Redis（八）：zset/zadd/zrange/zrembyscore 命令源码解析

/* ZSETs use a specialized version of Skiplists */
typedef struct zskiplistNode {
    sds ele;
    double score;
    struct zskiplistNode *backward;
    struct zskiplistLevel {
        struct zskiplistNode *forward;
        unsigned int span;
    } level[];
} zskiplistNode;
// 跳跃链表
typedef struct zskiplist {
    struct zskiplistNode *header, *tail;
    unsigned long length;
    int level;
} zskiplist;
// zset 主数据结构，dict + zskiplist
typedef struct zset {
    dict *dict;
    zskiplist *zsl;
} zset;
// zset 在合适场景下，将先使用 ziplist 存储数据
typedef struct zlentry {
    unsigned int prevrawlensize, prevrawlen;
    unsigned int lensize, len;
    unsigned int headersize;
    unsigned char encoding;
    unsigned char *p;
} zlentry;

// 用法: ZADD key score1 member1 [score2 member2]
// t_zset.c
void zaddCommand(client *c) {
    // zadd 的多个参数变形, 使用 flags 进行区分复用
    zaddGenericCommand(c,ZADD_NONE);
}
void zaddGenericCommand(client *c, int flags) {
    static char *nanerr = "resulting score is not a number (NaN)";
    robj *key = c->argv[1];
    robj *zobj;
    sds ele;
    double score = 0, *scores = NULL, curscore = 0.0;
    int j, elements;
    int scoreidx = 0;
    /* The following vars are used in order to track what the command actually
     * did during the execution, to reply to the client and to trigger the
     * notification of keyspace change. */
    int added = 0;      /* Number of new elements added. */
    int updated = 0;    /* Number of elements with updated score. */
    int processed = 0;  /* Number of elements processed, may remain zero with
                           options like XX. */

    /* Parse options. At the end 'scoreidx' is set to the argument position
     * of the score of the first score-element pair. */
    // 从第三位置开始尝试解析特殊标识（用法规范）
    // 按位与到 flags 中
    scoreidx = 2;
    while(scoreidx < c->argc) {
        char *opt = c->argv[scoreidx]->ptr;
        // NX: 不更新已存在的元素，只做添加操作
        if (!strcasecmp(opt,"nx")) flags |= ZADD_NX;
        // XX: 只做更新操作，不做添加操作
        else if (!strcasecmp(opt,"xx")) flags |= ZADD_XX;
        // CH: 将返回值从添加的新元素数修改为已更改元素的总数。 更改的元素是第添加的新元素以及已为其更新分数的现有元素。 因此，命令行中指定的具有与过去相同分数的元素将不计算在内。 注意：通常，ZADD的返回值仅计算添加的新元素的数量。
        else if (!strcasecmp(opt,"ch")) flags |= ZADD_CH;
        // INCR: 使用指定元素增加指定分数, 与 ZINCRBY 类似，此场景下，只允许操作一个元素
        else if (!strcasecmp(opt,"incr")) flags |= ZADD_INCR;
        else break;
        scoreidx++;
    }

    /* Turn options into simple to check vars. */
    int incr = (flags & ZADD_INCR) != 0;
    int nx = (flags & ZADD_NX) != 0;
    int xx = (flags & ZADD_XX) != 0;
    int ch = (flags & ZADD_CH) != 0;

    /* After the options, we expect to have an even number of args, since
     * we expect any number of score-element pairs. */
    // 把特殊标识去除后，剩下的参数列表应该2n数，即 score-element 一一配对的，否则语法错误
    elements = c->argc-scoreidx;
    if (elements % 2) {
        addReply(c,shared.syntaxerr);
        return;
    }
    elements /= 2; /* Now this holds the number of score-element pairs. */

    /* Check for incompatible options. */
    // 互斥项
    if (nx && xx) {
        addReplyError(c,
            "XX and NX options at the same time are not compatible");
        return;
    }
    // 语法检查，INCR 只能针对1个元素操作
    if (incr && elements > 1) {
        addReplyError(c,
            "INCR option supports a single increment-element pair");
        return;
    }

    /* Start parsing all the scores, we need to emit any syntax error
     * before executing additions to the sorted set, as the command should
     * either execute fully or nothing at all. */
    // 解析所有的 score 值为double类型，赋值到 scores 中
    scores = zmalloc(sizeof(double)*elements);
    for (j = 0; j < elements; j++) {
        if (getDoubleFromObjectOrReply(c,c->argv[scoreidx+j*2],&scores[j],NULL)
            != C_OK) goto cleanup;
    }

