/*      $NetBSD: readhappy.c,v 1.1 2015/05/13 07:07:36 pgoyette Exp $   */

/*-
 * Copyright (c) 2015 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: readhappy.c,v 1.1 2015/05/13 07:07:36 pgoyette Exp $");

#include <sys/param.h>
#include <sys/conf.h>
#include <sys/device.h>
#include <sys/kernel.h>
#include <sys/module.h>

/*
 * Create a device /dev/happy from which you can read sequential
 * happy numbers.
 *
 * To use this device you need to do:
 *     mknod /dev/happy c 210 0
 *
 * Commentary:
 * A happy number is a number defined by the following process: Starting with
 * any positive integer, replace the number by the sum of the squares of its
 * digits, and repeat the process until the number equals 1 (where it will
 * stay), or it loops endlessly in a cycle which does not include 1. Those
 * numbers for which this process ends in 1 are happy numbers, while those that
 * do not end in 1 are unhappy numbers (or sad numbers).
 *
 * For more information on happy numbers, and the algorithms, see
 *      http://en.wikipedia.org/wiki/Happy_number 
 *
 * The happy number generator is here only to have something that the user
 * can read from our device.  Any other arbitrary data generator could
 * have been used.  The algorithm is not critical to the implementation
 * of the module.
 */


#define HAPPY_NUMBER    1

/* If n is not happy then its sequence ends in the cycle:
 * 4, 16, 37, 58, 89, 145, 42, 20, 4, ... */
#define SAD_NUMBER      4

/* Calculate the sum of the squares of the digits of n */
static unsigned
dsum(unsigned n)
{
        unsigned sum, x;
        for (sum = 0; n; n /= 10) {
                x = n % 10;
                sum += x * x;
        }
        return sum;
}
 
static int
check_happy(unsigned n)
{
        for (;;) {
                unsigned total = dsum(n);

                if (total == HAPPY_NUMBER)
                        return 1;
                if (total == SAD_NUMBER)
                        return 0;

                n = total;
        }
}

dev_type_open(happy_open);
dev_type_close(happy_close);
dev_type_read(happy_read);

static struct cdevsw happy_cdevsw = {
        .d_open = happy_open,
        .d_close = happy_close,
        .d_read = happy_read,
        .d_write = nowrite,
        .d_ioctl = noioctl,
        .d_stop = nostop,
        .d_tty = notty,
        .d_poll = nopoll,
        .d_mmap = nommap,
        .d_kqfilter = nokqfilter,
        .d_discard = nodiscard,
        .d_flag = D_OTHER
};


struct happy_softc {
        int             refcnt;
        unsigned        last;
};

static struct happy_softc sc;

int
happy_open(dev_t self __unused, int flag __unused, int mode __unused,
           struct lwp *l __unused)
{
        if (sc.refcnt > 0)
                return EBUSY;

        sc.last = 0;
        ++sc.refcnt;

        return 0;
}

int
happy_close(dev_t self __unused, int flag __unused, int mode __unused,
            struct lwp *l __unused)
{
        --sc.refcnt;

        return 0;
}

int
happy_read(dev_t self __unused, struct uio *uio, int flags __unused)
{
        char line[80];

        /* Get next happy number */
        while (check_happy(++sc.last) == 0)
                continue;

        /* Print it into line[] with trailing \n */
        int len = snprintf(line, sizeof(line), "%u\n", sc.last);

        /* Is there room? */
        if (uio->uio_resid < len) {
                --sc.last; /* Step back */
                return EINVAL;
        }

        /* Send it to User-Space */
        int e;
        if ((e = uiomove(line, len, uio)))
                return e;

        return 0;
}

MODULE(MODULE_CLASS_MISC, happy, NULL);

static int
happy_modcmd(modcmd_t cmd, void *arg __unused)
{
        /* The major should be verified and changed if needed to avoid
         * conflicts with other devices. */
        int cmajor = 210, bmajor = -1;

        switch (cmd) {
        case MODULE_CMD_INIT:
                if (devsw_attach("happy", NULL, &bmajor, &happy_cdevsw,
                                 &cmajor))
                        return ENXIO;
                return 0;
        case MODULE_CMD_FINI:
                if (sc.refcnt > 0)
                        return EBUSY;

                devsw_detach(NULL, &happy_cdevsw);
                return 0;
        default:
                return ENOTTY;
        }
}