2)
{
  GdkRegionBox *	pNextRect;
    
  pNextRect = &pReg->rects[pReg->numRects];

#define MERGERECT(r) 					\
    if ((pReg->numRects != 0) &&  			\
	(pNextRect[-1].y1 == y1) &&  			\
	(pNextRect[-1].y2 == y2) &&  			\
	(pNextRect[-1].x2 >= r->x1))  			\
      {  						\
	if (pNextRect[-1].x2 < r->x2)  			\
	  {  						\
	    pNextRect[-1].x2 = r->x2;  			\
	    g_assert(pNextRect[-1].x1<pNextRect[-1].x2); 	\
	  }  						\
      }  						\
    else  						\
      {  						\
	MEMCHECK(pReg, pNextRect, pReg->rects); 	\
	pNextRect->y1 = y1;  				\
	pNextRect->y2 = y2;  				\
	pNextRect->x1 = r->x1;  			\
	pNextRect->x2 = r->x2;  			\
	pReg->numRects += 1;  				\
        pNextRect += 1;  				\
      }  						\
    g_assert(pReg->numRects<=pReg->size);			\
    r++;
    
    g_assert (y1<y2);
    while ((r1 != r1End) && (r2 != r2End))
    {
	if (r1->x1 < r2->x1)
	{
	    MERGERECT(r1);
	}
	else
	{
	    MERGERECT(r2);
	}
    }
    
    if (r1 != r1End)
    {
	do
	{
	    MERGERECT(r1);
	} while (r1 != r1End);
    }
    else while (r2 != r2End)
    {
	MERGERECT(r2);
    }
}

/**
 * gdk_region_union:
 * @source1:  a #GdkRegion
 * @source2: a #GdkRegion 
 * 
 * Sets the area of @source1 to the union of the areas of @source1 and
 * @source2. The resulting area is the set of pixels contained in
 * either @source1 or @source2.
 **/
void
gdk_region_union (GdkRegion *source1,
		  GdkRegion *source2)
{
  g_return_if_fail (source1 != NULL);
  g_return_if_fail (source2 != NULL);
  
  /*  checks all the simple cases */

  /*
   * source1 and source2 are the same or source2 is empty
   */
  if ((source1 == source2) || (!(source2->numRects)))
    return;

  /* 
   * source1 is empty
   */
  if (!(source1->numRects))
    {
      miRegionCopy (source1, source2);
      return;
    }
  
  /*
   * source1 completely subsumes source2
   */
  if ((source1->numRects == 1) && 
      (source1->extents.x1 <= source2->extents.x1) &&
      (source1->extents.y1 <= source2->extents.y1) &&
      (source1->extents.x2 >= source2->extents.x2) &&
      (source1->extents.y2 >= source2->extents.y2))
    return;

  /*
   * source2 completely subsumes source1
   */
  if ((source2->numRects == 1) && 
      (source2->extents.x1 <= source1->extents.x1) &&
      (source2->extents.y1 <= source1->extents.y1) &&
      (source2->extents.x2 >= source1->extents.x2) &&
      (source2->extents.y2 >= source1->extents.y2))
    {
      miRegionCopy(source1, source2);
      return;
    }

  miRegionOp (source1, source1, source2, miUnionO, 
	      miUnionNonO, miUnionNonO);

  source1->extents.x1 = MIN (source1->extents.x1, source2->extents.x1);
  source1->extents.y1 = MIN (source1->extents.y1, source2->extents.y1);
  source1->extents.x2 = MAX (source1->extents.x2, source2->extents.x2);
  source1->extents.y2 = MAX (source1->extents.y2, source2->extents.y2);
}


/*======================================================================
 * 	    	  Region Subtraction
 *====================================================================*/