    /* Lookup the key and create the sorted set if does not exist. */
    // 语法检查
    zobj = lookupKeyWrite(c->db,key);
    if (zobj == NULL) {
        if (xx) goto reply_to_client; /* No key + XX option: nothing to do. */
        // 创建原始key对象
        // 默认 zset_max_ziplist_entries=OBJ_ZSET_MAX_ZIPLIST_ENTRIES: 128
        // 默认 zset_max_ziplist_value=OBJ_ZSET_MAX_ZIPLIST_VALUE: 64
        // 所以此处默认主要是检查 第1个member的长度是大于 64
        if (server.zset_max_ziplist_entries == 0 ||
            server.zset_max_ziplist_value < sdslen(c->argv[scoreidx+1]->ptr))
        {
            // 2. 通用情况使用 dict+quicklist 型的zset 
            zobj = createZsetObject();
        } else {
            // 1. 元素比较小的情况下创建 ziplist 型的 zset
            zobj = createZsetZiplistObject();
        }
        // 将对象添加到db中，后续所有操作针对 zobj 操作即是对db的操作 (引用传递)
        dbAdd(c->db,key,zobj);
    } else {
        if (zobj->type != OBJ_ZSET) {
            addReply(c,shared.wrongtypeerr);
            goto cleanup;
        }
    }
    // 一个个元素循环添加
    for (j = 0; j < elements; j++) {
        score = scores[j];

        ele = c->argv[scoreidx+1+j*2]->ptr;
        // 分当前zobj的编码不同进行添加 (ziplist, skiplist)
        // 3. ZIPLIST 编码下的zset添加操作
        if (zobj->encoding == OBJ_ENCODING_ZIPLIST) {
            unsigned char *eptr;
            // 3.1. 查找是否存在要添加的元素 （确定添加或更新）
            if ((eptr = zzlFind(zobj->ptr,ele,&curscore)) != NULL) {
                if (nx) continue;
                if (incr) {
                    score += curscore;
                    if (isnan(score)) {
                        addReplyError(c,nanerr);
                        goto cleanup;
                    }
                }

                /* Remove and re-insert when score changed. */
                if (score != curscore) {
                    // 3.2. 元素更新操作，先删再插入
                    zobj->ptr = zzlDelete(zobj->ptr,eptr);
                    zobj->ptr = zzlInsert(zobj->ptr,ele,score);
                    server.dirty++;
                    updated++;
                }
                processed++;
            } else if (!xx) {
                /* Optimize: check if the element is too large or the list
                 * becomes too long *before* executing zzlInsert. */
                zobj->ptr = zzlInsert(zobj->ptr,ele,score);
                // 5. 超过一条件后，做 ziplist->skiplist 转换
                // 默认 元素个数>128, 当前元素>64
                // 这两个判断不会重复吗？？ 两个原因: 1. 转换函数内部会重新判定; 2. 下一次循环时不会再走当前逻辑;
                if (zzlLength(zobj->ptr) > server.zset_max_ziplist_entries)
                    zsetConvert(zobj,OBJ_ENCODING_SKIPLIST);
                if (sdslen(ele) > server.zset_max_ziplist_value)
                    zsetConvert(zobj,OBJ_ENCODING_SKIPLIST);
                server.dirty++;
                added++;
                processed++;
            }
        } 
        // 4. skiplist 下的zset元素添加
        else if (zobj->encoding == OBJ_ENCODING_SKIPLIST) {
            zset *zs = zobj->ptr;
            zskiplistNode *znode;
            dictEntry *de;
            // 判断ele是否已存在，使用hash查找，快速
            de = dictFind(zs->dict,ele);
            if (de != NULL) {
                if (nx) continue;
                curscore = *(double*)dictGetVal(de);

                if (incr) {
                    score += curscore;
                    if (isnan(score)) {
                        addReplyError(c,nanerr);
                        /* Don't need to check if the sorted set is empty
                         * because we know it has at least one element. */
                        goto cleanup;
                    }
                }

