/*
$Id: diskii_nybble.c,v 1.3 2005/03/21 22:13:00 eric Exp eric $
Copyright 2005 Eric Smith <eric@brouhaha.com>

diskii_nybble is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License version 2 as
published by the Free Software Foundation.  Note that I am not
granting permission to redistribute or modify diskii_nybble under the
terms of any later version of the General Public License.

diskii_nybble 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 (in the file "COPYING"); if not, write to the
Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
MA 02111, USA.
*/


#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>


bool valid_nybble (uint8_t n, int sectors)
{
  uint8_t n2;
  int i;
  int c;

  // A valid nybble must always have the MSB set.
  if (! (n & 0x80))
    return false;

  // Aside from the D5 and AA nybbles used for address and data marks,
  // valid nybbles always have at least one pair of consecutive one
  // bits (not including the MSB).  This is a somewhat arbitrary
  // restriction, though in the 16-sector boot ROM it helps to reduce
  // the number of valid nybbles in a manner that simplifies automatic
  // table generation by the boot PROM.
  n2 = n;
  c = 0;
  for (i = 0; i < 6; i++)
    {
      if ((n2 & 0x03) == 0x03)
	c++;
      n2 >>= 1;
    }
  if (c < 1)
    return false;

  if (sectors == 13)
    {
      // Valid 13-sector nybbles never have two consecutive zero bits.
      n2 = n;
      for (i = 0; i < 6; i++)
	{
	  if ((n2 & 0x03) == 0)
	    return (false);
	  n2 >>= 1;
	}
      return true;
    }

  if (sectors != 16)
    return false;

  // Valid 16-sector nybbles never have three consecutive zero bits.
  n2 = n;
  for (i = 0; i < 5; i++)
    {
      if ((n2 & 0x07) == 0)
	return (false);
      n2 >>= 1;
    }

  // Valid 16-sector nybbles never have more than one pair of
  // consecutive zero bits.  This is a somewhat arbitrary restriction
  // to reduce the number of valid nybbles in a manner that simplifies
  // with automatic table generation by the boot PROM.
  n2 = n;
  c = 0;
  for (i = 0; i < 6; i++)
    {
      if ((n2 & 0x03) == 0)
	c++;
      n2 >>= 1;
    }
  if (c > 1)
    return (false);

  return (true);
}


int main (int argc, char *argv[])
{
  int c;
  int sectors;
  uint8_t n;

  for (sectors = 13; sectors <= 16; sectors += 3)
    {
      printf ("%d sector nybbles:\n", sectors);

      c = 0;
      for (n = 0x80; n >= 0x80; n++)
	{
	  if (valid_nybble (n, sectors))
	    {
	      if ((c % 10) == 0)
		printf ("  %2d: ", c);
	      printf (" %02x", n);
	      if ((++c % 10) == 0)
		printf ("\n");
	    }
	}
      if (c % 10)
	printf ("\n");
      
      printf ("\n");
    }

  exit (0);
}