The netpbm-1mar1994 release of ppmquant suffers from an overflow bug.
(The "distance" between colors is computed as a long, but it won't
fit in a 32-bit long if maxval is 65535.)

You can test for this bug by running

    ppmmake red 10 10 | ppmquant 256 | ppmhist

If you get this output:

    r   g   b      lum     count
   --- --- ---     ---     -----
     0   0   0     0       100

then you've got the bug.  The fixed version will
produce the correct output (all red):

    r   g   b      lum     count
   --- --- ---     ---     -----
   65535   0   0   19595   100

- Darrell Kindred (dkindred@cmu.edu), 15 March 1997


--- ppm/ppmquant.c.orig	Mon Oct  4 05:12:30 1993
+++ ppm/ppmquant.c	Sat Mar 15 15:39:54 1997
@@ -247,20 +247,22 @@
 	    ind = ppm_lookupcolor( cht, pP );
 	    if ( ind == -1 )
 		{ /* No; search colormap for closest match. */
-		register int i, r1, g1, b1, r2, g2, b2;
-		register long dist, newdist;
+		int i;
+		pixval r1, g1, b1, r2, g2, b2;
+		double dist, newdist;
 		r1 = PPM_GETR( *pP );
 		g1 = PPM_GETG( *pP );
 		b1 = PPM_GETB( *pP );
-		dist = 2000000000;
+                /* 3 * maxval^2 is the largest distance possible */
+		dist = (double) maxval * maxval * 4;	
 		for ( i = 0; i < newcolors; ++i )
 		    {
 		    r2 = PPM_GETR( colormap[i].color );
 		    g2 = PPM_GETG( colormap[i].color );
 		    b2 = PPM_GETB( colormap[i].color );
-		    newdist = ( r1 - r2 ) * ( r1 - r2 ) +
-			      ( g1 - g2 ) * ( g1 - g2 ) +
-			      ( b1 - b2 ) * ( b1 - b2 );
+		    newdist = (double) ( r1 - r2 ) * ( r1 - r2 ) +
+			      (double) ( g1 - g2 ) * ( g1 - g2 ) +
+			      (double) ( b1 - b2 ) * ( b1 - b2 );
 		    if ( newdist < dist )
 			{
 			ind = i;