toxcore/toxcore/list.c

/* list.h
 *
 * Simple struct with functions to create a list which associates ids with data
 * -Allows for finding ids associated with data such as IPs or public keys in a short time
 * -Should only be used if there are relatively few add/remove calls to the list
 *
 *  Copyright (C) 2014 Tox project All Rights Reserved.
 *
 *  This file is part of Tox.
 *
 *  Tox 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 3 of the License, or
 *  (at your option) any later version.
 *
 *  Tox 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 Tox.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

#include "list.h"

/* Basically, the elements in the list are placed in order so that they can be searched for easily
 * -each element is seen as a big-endian integer when ordering them
 * -the ids array is maintained so that each id always matches
 * -the search algorithm cuts down the time to find the id associated with a piece of data
 *   at the cost of slow add/remove functions for large lists
 * -Starts at 1/2 of the array, compares the element in the array with the data,
 *   then moves +/- 1/4 of the array depending on whether the value is greater or lower,
 *   then +- 1/8, etc, until the value is matched or its position where it should be in the array is found
 * -some considerations since the array size is never perfect
 */

#define INDEX(i) (~i)

/* Find data in list
 *
 * return value:
 *  >= 0 : index of data in array
 *  < 0  : no match, returns index (return value is INDEX(index)) where
 *         the data should be inserted
 */
static int find(const BS_LIST *list, const uint8_t *data)
{
    //should work well, but could be improved
    if (list->n == 0) {
        return INDEX(0);
    }

    uint32_t i = list->n / 2; //current position in the array
    uint32_t delta = i / 2;   //how much we move in the array

    if (!delta) {
        delta = 1;
    }

    int d = -1; //used to determine if closest match is found
    //closest match is found if we move back to where we have already been

    while (1) {
        int r = memcmp(data, list->data + list->element_size * i, list->element_size);

        if (r == 0) {
            return i;
        }

        if (r > 0) {
            //data is greater
            //move down
            i += delta;

            if (d == 0 || i == list->n) {
                //reached bottom of list, or closest match
                return INDEX(i);
            }

            delta = (delta) / 2;

            if (delta == 0) {
                delta = 1;
                d = 1;
            }
        } else {
            //data is smaller
            if (d == 1 || i == 0) {
                //reached top or list or closest match
                return INDEX(i);
            }

            //move up
            i -= delta;

            delta = (delta) / 2;

            if (delta == 0) {
                delta = 1;
                d = 0;
            }
        }
    }
}

/* Resized the list list
 *
 * return value:
 *  1 : success
 *  0 : failure
 */
static int resize(BS_LIST *list, uint32_t new_size)
{
    void *p;

    p = realloc(list->data, list->element_size * new_size);

    if (!p) {
        return 0;
    } else {
        list->data = p;
    }

    p = realloc(list->ids, sizeof(int) * new_size);

    if (!p) {
        return 0;
    } else {
        list->ids = p;
    }

    return 1;
}


int bs_list_init(BS_LIST *list, uint32_t element_size, uint32_t initial_capacity)
{
    //set initial values
    list->n = 0;
    list->element_size = element_size;
    list->capacity = 0;
    list->data = NULL;
    list->ids = NULL;

    if (initial_capacity != 0) {
        if (!resize(list, initial_capacity)) {
            return 0;
        }
    }

    list->capacity = initial_capacity;

    return 1;
}

void bs_list_free(BS_LIST *list)
{
    //free both arrays
    free(list->data);
    free(list->ids);
}

int bs_list_find(const BS_LIST *list, const uint8_t *data)
{
    int r = find(list, data);

    //return only -1 and positive values
    if (r < 0) {
        return -1;
    }

    return list->ids[r];
}

int bs_list_add(BS_LIST *list, const uint8_t *data, int id)
{
    //find where the new element should be inserted
    //see: return value of find()
    int i = find(list, data);

    if (i >= 0) {
        //already in list
        return 0;
    }

    i = ~i;

    //increase the size of the arrays if needed
    if (list->n == list->capacity) {
        // 1.5 * n + 1
        const uint32_t new_capacity = list->n + list->n / 2 + 1;

        if (!resize(list, new_capacity)) {
            return 0;
        }

        list->capacity = new_capacity;
    }

    //insert data to element array
    memmove(list->data + (i + 1) * list->element_size, list->data + i * list->element_size,
            (list->n - i) * list->element_size);
    memcpy(list->data + i * list->element_size, data, list->element_size);

    //insert id to id array
    memmove(&list->ids[i + 1], &list->ids[i], (list->n - i) * sizeof(int));
    list->ids[i] = id;

    //increase n
    list->n++;

    return 1;
}

int bs_list_remove(BS_LIST *list, const uint8_t *data, int id)
{
    int i = find(list, data);

    if (i < 0) {
        return 0;
    }

    if (list->ids[i] != id) {
        //this should never happen
        return 0;
    }

    //decrease the size of the arrays if needed
    if (list->n < list->capacity / 2) {
        const uint32_t new_capacity = list->capacity / 2;

        if (resize(list, new_capacity)) {
            list->capacity = new_capacity;
        }
    }

    list->n--;

    memmove(list->data + i * list->element_size, list->data + (i + 1) * list->element_size,
            (list->n - i) * list->element_size);
    memmove(&list->ids[i], &list->ids[i + 1], (list->n - i) * sizeof(int));

    return 1;
}

int bs_list_trim(BS_LIST *list)
{
    if (!resize(list, list->n)) {
        return 0;
    }

    list->capacity = list->n;
    return 1;
}