                /* Remove and re-insert when score changes. */
                // 先删再插入 skiplist
                if (score != curscore) {
                    zskiplistNode *node;
                    serverAssert(zslDelete(zs->zsl,curscore,ele,&node));
                    znode = zslInsert(zs->zsl,score,node->ele);
                    /* We reused the node->ele SDS string, free the node now
                     * since zslInsert created a new one. */
                    node->ele = NULL;
                    zslFreeNode(node);
                    /* Note that we did not removed the original element from
                     * the hash table representing the sorted set, so we just
                     * update the score. */
                    // 更新dict中的分数引用
                    dictGetVal(de) = &znode->score; /* Update score ptr. */
                    server.dirty++;
                    updated++;
                }
                processed++;
            } else if (!xx) {
                ele = sdsdup(ele);
                znode = zslInsert(zs->zsl,score,ele);
                // 添加skiplist的同时，也往 dict 中添加一份数据，因为hash的查找永远是最快的
                serverAssert(dictAdd(zs->dict,ele,&znode->score) == DICT_OK);
                server.dirty++;
                added++;
                processed++;
            }
        } else {
            serverPanic("Unknown sorted set encoding");
        }
    }

reply_to_client:
    if (incr) { /* ZINCRBY or INCR option. */
        if (processed)
            addReplyDouble(c,score);
        else
            addReply(c,shared.nullbulk);
    } else { /* ZADD. */
        addReplyLongLong(c,ch ? added+updated : added);
    }

cleanup:
    zfree(scores);
    if (added || updated) {
        signalModifiedKey(c->db,key);
        notifyKeyspaceEvent(NOTIFY_ZSET,
            incr ? "zincr" : "zadd", key, c->db->id);
    }
}

// 1. 元素比较小的情况下创建 ziplist 型的 zset
// object.c, 创建ziplist 的zset
robj *createZsetZiplistObject(void) {
    unsigned char *zl = ziplistNew();
    robj *o = createObject(OBJ_ZSET,zl);
    o->encoding = OBJ_ENCODING_ZIPLIST;
    return o;
}
// 2. 创建通用的 zset 实例
// object.c
robj *createZsetObject(void) {
    zset *zs = zmalloc(sizeof(*zs));
    robj *o;
    // zsetDictType 稍有不同
    zs->dict = dictCreate(&zsetDictType,NULL);
    // 首次遇到 skiplist, 咱去瞅瞅是如何创建的
    zs->zsl = zslCreate();
    o = createObject(OBJ_ZSET,zs);
    o->encoding = OBJ_ENCODING_SKIPLIST;
    return o;
}
// server.c, zset创建时使用的dict类型，与hash有不同
/* Sorted sets hash (note: a skiplist is used in addition to the hash table) */
dictType zsetDictType = {
    dictSdsHash,               /* hash function */
    NULL,                      /* key dup */
    NULL,                      /* val dup */
    dictSdsKeyCompare,         /* key compare */
    NULL,                      /* Note: SDS string shared & freed by skiplist */
    NULL                       /* val destructor */
};
// 创建 skiplist 对象
/* Create a new skiplist. */
zskiplist *zslCreate(void) {
    int j;
    zskiplist *zsl;

    zsl = zmalloc(sizeof(*zsl));
    zsl->level = 1;
    zsl->length = 0;
    // 创建header节点，ZSKIPLIST_MAXLEVEL 32
    zsl->header = zslCreateNode(ZSKIPLIST_MAXLEVEL,0,NULL);
    // 初始化header
    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;
}
/* 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, sds ele) {
    zskiplistNode *zn =
        zmalloc(sizeof(*zn)+level*sizeof(struct zskiplistLevel));
    zn->score = score;
    zn->ele = ele;
    return zn;
}


// 3. ZIPLIST 编码下的zset添加操作
// 3.1. 查找是否存在要添加的元素 （确定添加或更新）
// t_zset.c, 查找指定ele
unsigned char *zzlFind(unsigned char *zl, sds ele, double *score) {
    unsigned char *eptr = ziplistIndex(zl,0), *sptr;
    // 遍历所有ziplist
    // 可见，此时的ziplist并没有表现出有序啊
    while (eptr != NULL) {
        // eptr 相当于是 key
        // sptr 相当于score
        sptr = ziplistNext(zl,eptr);
        serverAssert(sptr != NULL);

        if (ziplistCompare(eptr,(unsigned char*)ele,sdslen(ele))) {
            /* Matching element, pull out score. */
            // 找到相应的 key 后，解析下一值，即 score
            if (score != NULL) *score = zzlGetScore(sptr);
            return eptr;
        }
        /* Move to next element. */
        // 移动两次对象，才会到下一元素(因为存储是 key-score 相邻存储)
        eptr = ziplistNext(zl,sptr);
    }
    return NULL;
}
// t_zset.c, 获取元素的score
double zzlGetScore(unsigned char *sptr) {
    unsigned char *vstr;
    unsigned int vlen;
    long long vlong;
    char buf[128];
    double score;

