/* generate lfsr output */
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>

#define MAX 1025

typedef unsigned char	uchar;

uchar	R[MAX];
int	T[MAX];

int	n = 10000;	/* number of bits to output */
int	l = -1;
int	taps = 0;
int	verbose;
char	*init;
char	*myname;

void
printreg(int r)
{
    register int	i;

    for (i = 0; i < l; ++i)
	putchar(R[(r+i) % l] + '0');
    putchar('\n');
}

int
main(int ac, char **av)
{
    register long		i, tap, r;

    myname = av[0]; --ac; ++av;
    while (ac > 0 && av[0][0] == '-') {
	if (ac > 0 && strcmp(av[0], "-v") == 0) {
	    ++verbose;
	    --ac, ++av;
	}
	else if (ac > 1 && strcmp(av[0], "-n") == 0) {
	    n = strtol(av[1], (char **)NULL, 0);
	    if (n <= 0) {
		fprintf(stderr, "%s: Can't have negative count: %d\n", myname, n);
		goto usage;
	    }
	    ac -= 2; av += 2;
	}
	else if (ac > 1 && strcmp(av[0], "-i") == 0) {
	    init = av[1];
	    /* further checking done later */
	    ac -= 2; av += 2;
	}
	else
	    goto usage;
    }
    if (ac < 2) {
    badtaps:
	fprintf(stderr,
	    "%s: Must specify (odd, >= 3, 0 implied) taps overall\n", myname);
    usage:
	fprintf(stderr,
	    "usage: %s [-v] [-n bitsout] [-i initial] t1 t2 ...\n", myname);
	fprintf(stderr,
	    "Register shifts left, taps count from 1 at left.\n");
	return 1;
    }

    tap = 0;
    for (i = 0; i < ac; ++i) {
	tap = strtol(av[i], (char **)NULL, 0);
	if (tap < 0 || MAX <= tap) {
	    fprintf(stderr, "%s: invalid tap %ld\n", myname, tap);
	    goto usage;
	}
	if (tap == 0)
	    continue;
	T[taps++] = tap;
	if (tap > l)
	    l = tap;
    }
    if (taps < 2 || (taps & 1) == 1)
	goto badtaps;
    if (init) {
	if (strlen(init) != l || strspn(init, "01") != l) {
	    fprintf(stderr,
		"%s: init string must be binary of right length\n", myname);
		goto usage;
	}
	for (i = 0; i < l; ++i)
	    R[i] = *init++ - '0';
    }
    else {
	/* initialise to give impulse sequence */
	R[l-1] = 1;
    }

    /* reverse taps to fit polynomial */
    for (i = 0; i < taps; ++i)
	if (T[i] != l)
	    T[i] = l - T[i];

    /* wow -- ready to generate bits finally. */
    r = 0;
    while (--n >= 0) {
	tap = R[r];
	if (verbose)
	    printreg(r);
	else
	    putchar(tap+'0');
	for (tap = i = 0; i < taps; ++i)
	    tap ^= R[(r+T[i]) % l];
	R[r] = tap;
	if (++r == l)
	    r = 0;
    }
    fflush(stdout);
    return 0;
}