#pragma once

#define ZSKIPLIST_MAXLEVEL 32 /* Should be enough for 2^64 elements */
#define ZSKIPLIST_P 0.25      /* Skiplist P = 1/4 */

/* ZSETs use a specialized version of Skiplists */
typedef struct zskiplistNode {
    char * ele;
    double score;
    struct zskiplistNode * backward;
    struct zskiplistLevel {
        struct zskiplistNode * forward;
        unsigned long span;
    } level[];
} zskiplistNode;

typedef struct zskiplist {
    struct zskiplistNode * header, * tail;
    unsigned long length;
    int level;
} zskiplist;

/*
 * Copyright (c) 2009-2012, Salvatore Sanfilippo <antirez at gmail dot com>
 * Copyright (c) 2009-2012, Pieter Noordhuis <pcnoordhuis at gmail dot com>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   * Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *   * 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.
 *   * Neither the name of Redis nor the names of its contributors may be used
 *     to endorse or promote products derived from this software without
 *     specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS 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 COPYRIGHT OWNER 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.
 */

/*-----------------------------------------------------------------------------
 * Sorted set API
 *----------------------------------------------------------------------------*/

/* ZSETs are ordered sets using two data structures to hold the same elements
 * in order to get O(log(N)) INSERT and REMOVE operations into a sorted
 * data structure.
 *
 * The elements are added to a hash table mapping Redis objects to scores.
 * At the same time the elements are added to a skip list mapping scores
 * to Redis objects (so objects are sorted by scores in this "view").
 *
 * Note that the SDS string representing the element is the same in both
 * the hash table and skiplist in order to save memory. What we do in order
 * to manage the shared SDS string more easily is to free the SDS string
 * only in zslFreeNode(). The dictionary has no value free method set.
 * So we should always remove an element from the dictionary, and later from
 * the skiplist.
 *
 * This skiplist implementation is almost a C translation of the original
 * algorithm described by William Pugh in "Skip Lists: A Probabilistic
 * Alternative to Balanced Trees", modified in three ways:
 * a) this implementation allows for repeated scores.
 * b) the comparison is not just by key (our ‘score‘) but by satellite data.
 * c) there is a back pointer, so it‘s a doubly linked list with the back
 * pointers being only at "level 1". This allows to traverse the list
 * from tail to head, useful for ZREVRANGE. */

#include "server.h"
#include "struct.h"

/*-----------------------------------------------------------------------------
 * Skiplist implementation of the low level API
 *----------------------------------------------------------------------------*/

/* Create a skiplist node with the specified number of levels.
 * The SDS string ‘ele‘ is referenced by the node after the call. */
zskiplistNode * zslCreateNode(int level, double score, char * ele) {
    zskiplistNode * zn = malloc(
        sizeof(struct zskiplistNode) + level * sizeof(struct zskiplistLevel)
    );
    if (!zn) {
        return NULL;
    } 
    zn->score = score;
    zn->ele = ele;
    return zn;
}

/* Create a new skiplist. */
zskiplist * zslCreate(void) {
    int j;
    zskiplist * zsl = malloc(sizeof(struct zskiplist));
    if (!zsl) {
        return NULL;
    }

    zsl->level = 1;
    zsl->length = 0;
    zsl->header = zslCreateNode(ZSKIPLIST_MAXLEVEL, 0, NULL);
    if (!zsl->header) {
        free(zsl);
        return NULL;
    }
    for (j = 0; j < ZSKIPLIST_MAXLEVEL; j++) {
        zsl->header->level[j].forward = NULL;
        zsl->header->level[j].span = 0;
    }
    zsl->header->backward = NULL;
    zsl->tail = NULL;
    return zsl;
}

/* Free the specified skiplist node. The referenced SDS string representation
 * of the element is freed too, unless node->ele is set to NULL before calling
 * this function. */
void zslFreeNode(zskiplistNode * node) {
    free(node->ele);
    free(node);
}

/* Free a whole skiplist. */
void zslFree(zskiplist * zsl) {
    zskiplistNode * next;
    zskiplistNode * node = zsl->header->level[0].forward;

    free(zsl->header);
    while (node) {
        next = node->level[0].forward;
        zslFreeNode(node);
        node = next;
    }
    free(zsl);
}

/* Returns a random level for the new skiplist node we are going to create.
 * The return value of this function is between 1 and ZSKIPLIST_MAXLEVEL
 * (both inclusive), with a powerlaw-alike distribution where higher
 * levels are less likely to be returned. */
int zslRandomLevel(void) {
    int level = 1;
    while ((random()&0xFFFF) < (ZSKIPLIST_P*0xFFFF) && ++level < ZSKIPLIST_MAXLEVEL)
        ;
    return level;
}

/* Insert a new node in the skiplist. Assumes the element does not already
 * exist (up to the caller to enforce that). The skiplist takes ownership
 * of the passed SDS string ‘ele‘. */
zskiplistNode * zslInsert(zskiplist * zsl, double score, char * ele) {
    zskiplistNode * update[ZSKIPLIST_MAXLEVEL], * x;
    unsigned long rank[ZSKIPLIST_MAXLEVEL];
    int i, level;

    assert(!isnan(score));
    x = zsl->header;
    for (i = zsl->level-1; i>=0; i--) {
        /* store rank that is crossed to reach the insert position */
        rank[i] = i == zsl->level-1 ? 0 : rank[i+1];
        while (
            x->level[i].forward && (
                x->level[i].forward->score < score || 
                    (x->level[i].forward->score == score && 
                        strcmp(x->level[i].forward->ele, ele) < 0)
            )
        ) {
            rank[i] += x->level[i].span;
            x = x->level[i].forward;
        }
        update[i] = x;
    }