/*-
 *-----------------------------------------------------------------------
 * miSubtractNonO --
 *	Deal with non-overlapping band for subtraction. Any parts from
 *	region 2 we discard. Anything from region 1 we add to the region.
 *
 * Results:
 *	None.
 *
 * Side Effects:
 *	pReg may be affected.
 *
 *-----------------------------------------------------------------------
 */
/* static void*/
static void
miSubtractNonO1 (GdkRegion    *pReg,
		 GdkRegionBox *r,
		 GdkRegionBox *rEnd,
		 gint          y1,
		 gint          y2)
{
  GdkRegionBox *	pNextRect;
	
  pNextRect = &pReg->rects[pReg->numRects];
	
  g_assert(y1<y2);

  while (r != rEnd)
    {
      g_assert (r->x1<r->x2);
      MEMCHECK (pReg, pNextRect, pReg->rects);
      pNextRect->x1 = r->x1;
      pNextRect->y1 = y1;
      pNextRect->x2 = r->x2;
      pNextRect->y2 = y2;
      pReg->numRects += 1;
      pNextRect++;

      g_assert (pReg->numRects <= pReg->size);

      r++;
    }
}

/*-
 *-----------------------------------------------------------------------
 * miSubtractO --
 *	Overlapping band subtraction. x1 is the left-most point not yet
 *	checked.
 *
 * Results:
 *	None.
 *
 * Side Effects:
 *	pReg may have rectangles added to it.
 *
 *-----------------------------------------------------------------------
 */
/* static void*/
static void
miSubtractO (GdkRegion    *pReg,
	     GdkRegionBox *r1,
	     GdkRegionBox *r1End,
	     GdkRegionBox *r2,
	     GdkRegionBox *r2End,
	     gint          y1,
	     gint          y2)
{
  GdkRegionBox *	pNextRect;
  int  	x1;
    
  x1 = r1->x1;
    
  g_assert(y1<y2);
  pNextRect = &pReg->rects[pReg->numRects];

  while ((r1 != r1End) && (r2 != r2End))
    {
      if (r2->x2 <= x1)
	{
	  /*
	   * Subtrahend missed the boat: go to next subtrahend.
	   */
	  r2++;
	}
      else if (r2->x1 <= x1)
	{
	  /*
	   * Subtrahend preceeds minuend: nuke left edge of minuend.
	   */
	  x1 = r2->x2;
	  if (x1 >= r1->x2)
	    {
	      /*
	       * Minuend completely covered: advance to next minuend and
	       * reset left fence to edge of new minuend.
	       */
	      r1++;
	      if (r1 != r1End)
		x1 = r1->x1;
	    }
	  else
	    {
	      /*
	       * Subtrahend now used up since it doesn't extend beyond
	       * minuend
	       */
	      r2++;
	    }
	}
      else if (r2->x1 < r1->x2)
	{
	  /*
	   * Left part of subtrahend covers part of minuend: add uncovered
	   * part of minuend to region and skip to next subtrahend.
	   */
	  g_assert(x1<r2->x1);
	  MEMCHECK(pReg, pNextRect, pReg->rects);
	  pNextRect->x1 = x1;
	  pNextRect->y1 = y1;
	  pNextRect->x2 = r2->x1;
	  pNextRect->y2 = y2;
	  pReg->numRects += 1;
	  pNextRect++;

	  g_assert(pReg->numRects<=pReg->size);

	  x1 = r2->x2;
	  if (x1 >= r1->x2)
	    {
	      /*
	       * Minuend used up: advance to new...
	       */
	      r1++;
	      if (r1 != r1End)
		x1 = r1->x1;
	    }
	  else
	    {
	      /*
	       * Subtrahend used up
	       */
	      r2++;
	    }
	}
      else
	{
	  /*
	   * Minuend used up: add any remaining piece before advancing.
	   */
	  if (r1->x2 > x1)
	    {
	      MEMCHECK(pReg, pNextRect, pReg->rects);
	      pNextRect->x1 = x1;
	      pNextRect->y1 = y1;
	      pNextRect->x2 = r1->x2;
	      pNextRect->y2 = y2;
	      pReg->numRects += 1;
	      pNextRect++;
	      g_assert(pReg->numRects<=pReg->size);
	    }
	  r1++;
	  if (r1 != r1End)
	    x1 = r1->x1;
	}
    }

