/* $XConsortium: zeroeven.c,v 1.3 94/01/12 20:11:23 rws Exp $ */
/**** module fax/zeroeven.c ****/
/******************************************************************************

Copyright (c) 1993, 1994  X Consortium

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				NOTICE
                              
This software is being provided by AGE Logic, Inc. under the
following license.  By obtaining, using and/or copying this software,
you agree that you have read, understood, and will comply with these
terms and conditions:

     Permission to use, copy, modify, distribute and sell this
     software and its documentation for any purpose and without
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     "Copyright 1993, 1994 by AGE Logic, Inc."
     
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     Logic, Inc.
*****************************************************************************
  
	fax/zeroeven.c -- DDXIE fax utility
  
	Ben && Larry -- AGE Logic, Inc. May, 1993
  
*****************************************************************************/


/***	zero_even.c - zero out even transitions. 

	The array wbtrans is arranged as a table of positions at which
	the input changes from white to black, then black to white, then
	white to black, and so on.  We assume all output pixels have
	already been set white. Therefore we only have to fill the
	black ranges with zeros...

	W->B   B->W   W->B   B->W   W->B   B->W   W->B   B->W

	12	17     29     30     65    68      90     92

	bit positions 12-16, 29, 65-67, and 90-91 in the above
	example are black.  It should be clear from the example
	that the size of the run length is t[w]-t[b],  where 
	w=b+1 and b  is the index of a transition from white to 
	black.  Note that if the line was only 92 pixels wide,
	setting the imaginary pixel with index = image width
	B->W still gives us the right answer.

***/

#include <xiemd.h>

/**********************************************************************/

zero_even(cp,t,nt,w,white)
register LogInt *cp;
register int	*t, nt,w,white;
{
unsigned int	sbit,ebit;
LogInt		*sbyt, *ebyt, black;

#define ONES ~((LogInt)0)
   
    memset(cp, white ? 0xff: 0, (w+7)>>3);
    black = white ? 0 : ONES; 

    for (nt = (nt>>1) - 1; nt >= 0; nt--) {
	sbit = *t++;
	ebit = *t++; /* if (!nt && (ebit > w)) ebit = w; */
	sbyt = cp + (sbit >> LOGSHIFT); sbit &= LOGMASK;
	ebyt = cp + (ebit >> LOGSHIFT); ebit &= LOGMASK;

	if (sbyt < ebyt) {
	    if (sbit)	    	*sbyt++ ^= BitRight(ONES,sbit);
	    while (sbyt < ebyt)	*sbyt++  = black;
	    if (ebit)		*sbyt   ^= ~BitRight(ONES,ebit);
	} else
	    *sbyt ^= BitRight(ONES,sbit) & ~(BitRight(ONES,ebit));
    }
}
/**** module fax/zeroeven.c ****/