    serverAssert(sptr != NULL);
    serverAssert(ziplistGet(sptr,&vstr,&vlen,&vlong));
    // 带小数点不带小数点
    if (vstr) {
        memcpy(buf,vstr,vlen);
        buf[vlen] = '\0';
        // 做类型转换
        score = strtod(buf,NULL);
    } else {
        score = vlong;
    }

    return score;
}

// 3.2. 元素更新操作，先删再插入
// t_zset.c
/* Delete (element,score) pair from ziplist. Use local copy of eptr because we
 * don't want to modify the one given as argument. */
unsigned char *zzlDelete(unsigned char *zl, unsigned char *eptr) {
    unsigned char *p = eptr;

    /* TODO: add function to ziplist API to delete N elements from offset. */
    zl = ziplistDelete(zl,&p);
    zl = ziplistDelete(zl,&p);
    return zl;
}
// 添加 ele-score 到 ziplist 中
/* Insert (element,score) pair in ziplist. This function assumes the element is
 * not yet present in the list. */
unsigned char *zzlInsert(unsigned char *zl, sds ele, double score) {
    unsigned char *eptr = ziplistIndex(zl,0), *sptr;
    double s;
    // 在上面查找时，我们看到ziplist也是遍历，以为是无序的ziplist
    // 然而实际上，插入时是维护了顺序的哟
    while (eptr != NULL) {
        sptr = ziplistNext(zl,eptr);
        serverAssert(sptr != NULL);
        s = zzlGetScore(sptr);
        // 找到第一个比score大的位置，在其前面插入 ele-score
        if (s > score) {
            /* First element with score larger than score for element to be
             * inserted. This means we should take its spot in the list to
             * maintain ordering. */
            zl = zzlInsertAt(zl,eptr,ele,score);
            break;
        } else if (s == score) {
            /* Ensure lexicographical ordering for elements. */
            // 当分数相同时，按字典顺序排列
            if (zzlCompareElements(eptr,(unsigned char*)ele,sdslen(ele)) > 0) {
                zl = zzlInsertAt(zl,eptr,ele,score);
                break;
            }
        }

        /* Move to next element. */
        eptr = ziplistNext(zl,sptr);
    }

    /* Push on tail of list when it was not yet inserted. */
    // 以上遍历完成都没有找到相应位置，说明当前score是最大值，将其插入尾部
    if (eptr == NULL)
        zl = zzlInsertAt(zl,NULL,ele,score);
    return zl;
}
// 在eptr的前面插入 ele-score
unsigned char *zzlInsertAt(unsigned char *zl, unsigned char *eptr, sds ele, double score) {
    unsigned char *sptr;
    char scorebuf[128];
    int scorelen;
    size_t offset;

    scorelen = d2string(scorebuf,sizeof(scorebuf),score);
    if (eptr == NULL) {
        // 直接插入到尾部
        zl = ziplistPush(zl,(unsigned char*)ele,sdslen(ele),ZIPLIST_TAIL);
        zl = ziplistPush(zl,(unsigned char*)scorebuf,scorelen,ZIPLIST_TAIL);
    } else {
        /* Keep offset relative to zl, as it might be re-allocated. */
        offset = eptr-zl;
        // 直接在 eptr 位置添加 ele, 其他元素后移
        zl = ziplistInsert(zl,eptr,(unsigned char*)ele,sdslen(ele));
        eptr = zl+offset;

        /* Insert score after the element. */
        // 此时的 eptr 已经插入ele之后的位置，后移一位后，就可以找到 score 的存储位置
        serverAssert((sptr = ziplistNext(zl,eptr)) != NULL);
        zl = ziplistInsert(zl,sptr,(unsigned char*)scorebuf,scorelen);
    }
    return zl;
}

