cache.c

/*
 * cache.c: This file is part of the `djbdns' project, originally written
 * by Dr. D J Bernstein and later released under public-domain since late
 * December 2007 (http://cr.yp.to/distributors.html).
 *
 * Copyright (C) 2009 - 2012 Prasad J Pandit
 *
 * This program is a free software; you can redistribute it and/or modify
 * it under the terms of GNU General Public License as published by Free
 * Software Foundation; either version 2 of the license or (at your option)
 * any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * of 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; if not, write to Free Software Foundation, Inc., 51
 * Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#include <stdlib.h>

#include "tai.h"
#include "byte.h"
#include "alloc.h"
#include "cache.h"
#include "uint32.h"

uint64 cache_motion = 0;

static char *x = 0;
static uint32 size;
static uint32 hsize;
static uint32 writer;
static uint32 oldest;
static uint32 unused;

/*
100 <= size <= 1000000000.
4 <= hsize <= size/16.
hsize is a power of 2.

hsize <= writer <= oldest <= unused <= size.
If oldest == unused then unused == size.

x is a hash table with the following structure:
x[0...hsize-1]: hsize/4 head links.
x[hsize...writer-1]: consecutive entries, newest entry on the right.
x[writer...oldest-1]: free space for new entries.
x[oldest...unused-1]: consecutive entries, oldest entry on the left.
x[unused...size-1]: unused.

Each hash bucket is a linked list containing the following items:
the head link, the newest entry, the second-newest entry, etc.
Each link is a 4-byte number giving the xor of
the positions of the adjacent items in the list.

Entries are always inserted immediately after the head and removed at the tail.

Each entry contains the following information:
4-byte link; 4-byte keylen; 4-byte datalen; 8-byte expire time; key; data.
*/

#define MAXKEYLEN 1000
#define MAXDATALEN 1000000

static void
cache_impossible (void)
{
    exit (111);
}

static void
set4 (uint32 pos, uint32 u)
{
    if (pos > size - 4)
        cache_impossible ();

    uint32_pack (x + pos, u);
}

static
uint32 get4 (uint32 pos)
{
    uint32 result = 0;

    if (pos > size - 4)
        cache_impossible ();
    uint32_unpack (x + pos, &result);

    return result;
}

static unsigned int
hash (const char *key, unsigned int keylen)
{
    unsigned int result = 5381;

    while (keylen)
    {
        result = (result << 5) + result;
        result ^= (unsigned char) *key;
        ++key;
        --keylen;
    }
    result <<= 2;
    result &= hsize - 4;

    return result;
}

char *
cache_get (const char *key, unsigned int keylen,
                            unsigned int *datalen, uint32 *ttl)
{
    struct tai now;
    struct tai expire;

    uint32 u = 0, pos = 0;
    uint32 prevpos = 0, nextpos = 0;

    double d = 0.0;
    unsigned int loop = 0;

    if (!x)
        return 0;
    if (keylen > MAXKEYLEN)
        return 0;

    prevpos = hash (key, keylen);
    pos = get4 (prevpos);

    while (pos)
    {
        if (get4(pos + 4) == keylen)
        {
            if (pos + 20 + keylen > size)
                cache_impossible ();
            if (byte_equal (key, keylen, x + pos + 20))
            {
                tai_unpack (x + pos + 12, &expire);
                tai_now (&now);
                if (tai_less (&expire, &now))
                    return 0;

                tai_sub (&expire, &expire, &now);
                d = tai_approx (&expire);
                if (d > 604800)
                    d = 604800;
                *ttl = d;

                u = get4 (pos + 8);
                if (u > size - pos - 20 - keylen)
                    cache_impossible ();
                *datalen = u;

                return x + pos + 20 + keylen;
            }
        }
        nextpos = prevpos ^ get4 (pos);
        prevpos = pos;
        pos = nextpos;
        if (++loop > 100)
            return 0; /* to protect against hash flooding */
    }

    return 0;
}

void
cache_set (const char *key, unsigned int keylen,
                            const char *data, unsigned int datalen, uint32 ttl)
{
    uint32 pos = 0;
    struct tai now;
    struct tai expire;

    unsigned int keyhash = 0;
    unsigned int entrylen = 0;

    if (!x)
      return;
    if (keylen > MAXKEYLEN)
        return;
    if (datalen > MAXDATALEN)
        return;

    if (!ttl)
        return;
    if (ttl > 604800)
        ttl = 604800;

    entrylen = keylen + datalen + 20;

    while (writer + entrylen > oldest)
    {
        if (oldest == unused)
        {
            if (writer <= hsize)
                return;
            unused = writer;
            oldest = hsize;
            writer = hsize;
        }

        pos = get4 (oldest);
        set4 (pos, get4 (pos) ^ oldest);

        oldest += get4 (oldest + 4) + get4 (oldest + 8) + 20;
        if (oldest > unused)
            cache_impossible ();
        if (oldest == unused)
        {
            unused = size;
            oldest = size;
        }
    }

    keyhash = hash (key,keylen);

    tai_now (&now);
    tai_uint (&expire, ttl);
    tai_add (&expire, &expire, &now);

    pos = get4 (keyhash);
    if (pos)
        set4 (pos, get4 (pos) ^ keyhash ^ writer);

    set4 (writer, pos ^ keyhash);
    set4 (writer + 4, keylen);
    set4 (writer + 8, datalen);
    tai_pack (x + writer + 12, &expire);
    byte_copy (x + writer + 20, keylen, key);
    byte_copy (x + writer + 20 + keylen, datalen, data);

    set4 (keyhash,writer);
    writer += entrylen;
    cache_motion += entrylen;
}

int
cache_init (unsigned int cachesize)
{
    if (x)
    {
        alloc_free (x);
        x = 0;
    }

    if (cachesize > 1000000000)
        cachesize = 1000000000;
    if (cachesize < 100)
        cachesize = 100;
    size = cachesize;

    hsize = 4;
    while (hsize <= (size >> 5))
        hsize <<= 1;

    if (!(x = alloc (size)))
        return 0;

    writer = hsize;
    oldest = size;
    unused = size;

    return 1;
}