    /* we assume the element is not already inside, since we allow duplicated
     * scores, reinserting the same element should never happen since the
     * caller of zslInsert() should test in the hash table if the element is
     * already inside or not. */
    level = zslRandomLevel();
    if (level > zsl->level) {
        for (i = zsl->level; i < level; i++) {
            rank[i] = 0;
            update[i] = zsl->header;
            update[i]->level[i].span = zsl->length;
        }
        zsl->level = level;
    }
    x = zslCreateNode(level, score, ele);
    if (!x) {
        return NULL;
    }
    for (i = 0; i < level; i++) {
        x->level[i].forward = update[i]->level[i].forward;
        update[i]->level[i].forward = x;

        /* update span covered by update[i] as x is inserted here */
        x->level[i].span = update[i]->level[i].span - (rank[0] - rank[i]);
        update[i]->level[i].span = (rank[0] - rank[i]) + 1;
    }

    /* increment span for untouched levels */
    for (i = level; i < zsl->level; i++) {
        update[i]->level->span++;
    }

    x->backward = (update[0] == zsl->header) ? NULL : update[0];
    if (x->level[0].forward)
        x->level[0].forward->backward = x;
    else
        zsl->tail = x;
    zsl->length++;
    return x;
}

/* Internal function used by zslDelete, zslDeleteRangeByScore and
 * zslDeleteRangeByRank. */
void zslDeleteNode(zskiplist * zsl, zskiplistNode * x, zskiplistNode ** update) {
    int i;
    
    for (i = 0; i < zsl->level; i++) {
        if (update[i]->level[i].forward == x) {
            update[i]->level[i].span += x->level[i].span - 1;
            update[i]->level[i].forward = x->level[i].forward;
        } else {
            update[i]->level[i].span -= 1;
        }
    }
    if (x->level[0].forward) {
        x->level[0].forward->backward = x->backward;
    } else {
        zsl->tail = x->backward;
    }
    while (zsl->level > 1 && zsl->header->level[zsl->level-1].forward == NULL)
        zsl->length--;
    zsl->length--;
}

/* Delete an element with matching score/element from the skiplist.
 * The function returns 1 if the node was found and deleted, otherwise
 * 0 is returned.
 *
 * If ‘node‘ is NULL the deleted node is freed by zslFreeNode(), otherwise
 * it is not freed (but just unlinked) and *node is set to the node pointer,
 * so that it is possible for the caller to reuse the node (including the
 * referenced SDS string at node->ele). */
int zslDelete(zskiplist * zsl, double score, char * ele, zskiplistNode ** node) {
    zskiplistNode * update[ZSKIPLIST_MAXLEVEL], * x;
    int i;

    x = zsl->header;
    for (i = zsl->level-1; i >= 0; i--) {
        while (x->level[i].forward &&
                (x->level[i].forward->score < score ||
                    (x->level[i].forward->score == score && 
                        strcmp(x->level[i].forward->ele, ele) < 0))) {
            x = x->level[i].forward;
        }
        update[i] = x;
    }

    /* We may have multiple elements with the same score, what we need
     * is to find the element with both the right score and object. */
    x = x->level[0].forward;
    if (x && score == x->score && strcmp(x->ele, ele) == 0) {
        zslDeleteNode(zsl, x, update);
        if (!node)
            zslFreeNode(x);
        else
            *node = x;
        return 1;
    }
    return 0; /* not found */
}

/* Update the score of an element inside the sorted set skiplist.
 * Note that the element must exist and must match ‘score‘.
 * This function does not update the score in the hash table side, the
 * caller should take care of it.
 *
 * Note that this function attempts to just update the node, in case after
 * the score update, the node would be exactly at the same position.
 * Otherwise the skiplist is modified by removing and re-adding a new
 * element, which is more costly.
 *
 * The function returns the updated element skiplist node pointer. */
zskiplistNode * zslUpdateScore(zskiplist * zsl, double curscore, char * ele, double newscore) {
    zskiplistNode * update[ZSKIPLIST_MAXLEVEL], * x;
    int i;

    /* We need to seek to element to update to start: this is useful anyway,
     * we‘ll have to update or remove it. */
    x = zsl->header;
    for (i = zsl->level-1; i >= 0; i--) {
        while (x->level[i].forward &&
                (x->level[i].forward->score < curscore ||
                    (x->level[i].forward->score == curscore && 
                        strcmp(x->level[i].forward->ele, ele) < 0))) {
            x = x->level[i].forward;
        }
        update[i] = x;
    }

    /* Jump to our element: note that this function assumes that the
     * element with the matching score exists. */
    x = x->level[0].forward;
    assert(x && curscore == x->score && strcmp(x->ele, ele) == 0);

    /* If the node, after the score update, would be still exactly
     * at the same position, we can just update the score without
     * actually removing and re-inserting the element in the skiplist. */
    if ((!x->backward || x->backward->score < newscore) && 
        (!x->level[0].forward || x->level[0].forward->score > newscore)) {
        x->score = newscore;
        return x;
    }

    /* No way to reuse the old node: we need to remove and insert a new
     * one at a different place. */
    zslDeleteNode(zsl, x, update);
    zskiplistNode * newnode = zslInsert(zsl, newscore, x->ele);
    /* We reused the old node x->ele SDS string, free the node now
     * since zslInsert created a new one. */
    x->ele = NULL;
    zslFreeNode(x);
    return newnode;
}