// 4. skiplist 下的zset元素添加
// 4.1. 添加元素
// t_zset.c, 添加 ele-score 到 skiplist 中
/* 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, sds ele) {
    // ZSKIPLIST_MAXLEVEL 32
    zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x;
    unsigned int rank[ZSKIPLIST_MAXLEVEL];
    int i, level;

    serverAssert(!isnan(score));
    x = zsl->header;
    // 初始 zsl->level = 1
    // 从header的最高层开始遍历
    for (i = zsl->level-1; i >= 0; i--) {
        /* store rank that is crossed to reach the insert position */
        // 计算出每层可以插入的位置
        rank[i] = i == (zsl->level-1) ? 0 : rank[i+1];
        // 当前level的score小于需要添加的元素时，往前推进skiplist
        while (x->level[i].forward &&
                (x->level[i].forward->score < score ||
                    (x->level[i].forward->score == score &&
                    sdscmp(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. */
    // 得到一随机的level, 决定要写的节点数
    // 如果当前的level过小，则变更level, 重新初始化大的level
    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;
    }
    // 构建新的 skiplist 节点，为每一层节点添加同样的数据
    x = zslCreateNode(level,score,ele);
    for (i = 0; i < level; i++) {
        // 让i层的节点与x关联
        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 */
    // 如果当前level较小，则存在有的level未赋值情况，需要主动+1
    for (i = level; i < zsl->level; i++) {
        update[i]->level[i].span++;
    }
    // 关联好header后，设置backward指针
    x->backward = (update[0] == zsl->header) ? NULL : update[0];
    if (x->level[0].forward)
        x->level[0].forward->backward = x;
    else
        // 同有后继节点，说明是尾节点，赋值tail
        zsl->tail = x;
    zsl->length++;
    return x;
}

// 补充，我们看一下随机level的计算算法
// t_zset.c
/* 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;
    // n次随机值得到 level, ZSKIPLIST_P:0.25
    // 按随机概率，应该是有1/4的命中概率（如果不是呢？？）
    while ((random()&0xFFFF) < (ZSKIPLIST_P * 0xFFFF))
        level += 1;
    return (level<ZSKIPLIST_MAXLEVEL) ? level : ZSKIPLIST_MAXLEVEL;
}

// t_zset.c, 删除skiplist的指定节点
/* 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, sds ele, zskiplistNode **node) {
    zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x;
    int i;

    x = zsl->header;
    // 与添加时查找对应位置一样，先进行遍历，找到最每个层级最接近 node 的位置
    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 &&
                     sdscmp(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. */
    // 进行精确比对，相同才进行删除
    x = x->level[0].forward;
    if (x && score == x->score && sdscmp(x->ele,ele) == 0) {
        // 执行删除动作
        zslDeleteNode(zsl, x, update);
        if (!node)
            zslFreeNode(x);
        else
            *node = x;
        return 1;
    }
    return 0; /* not found */
}
// 删除 x对应的节点
// update 是node的每一层级对应的前驱节点
/* Internal function used by zslDelete, zslDeleteByScore and zslDeleteByRank */
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 {
            // 不相等说明该层不存在指向 x 的引用
            update[i]->level[i].span -= 1;
        }
    }
    // 更新第0层尾节点指针
    if (x->level[0].forward) {
        x->level[0].forward->backward = x->backward;
    } else {
        zsl->tail = x->backward;
    }
    // 降低 skiplist 的层级，直到第一个非空的节点为止
    while(zsl->level > 1 && zsl->header->level[zsl->level-1].forward == NULL)
        zsl->level--;
    zsl->length--;
}

// t_zset.c, 编码类型转换
void zsetConvert(robj *zobj, int encoding) {
    zset *zs;
    zskiplistNode *node, *next;
    sds ele;
    double score;
    // 编码相同，直接返回
    if (zobj->encoding == encoding) return;
    // ziplist -> skiplist 转换
    if (zobj->encoding == OBJ_ENCODING_ZIPLIST) {
        unsigned char *zl = zobj->ptr;
        unsigned char *eptr, *sptr;
        unsigned char *vstr;
        unsigned int vlen;
        long long vlong;

        if (encoding != OBJ_ENCODING_SKIPLIST)
            serverPanic("Unknown target encoding");

        zs = zmalloc(sizeof(*zs));
        zs->dict = dictCreate(&zsetDictType,NULL);
        zs->zsl = zslCreate();

        eptr = ziplistIndex(zl,0);
        serverAssertWithInfo(NULL,zobj,eptr != NULL);
        sptr = ziplistNext(zl,eptr);
        serverAssertWithInfo(NULL,zobj,sptr != NULL);

        while (eptr != NULL) {
            score = zzlGetScore(sptr);
            serverAssertWithInfo(NULL,zobj,ziplistGet(eptr,&vstr,&vlen,&vlong));
            if (vstr == NULL)
                ele = sdsfromlonglong(vlong);
            else
                ele = sdsnewlen((char*)vstr,vlen);
            // 依次插入 skiplist 和 dict 中即可
            node = zslInsert(zs->zsl,score,ele);
            serverAssert(dictAdd(zs->dict,ele,&node->score) == DICT_OK);
            // zzlNext 封装了同时迭代 eptr 和 sptr 方法
            zzlNext(zl,&eptr,&sptr);
        }