  /*
     * Add remaining minuend rectangles to region.
     */
  while (r1 != r1End)
    {
      g_assert(x1<r1->x2);
      MEMCHECK(pReg, pNextRect, pReg->rects);
      pNextRect->x1 = x1;
      pNextRect->y1 = y1;
      pNextRect->x2 = r1->x2;
      pNextRect->y2 = y2;
      pReg->numRects += 1;
      pNextRect++;

      g_assert(pReg->numRects<=pReg->size);

      r1++;
      if (r1 != r1End)
	{
	  x1 = r1->x1;
	}
    }
}

/**
 * gdk_region_subtract:
 * @source1: a #GdkRegion
 * @source2: another #GdkRegion
 *
 * Subtracts the area of @source2 from the area @source1. The resulting
 * area is the set of pixels contained in @source1 but not in @source2.
 **/
void
gdk_region_subtract (GdkRegion *source1,
		     GdkRegion *source2)
{
  g_return_if_fail (source1 != NULL);
  g_return_if_fail (source2 != NULL);
  
  /* check for trivial reject */
  if ((!(source1->numRects)) || (!(source2->numRects)) ||
      (!EXTENTCHECK(&source1->extents, &source2->extents)))
    return;
 
  miRegionOp (source1, source1, source2, miSubtractO,
	      miSubtractNonO1, (nonOverlapFunc) NULL);

  /*
   * Can't alter source1's extents before we call miRegionOp because miRegionOp
   * depends on the extents of those regions being the unaltered. Besides, this
   * way there's no checking against rectangles that will be nuked
   * due to coalescing, so we have to examine fewer rectangles.
   */
  miSetExtents (source1);
}

/**
 * gdk_region_xor:
 * @source1: a #GdkRegion
 * @source2: another #GdkRegion
 *
 * Sets the area of @source1 to the exclusive-OR of the areas of @source1
 * and @source2. The resulting area is the set of pixels contained in one
 * or the other of the two sources but not in both.
 **/
void
gdk_region_xor (GdkRegion *source1,
		GdkRegion *source2)
{
  GdkRegion *trb;

  g_return_if_fail (source1 != NULL);
  g_return_if_fail (source2 != NULL);

  trb = gdk_region_copy (source2);

  gdk_region_subtract (trb, source1);
  gdk_region_subtract (source1, source2);

  gdk_region_union (source1, trb);
  
  gdk_region_destroy (trb);
}

/**
 * gdk_region_empty: 
 * @region: a #GdkRegion
 *
 * Finds out if the #GdkRegion is empty.
 *
 * Returns: %TRUE if @region is empty.
 */
gboolean
gdk_region_empty (GdkRegion *region)
{
  g_return_val_if_fail (region != NULL, FALSE);
  
  if (region->numRects == 0)
    return TRUE;
  else
    return FALSE;
}

/**
 * gdk_region_equal:
 * @region1: a #GdkRegion
 * @region2: a #GdkRegion
 *
 * Finds out if the two regions are the same.
 *
 * Returns: %TRUE if @region1 and @region2 are equal.
 */
gboolean
gdk_region_equal (GdkRegion *region1,
		  GdkRegion *region2)
{
  int i;

  g_return_val_if_fail (region1 != NULL, FALSE);
  g_return_val_if_fail (region2 != NULL, FALSE);

  if (region1->numRects != region2->numRects) return FALSE;
  else if (region1->numRects == 0) return TRUE;
  else if (region1->extents.x1 != region2->extents.x1) return FALSE;
  else if (region1->extents.x2 != region2->extents.x2) return FALSE;
  else if (region1->extents.y1 != region2->extents.y1) return FALSE;
  else if (region1->extents.y2 != region2->extents.y2) return FALSE;
  else
    for(i = 0; i < region1->numRects; i++ )
      {
	if (region1->rects[i].x1 != region2->rects[i].x1) return FALSE;
	else if (region1->rects[i].x2 != region2->rects[i].x2) return FALSE;
	else if (region1->rects[i].y1 != region2->rects[i].y1) return FALSE;
	else if (region1->rects[i].y2 != region2->rects[i].y2) return FALSE;
      }
  return TRUE;
}

