pkghash.c

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/*
 *  pkghash.c
 *
 *  Copyright (c) 2011 Pacman Development Team <pacman-dev@archlinux.org>
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the 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 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.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <errno.h>

#include "pkghash.h"
#include "util.h"

/* List of primes for possible sizes of hash tables.
 *
 * The maximum table size is the last prime under 1,000,000.  That is
 * more than an order of magnitude greater than the number of packages
 * in any Linux distribution, and well under UINT_MAX.
 */
static const unsigned int prime_list[] =
{
    11u, 13u, 17u, 19u, 23u, 29u, 31u, 37u, 41u, 43u, 47u,
    53u, 59u, 61u, 67u, 71u, 73u, 79u, 83u, 89u, 97u, 103u,
    109u, 113u, 127u, 137u, 139u, 149u, 157u, 167u, 179u, 193u,
    199u, 211u, 227u, 241u, 257u, 277u, 293u, 313u, 337u, 359u,
    383u, 409u, 439u, 467u, 503u, 541u, 577u, 619u, 661u, 709u,
    761u, 823u, 887u, 953u, 1031u, 1109u, 1193u, 1289u, 1381u,
    1493u, 1613u, 1741u, 1879u, 2029u, 2179u, 2357u, 2549u,
    2753u, 2971u, 3209u, 3469u, 3739u, 4027u, 4349u, 4703u,
    5087u, 5503u, 5953u, 6427u, 6949u, 7517u, 8123u, 8783u,
    9497u, 10273u, 11113u, 12011u, 12983u, 14033u, 15173u,
    16411u, 17749u, 19183u, 20753u, 22447u, 24281u, 26267u,
    28411u, 30727u, 33223u, 35933u, 38873u, 42043u, 45481u,
    49201u, 53201u, 57557u, 62233u, 67307u, 72817u, 78779u,
    85229u, 92203u, 99733u, 107897u, 116731u, 126271u, 136607u,
    147793u, 159871u, 172933u, 187091u, 202409u, 218971u, 236897u,
    256279u, 277261u, 299951u, 324503u, 351061u, 379787u, 410857u,
    444487u, 480881u, 520241u, 562841u, 608903u, 658753u, 712697u,
    771049u, 834181u, 902483u, 976369u
};

/* How far forward do we look when linear probing for a spot? */
static const unsigned int stride = 1;
/* What is the maximum load percentage of our hash table? */
static const double max_hash_load = 0.68;
/* Initial load percentage given a certain size */
static const double initial_hash_load = 0.58;

/* Allocate a hash table with space for at least "size" elements */
alpm_pkghash_t *_alpm_pkghash_create(unsigned int size)
{
    alpm_pkghash_t *hash = NULL;
    unsigned int i, loopsize;

    CALLOC(hash, 1, sizeof(alpm_pkghash_t), return NULL);
    size = size / initial_hash_load + 1;

    loopsize = sizeof(prime_list) / sizeof(*prime_list);
    for(i = 0; i < loopsize; i++) {
        if(prime_list[i] > size) {
            hash->buckets = prime_list[i];
            hash->limit = hash->buckets * max_hash_load;
            break;
        }
    }

    if(hash->buckets < size) {
        errno = ERANGE;
        free(hash);
        return NULL;
    }

    CALLOC(hash->hash_table, hash->buckets, sizeof(alpm_list_t *), \
                free(hash); return NULL);

    return hash;
}

static unsigned int get_hash_position(unsigned long name_hash,
        alpm_pkghash_t *hash)
{
    unsigned int position;

    position = name_hash % hash->buckets;

    /* collision resolution using open addressing with linear probing */
    while(hash->hash_table[position] != NULL) {
        position += stride;
        while(position >= hash->buckets) {
            position -= hash->buckets;
        }
    }

    return position;
}

/* Expand the hash table size to the next increment and rebin the entries */
static alpm_pkghash_t *rehash(alpm_pkghash_t *oldhash)
{
    alpm_pkghash_t *newhash;
    unsigned int newsize, i;

    /* Hash tables will need resized in two cases:
     *  - adding packages to the local database
     *  - poor estimation of the number of packages in sync database
     *
     * For small hash tables sizes (<500) the increase in size is by a
     * minimum of a factor of 2 for optimal rehash efficiency.  For
     * larger database sizes, this increase is reduced to avoid excess
     * memory allocation as both scenarios requiring a rehash should not
     * require a table size increase that large. */
    if(oldhash->buckets < 500) {
        newsize = oldhash->buckets * 2;
    } else if(oldhash->buckets < 2000) {
        newsize = oldhash->buckets * 3 / 2;
    } else if(oldhash->buckets < 5000) {
        newsize = oldhash->buckets * 4 / 3;
    } else {
        newsize = oldhash->buckets + 1;
    }

    newhash = _alpm_pkghash_create(newsize);
    if(newhash == NULL) {
        /* creation of newhash failed, stick with old one... */
        return oldhash;
    }

    newhash->list = oldhash->list;
    oldhash->list = NULL;

    for(i = 0; i < oldhash->buckets; i++) {
        if(oldhash->hash_table[i] != NULL) {
            alpm_pkg_t *package = oldhash->hash_table[i]->data;
            unsigned int position = get_hash_position(package->name_hash, newhash);

            newhash->hash_table[position] = oldhash->hash_table[i];
            oldhash->hash_table[i] = NULL;
        }
    }

    newhash->entries = oldhash->entries;