        zfree(zobj->ptr);
        zobj->ptr = zs;
        zobj->encoding = OBJ_ENCODING_SKIPLIST;
    }
    // skiplist -> ziplist 逆向转换
    else if (zobj->encoding == OBJ_ENCODING_SKIPLIST) {
        unsigned char *zl = ziplistNew();
        if (encoding != OBJ_ENCODING_ZIPLIST)
            serverPanic("Unknown target encoding");

        /* Approach similar to zslFree(), since we want to free the skiplist at
         * the same time as creating the ziplist. */
        zs = zobj->ptr;
        dictRelease(zs->dict);
        node = zs->zsl->header->level[0].forward;
        zfree(zs->zsl->header);
        zfree(zs->zsl);
        // 正向迭代转换
        while (node) {
            zl = zzlInsertAt(zl,NULL,node->ele,node->score);
            next = node->level[0].forward;
            zslFreeNode(node);
            node = next;
        }

        zfree(zs);
        zobj->ptr = zl;
        zobj->encoding = OBJ_ENCODING_ZIPLIST;
    } else {
        serverPanic("Unknown sorted set encoding");
    }
}
// 基于ziplist, 同时迭代 ele-score
/* Move to next entry based on the values in eptr and sptr. Both are set to
 * NULL when there is no next entry. */
void zzlNext(unsigned char *zl, unsigned char **eptr, unsigned char **sptr) {
    unsigned char *_eptr, *_sptr;
    serverAssert(*eptr != NULL && *sptr != NULL);

    _eptr = ziplistNext(zl,*sptr);
    if (_eptr != NULL) {
        _sptr = ziplistNext(zl,_eptr);
        serverAssert(_sptr != NULL);
    } else {
        /* No next entry. */
        _sptr = NULL;
    }

    *eptr = _eptr;
    *sptr = _sptr;
}

// 用法: ZRANGE key start stop [WITHSCORES]
// t_zset.c
void zrangeCommand(client *c) {
    zrangeGenericCommand(c,0);
}

void zrangeGenericCommand(client *c, int reverse) {
    robj *key = c->argv[1];
    robj *zobj;
    int withscores = 0;
    long start;
    long end;
    int llen;
    int rangelen;

    if ((getLongFromObjectOrReply(c, c->argv[2], &start, NULL) != C_OK) ||
        (getLongFromObjectOrReply(c, c->argv[3], &end, NULL) != C_OK)) return;

    if (c->argc == 5 && !strcasecmp(c->argv[4]->ptr,"withscores")) {
        withscores = 1;
    } else if (c->argc >= 5) {
        addReply(c,shared.syntaxerr);
        return;
    }

    if ((zobj = lookupKeyReadOrReply(c,key,shared.emptymultibulk)) == NULL
         || checkType(c,zobj,OBJ_ZSET)) return;

    /* Sanitize indexes. */
    // 小于0，则代表反向查询，但实际的输出顺序不是按此值运算的（提供了 reverse 方法）
    llen = zsetLength(zobj);
    if (start < 0) start = llen+start;
    if (end < 0) end = llen+end;
    if (start < 0) start = 0;

    /* Invariant: start >= 0, so this test will be true when end < 0.
     * The range is empty when start > end or start >= length. */
    if (start > end || start >= llen) {
        addReply(c,shared.emptymultibulk);
        return;
    }
    if (end >= llen) end = llen-1;
    rangelen = (end-start)+1;

    /* Return the result in form of a multi-bulk reply */
    addReplyMultiBulkLen(c, withscores ? (rangelen*2) : rangelen);
    // 同样，分 ZIPLIST 和 SKIPLIST 编码分别实现
    if (zobj->encoding == OBJ_ENCODING_ZIPLIST) {
        unsigned char *zl = zobj->ptr;
        unsigned char *eptr, *sptr;
        unsigned char *vstr;
        unsigned int vlen;
        long long vlong;
        // ziplist 以 ele-score 方式存储，所以步长是 2
        if (reverse)
            eptr = ziplistIndex(zl,-2-(2*start));
        else
            eptr = ziplistIndex(zl,2*start);

