skiplist.c

/*
 *  author: xjdrew
 *  date: 2014-06-03 20:38
 */

// skiplist similar with the version in redis
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

#include "skiplist.h"


skiplistNode *slCreateNode(int level, double score, slobj *obj) {
    skiplistNode *n = malloc(sizeof(*n) + level * sizeof(struct skiplistLevel));
    n->score = score;
    n->obj   = obj;
    return n;
}

skiplist *slCreate(void) {
    int j;
    skiplist *sl;

    sl = malloc(sizeof(*sl));
    sl->level = 1;
    sl->length = 0;
    sl->header = slCreateNode(SKIPLIST_MAXLEVEL, 0, NULL);
    for (j=0; j < SKIPLIST_MAXLEVEL; j++) {
        sl->header->level[j].forward = NULL;
        sl->header->level[j].span = 0;
    }
    sl->header->backward = NULL;
    sl->tail = NULL;
    return sl;
}

slobj* slCreateObj(const char* ptr, size_t length) {
    slobj *obj = malloc(sizeof(*obj));
    obj->ptr    = malloc(length + 1);

    if(ptr) {
        memcpy(obj->ptr, ptr, length);
    }
    obj->ptr[length] = '\0';

    obj->length = length;
    return obj;
}

void slFreeObj(slobj *obj) {
    free(obj->ptr);
    free(obj);
}

void slFreeNode(skiplistNode *node) {
    slFreeObj(node->obj);
    free(node);
}

void slFree(skiplist *sl) {
    skiplistNode *node = sl->header->level[0].forward, *next;

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

int slRandomLevel(void) {
    int level = 1;
    while((random() & 0xffff) < (SKIPLIST_P * 0xffff))
        level += 1;
    return (level < SKIPLIST_MAXLEVEL) ? level : SKIPLIST_MAXLEVEL;
}

int compareslObj(slobj *a, slobj *b) {
    int cmp = memcmp(a->ptr, b->ptr, a->length <= b->length ? a->length : b->length);
    if(cmp == 0) return a->length - b->length;
    return cmp;
}

int equalslObj(slobj *a, slobj *b) {
    return compareslObj(a, b) == 0;
}

void slInsert(skiplist *sl, double score, slobj *obj) {
    skiplistNode *update[SKIPLIST_MAXLEVEL], *x;
    unsigned int rank[SKIPLIST_MAXLEVEL];
    int i, level;

    x = sl->header;
    for (i = sl->level-1; i >= 0; i--) {
        /* store rank that is crossed to reach the insert position */
        rank[i] = i == (sl->level-1) ? 0 : rank[i+1];
        while (x->level[i].forward &&
            (x->level[i].forward->score < score ||
                (x->level[i].forward->score == score &&
                compareslObj(x->level[i].forward->obj,obj) < 0))) {
            rank[i] += x->level[i].span;
            x = x->level[i].forward;
        }
        update[i] = x;
    }
    /* we assume the key is not already inside, since we allow duplicated
     * scores, and the re-insertion of score and redis object should never
     * happen since the caller of slInsert() should test in the hash table
     * if the element is already inside or not. */
    level = slRandomLevel();
    if (level > sl->level) {
        for (i = sl->level; i < level; i++) {
            rank[i] = 0;
            update[i] = sl->header;
            update[i]->level[i].span = sl->length;
        }
        sl->level = level;
    }
    x = slCreateNode(level,score,obj);
    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 < sl->level; i++) {
        update[i]->level[i].span++;
    }

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

/* Internal function used by slDelete, slDeleteByScore */
void slDeleteNode(skiplist *sl, skiplistNode *x, skiplistNode **update) {
    int i;
    for (i = 0; i < sl->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 {
        sl->tail = x->backward;
    }
    while(sl->level > 1 && sl->header->level[sl->level-1].forward == NULL)
        sl->level--;
    sl->length--;
}