    _alpm_pkghash_free(oldhash);

    return newhash;
}

static alpm_pkghash_t *pkghash_add_pkg(alpm_pkghash_t *hash, alpm_pkg_t *pkg,
        int sorted)
{
    alpm_list_t *ptr;
    unsigned int position;

    if(pkg == NULL || hash == NULL) {
        return hash;
    }

    if(hash->entries >= hash->limit) {
        hash = rehash(hash);
    }

    position = get_hash_position(pkg->name_hash, hash);

    ptr = malloc(sizeof(alpm_list_t));
    if(ptr == NULL) {
        return hash;
    }

    ptr->data = pkg;
    ptr->prev = ptr;
    ptr->next = NULL;

    hash->hash_table[position] = ptr;
    if(!sorted) {
        hash->list = alpm_list_join(hash->list, ptr);
    } else {
        hash->list = alpm_list_mmerge(hash->list, ptr, _alpm_pkg_cmp);
    }

    hash->entries += 1;
    return hash;
}

alpm_pkghash_t *_alpm_pkghash_add(alpm_pkghash_t *hash, alpm_pkg_t *pkg)
{
    return pkghash_add_pkg(hash, pkg, 0);
}

alpm_pkghash_t *_alpm_pkghash_add_sorted(alpm_pkghash_t *hash, alpm_pkg_t *pkg)
{
    return pkghash_add_pkg(hash, pkg, 1);
}

static unsigned int move_one_entry(alpm_pkghash_t *hash,
        unsigned int start, unsigned int end)
{
    /* Iterate backwards from 'end' to 'start', seeing if any of the items
     * would hash to 'start'. If we find one, we move it there and break.  If
     * we get all the way back to position and find none that hash to it, we
     * also end iteration. Iterating backwards helps prevent needless shuffles;
     * we will never need to move more than one item per function call.  The
     * return value is our current iteration location; if this is equal to
     * 'start' we can stop this madness. */
    while(end != start) {
        alpm_list_t *i = hash->hash_table[end];
        alpm_pkg_t *info = i->data;
        unsigned int new_position = get_hash_position(info->name_hash, hash);

        if(new_position == start) {
            hash->hash_table[start] = i;
            hash->hash_table[end] = NULL;
            break;
        }

        /* the odd math ensures we are always positive, e.g.
         * e.g. (0 - 1) % 47      == -1
         * e.g. (47 + 0 - 1) % 47 == 46 */
        end = (hash->buckets + end - stride) % hash->buckets;
    }
    return end;
}

/**
 * @brief Remove a package from a pkghash.
 *
 * @param hash     the hash to remove the package from
 * @param pkg      the package we are removing
 * @param data     output parameter containing the removed item
 *
 * @return the resultant hash
 */
alpm_pkghash_t *_alpm_pkghash_remove(alpm_pkghash_t *hash, alpm_pkg_t *pkg,
        alpm_pkg_t **data)
{
    alpm_list_t *i;
    unsigned int position;

    if(data) {
        *data = NULL;
    }

    if(pkg == NULL || hash == NULL) {
        return hash;
    }

    position = pkg->name_hash % hash->buckets;
    while((i = hash->hash_table[position]) != NULL) {
        alpm_pkg_t *info = i->data;

        if(info->name_hash == pkg->name_hash &&
                    strcmp(info->name, pkg->name) == 0) {
            unsigned int stop, prev;

            /* remove from list and hash */
            hash->list = alpm_list_remove_item(hash->list, i);
            if(data) {
                *data = info;
            }
            hash->hash_table[position] = NULL;
            free(i);
            hash->entries -= 1;

            /* Potentially move entries following removed entry to keep open
             * addressing collision resolution working. We start by finding the
             * next null bucket to know how far we have to look. */
            stop = position + stride;
            while(stop >= hash->buckets) {
                stop -= hash->buckets;
            }
            while(hash->hash_table[stop] != NULL && stop != position) {
                stop += stride;
                while(stop >= hash->buckets) {
                    stop -= hash->buckets;
                }
            }
            stop = (hash->buckets + stop - stride) % hash->buckets;

            /* We now search backwards from stop to position. If we find an
             * item that now hashes to position, we will move it, and then try
             * to plug the new hole we just opened up, until we finally don't
             * move anything. */
            while((prev = move_one_entry(hash, position, stop)) != position) {
                position = prev;
            }

            return hash;
        }

        position += stride;
        while(position >= hash->buckets) {
            position -= hash->buckets;
        }
    }

    return hash;
}

void _alpm_pkghash_free(alpm_pkghash_t *hash)
{
    if(hash != NULL) {
        unsigned int i;
        for(i = 0; i < hash->buckets; i++) {
            free(hash->hash_table[i]);
        }
        free(hash->hash_table);
    }
    free(hash);
}

alpm_pkg_t *_alpm_pkghash_find(alpm_pkghash_t *hash, const char *name)
{
    alpm_list_t *lp;
    unsigned long name_hash;
    unsigned int position;

    if(name == NULL || hash == NULL) {
        return NULL;
    }

    name_hash = _alpm_hash_sdbm(name);

    position = name_hash % hash->buckets;

    while((lp = hash->hash_table[position]) != NULL) {
        alpm_pkg_t *info = lp->data;

        if(info->name_hash == name_hash && strcmp(info->name, name) == 0) {
            return info;
        }

        position += stride;
        while(position >= hash->buckets) {
            position -= hash->buckets;
        }
    }

    return NULL;
}

/* vim: set ts=2 sw=2 noet: */