        serverAssertWithInfo(c,zobj,eptr != NULL);
        sptr = ziplistNext(zl,eptr);
        // 依次迭代输出
        while (rangelen--) {
            serverAssertWithInfo(c,zobj,eptr != NULL && sptr != NULL);
            serverAssertWithInfo(c,zobj,ziplistGet(eptr,&vstr,&vlen,&vlong));
            if (vstr == NULL)
                addReplyBulkLongLong(c,vlong);
            else
                addReplyBulkCBuffer(c,vstr,vlen);

            if (withscores)
                addReplyDouble(c,zzlGetScore(sptr));
            // ziplist 提供正向迭代，返回迭代功能，其实就是 offset的加减问题
            if (reverse)
                zzlPrev(zl,&eptr,&sptr);
            else
                zzlNext(zl,&eptr,&sptr);
        }

    } else if (zobj->encoding == OBJ_ENCODING_SKIPLIST) {
        zset *zs = zobj->ptr;
        zskiplist *zsl = zs->zsl;
        zskiplistNode *ln;
        sds ele;

        /* Check if starting point is trivial, before doing log(N) lookup. */
        // 反向使用 tail 迭代，否则使用header迭代
        if (reverse) {
            ln = zsl->tail;
            if (start > 0)
                // 获取下标元素应该只是一个迭代循环问题,不过还是稍微细看一下skiplist实现
                ln = zslGetElementByRank(zsl,llen-start);
        } else {
            ln = zsl->header->level[0].forward;
            if (start > 0)
                ln = zslGetElementByRank(zsl,start+1);
        }

        while(rangelen--) {
            serverAssertWithInfo(c,zobj,ln != NULL);
            ele = ln->ele;
            addReplyBulkCBuffer(c,ele,sdslen(ele));
            if (withscores)
                addReplyDouble(c,ln->score);
            // 直接正向或反向迭代即可
            ln = reverse ? ln->backward : ln->level[0].forward;
        }
    } else {
        serverPanic("Unknown sorted set encoding");
    }
}
// 根据排名查找元素
/* Finds an element by its rank. The rank argument needs to be 1-based. */
zskiplistNode* zslGetElementByRank(zskiplist *zsl, unsigned long rank) {
    zskiplistNode *x;
    unsigned long traversed = 0;
    int i;

    x = zsl->header;
    // 好像没有相像中的简单哦
    // 请仔细品
    for (i = zsl->level-1; i >= 0; i--) {
        while (x->level[i].forward && (traversed + x->level[i].span) <= rank)
        {
            // span 的作用？？
            traversed += x->level[i].span;
            x = x->level[i].forward;
        }
        if (traversed == rank) {
            return x;
        }
    }
    return NULL;
}

// t_zset.c, 
void zremrangebyscoreCommand(client *c) {
    // 几个范围删除，都复用 zremrangeGenericCommand
    // ZRANGE_RANK/ZRANGE_SCORE/ZRANGE_LEX
    zremrangeGenericCommand(c,ZRANGE_SCORE);
}
void zremrangeGenericCommand(client *c, int rangetype) {
    robj *key = c->argv[1];
    robj *zobj;
    int keyremoved = 0;
    unsigned long deleted = 0;
    // score 存储使用另外的数据结构
    zrangespec range;
    zlexrangespec lexrange;
    long start, end, llen;

    /* Step 1: Parse the range. */
    // 解析参数，除了 rank 方式的查询，其他两个都使用 另外的专门数据结构存储参数
    if (rangetype == ZRANGE_RANK) {
        if ((getLongFromObjectOrReply(c,c->argv[2],&start,NULL) != C_OK) ||
            (getLongFromObjectOrReply(c,c->argv[3],&end,NULL) != C_OK))
            return;
    } else if (rangetype == ZRANGE_SCORE) {
        if (zslParseRange(c->argv[2],c->argv[3],&range) != C_OK) {
            addReplyError(c,"min or max is not a float");
            return;
        }
    } else if (rangetype == ZRANGE_LEX) {
        if (zslParseLexRange(c->argv[2],c->argv[3],&lexrange) != C_OK) {
            addReplyError(c,"min or max not valid string range item");
            return;
        }
    }

    /* Step 2: Lookup & range sanity checks if needed. */
    if ((zobj = lookupKeyWriteOrReply(c,key,shared.czero)) == NULL ||
        checkType(c,zobj,OBJ_ZSET)) goto cleanup;

    if (rangetype == ZRANGE_RANK) {
        /* Sanitize indexes. */
        llen = zsetLength(zobj);
        if (start < 0) start = llen+start;
        if (end < 0) end = llen+end;
        if (start < 0) start = 0;