/* Delete an element with matching score/object from the skiplist. */
int slDelete(skiplist *sl, double score, slobj *obj) {
    skiplistNode *update[SKIPLIST_MAXLEVEL], *x;
    int i;

    x = sl->header;
    for (i = sl->level-1; i >= 0; i--) {
        while (x->level[i].forward &&
            (x->level[i].forward->score < score ||
                (x->level[i].forward->score == score &&
                compareslObj(x->level[i].forward->obj,obj) < 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 && equalslObj(x->obj,obj)) {
        slDeleteNode(sl, x, update);
        slFreeNode(x);
        return 1;
    } else {
        return 0; /* not found */
    }
    return 0; /* not found */
}


/* Find the rank for an element by both score and key.
 * Returns 0 when the element cannot be found, rank otherwise.
 * Note that the rank is 1-based due to the span of sl->header to the
 * first element. */
unsigned long slGetRank(skiplist *sl, double score, slobj *o) {
    skiplistNode *x;
    unsigned long rank = 0;
    int i;

    x = sl->header;
    for (i = sl->level-1; i >= 0; i--) {
        while (x->level[i].forward &&
            (x->level[i].forward->score < score ||
                (x->level[i].forward->score == score &&
                compareslObj(x->level[i].forward->obj,o) <= 0))) {
            rank += x->level[i].span;
            x = x->level[i].forward;
        }

        /* x might be equal to sl->header, so test if obj is non-NULL */
        if (x->obj && equalslObj(x->obj, o)) {
            return rank;
        }
    }
    return 0;
}

/* Finds an element by its rank. The rank argument needs to be 1-based. */
skiplistNode* slGetNodeByRank(skiplist *sl, unsigned long rank) {
    if(rank == 0 || rank > sl->length) {
        return NULL;
    }

    skiplistNode *x;
    unsigned long traversed = 0;
    int i;

    x = sl->header;
    for (i = sl->level-1; i >= 0; i--) {
        while (x->level[i].forward && (traversed + x->level[i].span) <= rank)
        {
            traversed += x->level[i].span;
            x = x->level[i].forward;
        }
        if (traversed == rank) {
            return x;
        }
    }

    return NULL;
}

/* range [min, max], left & right both include */
/* Returns if there is a part of the zset is in range. */
int slIsInRange(skiplist *sl, double min, double max) {
    skiplistNode *x;

    /* Test for ranges that will always be empty. */
    if(min > max) {
        return 0;
    }
    x = sl->tail;
    if (x == NULL || x->score < min)
        return 0;

    x = sl->header->level[0].forward;
    if (x == NULL || x->score > max)
        return 0;
    return 1;
}

/* Find the first node that is contained in the specified range.
 * Returns NULL when no element is contained in the range. */
skiplistNode *slFirstInRange(skiplist *sl, double min, double max) {
    skiplistNode *x;
    int i;

    /* If everything is out of range, return early. */
    if (!slIsInRange(sl,min, max)) return NULL;

    x = sl->header;
    for (i = sl->level-1; i >= 0; i--) {
        /* Go forward while *OUT* of range. */
        while (x->level[i].forward && x->level[i].forward->score < min)
                x = x->level[i].forward;
    }

    /* This is an inner range, so the next node cannot be NULL. */
    x = x->level[0].forward;
    return x;
}

/* Find the last node that is contained in the specified range.
 * Returns NULL when no element is contained in the range. */
skiplistNode *slLastInRange(skiplist *sl, double min, double max) {
    skiplistNode *x;
    int i;

    /* If everything is out of range, return early. */
    if (!slIsInRange(sl, min, max)) return NULL;

    x = sl->header;
    for (i = sl->level-1; i >= 0; i--) {
        /* Go forward while *IN* range. */
        while (x->level[i].forward &&
            x->level[i].forward->score <= max)
                x = x->level[i].forward;
    }

    /* This is an inner range, so this node cannot be NULL. */
    return x;
}

void slDump(skiplist *sl) {
    skiplistNode *x;
    int i;

    x = sl->header;
    i = 0;
    while(x->level[0].forward) {
        x = x->level[0].forward;
        i++;
        printf("node %d: score:%f, member:%s\n", i, x->score, x->obj->ptr);
    }
}