/**
 * gdk_region_point_in:
 * @region: a #GdkRegion
 * @x: the x coordinate of a point
 * @y: the y coordinate of a point
 *
 * Finds out if a point is in a region.
 *
 * Returns: %TRUE if the point is in @region.
 */
gboolean
gdk_region_point_in (GdkRegion *region,
		     int        x,
		     int        y)
{
  int i;

  g_return_val_if_fail (region != NULL, FALSE);

  if (region->numRects == 0)
    return FALSE;
  if (!INBOX(region->extents, x, y))
    return FALSE;
  for (i = 0; i < region->numRects; i++)
    {
      if (INBOX (region->rects[i], x, y))
	return TRUE;
    }
  return FALSE;
}

/**
 * gdk_region_rect_in: 
 * @region: a #GdkRegion.
 * @rectangle: a #GdkRectangle.
 *
 * Tests whether a rectangle is within a region.
 *
 * Returns: %GDK_OVERLAP_RECTANGLE_IN, %GDK_OVERLAP_RECTANGLE_OUT, or
 *   %GDK_OVERLAP_RECTANGLE_PART, depending on whether the rectangle is inside,
 *   outside, or partly inside the #GdkRegion, respectively.
 */
GdkOverlapType
gdk_region_rect_in (GdkRegion    *region,
		    GdkRectangle *rectangle)
{
  GdkRegionBox *pbox;
  GdkRegionBox *pboxEnd;
  GdkRegionBox  rect;
  GdkRegionBox *prect = &rect;
  gboolean      partIn, partOut;
  gint rx, ry;

  g_return_val_if_fail (region != NULL, GDK_OVERLAP_RECTANGLE_OUT);
  g_return_val_if_fail (rectangle != NULL, GDK_OVERLAP_RECTANGLE_OUT);

  rx = rectangle->x;
  ry = rectangle->y;
  
  prect->x1 = rx;
  prect->y1 = ry;
  prect->x2 = rx + rectangle->width;
  prect->y2 = ry + rectangle->height;
    
    /* this is (just) a useful optimization */
  if ((region->numRects == 0) || !EXTENTCHECK (&region->extents, prect))
    return GDK_OVERLAP_RECTANGLE_OUT;

  partOut = FALSE;
  partIn = FALSE;

    /* can stop when both partOut and partIn are TRUE, or we reach prect->y2 */
  for (pbox = region->rects, pboxEnd = pbox + region->numRects;
       pbox < pboxEnd;
       pbox++)
    {

      if (pbox->y2 <= ry)
	continue;	/* getting up to speed or skipping remainder of band */

      if (pbox->y1 > ry)
	{
	  partOut = TRUE;	/* missed part of rectangle above */
	  if (partIn || (pbox->y1 >= prect->y2))
	    break;
	  ry = pbox->y1;	/* x guaranteed to be == prect->x1 */
	}

      if (pbox->x2 <= rx)
	continue;		/* not far enough over yet */

      if (pbox->x1 > rx)
	{
	  partOut = TRUE;	/* missed part of rectangle to left */
	  if (partIn)
	    break;
	}

      if (pbox->x1 < prect->x2)
	{
	  partIn = TRUE;	/* definitely overlap */
	  if (partOut)
	    break;
	}

      if (pbox->x2 >= prect->x2)
	{
	  ry = pbox->y2;	/* finished with this band */
	  if (ry >= prect->y2)
	    break;
	  rx = prect->x1;	/* reset x out to left again */
	}
      else
	{
	  /*
	   * Because boxes in a band are maximal width, if the first box
	   * to overlap the rectangle doesn't completely cover it in that
	   * band, the rectangle must be partially out, since some of it
	   * will be uncovered in that band. partIn will have been set true
	   * by now...
	   */
	  break;
	}

    }

  return (partIn ?
	     ((ry < prect->y2) ?
	      GDK_OVERLAP_RECTANGLE_PART : GDK_OVERLAP_RECTANGLE_IN) : 
	  GDK_OVERLAP_RECTANGLE_OUT);
}


static void
gdk_region_unsorted_spans_intersect_foreach (GdkRegion *region,
					     GdkSpan   *spans,
					     int        n_spans,
					     GdkSpanFunc function,
					     gpointer data)
{
  gint i, left, right, y;
  gint clipped_left, clipped_right;
  GdkRegionBox *pbox;
  GdkRegionBox *pboxEnd;
  GdkSpan out_span;

  if (!region->numRects)
    return;

  for (i=0;i<n_spans;i++)
    {
      y = spans[i].y;
      left = spans[i].x;
      right = left + spans[i].width; /* right is not in the span! */
    
      if (! ((region->extents.y1 <= y) &&
	     (region->extents.y2 > y) &&
	     (region->extents.x1 < right) &&
	     (region->extents.x2 > left)) ) 
	continue;

