#include "stdio.h"
#include "math.h"

//---------------------------------------------------------------------------

#define prntvalue(x,y) value=0; for(i=0;i < bits;i++) if (gray_scale[i]=='1') value = value + pow(2,i); printf("%8ld:   ",value)
#define prntbits(x,y) for(i=0;i < bits;i++) printf("%c",gray_scale[i])

//---------------------------------------------------------------------------
int main(int argc, char **argv)
{
    #define MAXBITS 32
    char gray_scale[MAXBITS];

    int i,j;
    int bits;
    long int    index;
    long int    value;


    if (argc==2)
    {
        sscanf(argv[1],"%d",&bits);
    }
    else
    {
        printf("Please enter the number of gray scale bits to generate: ");
        scanf("%d",&bits);
    }

    if ((bits<=0) || (bits>MAXBITS))
    {
        printf("gray code bits must be between 1 and %d\n",MAXBITS);
    }
    else
    {
        for (i=0;i<MAXBITS;i++)
        {
          gray_scale[i] = '0';
        }
        
        index = 0;
        printf("Position:      gray:   BINARY\n");
        printf("%8d:  ", index);
        prntvalue(bits,gray_scale);
        prntbits(bits,gray_scale);
        printf("\n");

        for (index=1;index<pow(2,bits);index++) 
        {
            for (i=bits;
                (i>0);
                i--)
            {
              j = pow(2,i);
              if (!(index%j)) break;
            };
            if (gray_scale[i]=='0')  gray_scale[i]='1';
            else                     gray_scale[i]='0';
            printf("%8d:  ", index);
            prntvalue(bits,gray_scale);
            prntbits(bits,gray_scale);
            printf("\n");
        }
   }
    return 0;
}