      /* can stop when we passed y */
      for (pbox = region->rects, pboxEnd = pbox + region->numRects;
	   pbox < pboxEnd;
	   pbox++)
	{
	  if (pbox->y2 <= y)
	    continue; /* Not quite there yet */
	  
	  if (pbox->y1 > y)
	    break; /* passed the spanline */
	  
	  if ((right > pbox->x1) && (left < pbox->x2)) 
	    {
	      clipped_left = MAX (left, pbox->x1);
	      clipped_right = MIN (right, pbox->x2);
	      
	      out_span.y = y;
	      out_span.x = clipped_left;
	      out_span.width = clipped_right - clipped_left;
	      (*function) (&out_span, data);
	    }
	}
    }
}

/**
 * gdk_region_spans_intersect_foreach:
 * @region: a #GdkRegion
 * @spans: an array of #GdkSpans
 * @n_spans: the length of @spans
 * @sorted: %TRUE if @spans is sorted wrt. the y coordinate
 * @function: function to call on each span in the intersection
 * @data: data to pass to @function
 *
 * Calls a function on each span in the intersection of @region and @spans.
 */
void
gdk_region_spans_intersect_foreach (GdkRegion  *region,
				    GdkSpan    *spans,
				    int         n_spans,
				    gboolean    sorted,
				    GdkSpanFunc function,
				    gpointer    data)
{
  gint left, right, y;
  gint clipped_left, clipped_right;
  GdkRegionBox *pbox;
  GdkRegionBox *pboxEnd;
  GdkSpan *span, *tmpspan;
  GdkSpan *end_span;
  GdkSpan out_span;

  g_return_if_fail (region != NULL);
  g_return_if_fail (spans != NULL);

  if (!sorted)
    {
      gdk_region_unsorted_spans_intersect_foreach (region,
						   spans,
						   n_spans,
						   function,
						   data);
      return;
    }
  
  if ((!region->numRects) || (n_spans == 0))
    return;

  /* The main method here is to step along the
   * sorted rectangles and spans in lock step, and
   * clipping the spans that are in the current
   * rectangle before going on to the next rectangle.
   */

  span = spans;
  end_span = spans + n_spans;
  pbox = region->rects;
  pboxEnd = pbox + region->numRects;
  while (pbox < pboxEnd)
    {
      while ((pbox->y2 < span->y) || (span->y < pbox->y1))
	{
	  /* Skip any rectangles that are above the current span */
	  if (pbox->y2 < span->y)
	    {
	      pbox++;
	      if (pbox == pboxEnd)
		return;
	    }
	  /* Skip any spans that are above the current rectangle */
	  if (span->y < pbox->y1)
	    {
	      span++;
	      if (span == end_span)
		return;
	    }
	}
      
      /* Ok, we got at least one span that might intersect this rectangle. */
      tmpspan = span;
      while ((tmpspan < end_span) &&
	     (tmpspan->y < pbox->y2))
	{
	  y = tmpspan->y;
	  left = tmpspan->x;
	  right = left + tmpspan->width; /* right is not in the span! */
	  
	  if ((right > pbox->x1) && (left < pbox->x2))
	    {
	      clipped_left = MAX (left, pbox->x1);
	      clipped_right = MIN (right, pbox->x2);
	      
	      out_span.y = y;
	      out_span.x = clipped_left;
	      out_span.width = clipped_right - clipped_left;
	      (*function) (&out_span, data);
	    }
	  
	  tmpspan++;
	}

      /* Finished this rectangle.
       * The spans could still intersect the next one
       */
      pbox++;
    }
}

#define __GDK_REGION_GENERIC_C__
#include "gdkaliasdef.c"