        /* Invariant: start >= 0, so this test will be true when end < 0.
         * The range is empty when start > end or start >= length. */
        if (start > end || start >= llen) {
            addReply(c,shared.czero);
            goto cleanup;
        }
        if (end >= llen) end = llen-1;
    }

    /* Step 3: Perform the range deletion operation. */
    if (zobj->encoding == OBJ_ENCODING_ZIPLIST) {
        // 针对不同的删除类型，使用不同的删除方法
        // 所以，这段代码的复用体现在哪里呢？？？
        switch(rangetype) {
        case ZRANGE_RANK:
            zobj->ptr = zzlDeleteRangeByRank(zobj->ptr,start+1,end+1,&deleted);
            break;
        case ZRANGE_SCORE:
            // 3.1. 我们只看 score 的删除 --ziplist
            zobj->ptr = zzlDeleteRangeByScore(zobj->ptr,&range,&deleted);
            break;
        case ZRANGE_LEX:
            zobj->ptr = zzlDeleteRangeByLex(zobj->ptr,&lexrange,&deleted);
            break;
        }
        if (zzlLength(zobj->ptr) == 0) {
            dbDelete(c->db,key);
            keyremoved = 1;
        }
    } else if (zobj->encoding == OBJ_ENCODING_SKIPLIST) {
        zset *zs = zobj->ptr;
        switch(rangetype) {
        case ZRANGE_RANK:
            deleted = zslDeleteRangeByRank(zs->zsl,start+1,end+1,zs->dict);
            break;
        case ZRANGE_SCORE:
            // 3.2. skiplist 的删除rangeByScore 方法
            deleted = zslDeleteRangeByScore(zs->zsl,&range,zs->dict);
            break;
        case ZRANGE_LEX:
            deleted = zslDeleteRangeByLex(zs->zsl,&lexrange,zs->dict);
            break;
        }
        if (htNeedsResize(zs->dict)) dictResize(zs->dict);
        if (dictSize(zs->dict) == 0) {
            dbDelete(c->db,key);
            keyremoved = 1;
        }
    } else {
        serverPanic("Unknown sorted set encoding");
    }

    /* Step 4: Notifications and reply. */
    if (deleted) {
        char *event[3] = {"zremrangebyrank","zremrangebyscore","zremrangebylex"};
        signalModifiedKey(c->db,key);
        notifyKeyspaceEvent(NOTIFY_ZSET,event[rangetype],key,c->db->id);
        if (keyremoved)
            notifyKeyspaceEvent(NOTIFY_GENERIC,"del",key,c->db->id);
    }
    server.dirty += deleted;
    addReplyLongLong(c,deleted);

cleanup:
    if (rangetype == ZRANGE_LEX) zslFreeLexRange(&lexrange);
}
// server.h, 范围查询参数存储
/* Struct to hold a inclusive/exclusive range spec by score comparison. */
typedef struct {
    double min, max;
    int minex, maxex; /* are min or max exclusive? */
} zrangespec;

// 3.1. ziplist 的删除range方法
// t_zset.c
unsigned char *zzlDeleteRangeByScore(unsigned char *zl, zrangespec *range, unsigned long *deleted) {
    unsigned char *eptr, *sptr;
    double score;
    unsigned long num = 0;
    if (deleted != NULL) *deleted = 0;
    // 找到首个在范围内的指针，进行迭代
    eptr = zzlFirstInRange(zl,range);
    if (eptr == NULL) return zl;

    /* When the tail of the ziplist is deleted, eptr will point to the sentinel
     * byte and ziplistNext will return NULL. */
    while ((sptr = ziplistNext(zl,eptr)) != NULL) {
        score = zzlGetScore(sptr);
        // 肯定是比 min 大的，所以只需确认比 max 小即可
        if (zslValueLteMax(score,range)) {
            /* Delete both the element and the score. */
            zl = ziplistDelete(zl,&eptr);
            zl = ziplistDelete(zl,&eptr);
            num++;
        } else {
            /* No longer in range. */
            break;
        }
    }

    if (deleted != NULL) *deleted = num;
    return zl;
}

/* Find pointer to the first element contained in the specified range.
 * Returns NULL when no element is contained in the range. */
unsigned char *zzlFirstInRange(unsigned char *zl, zrangespec *range) {
    unsigned char *eptr = ziplistIndex(zl,0), *sptr;
    double score;

    /* If everything is out of range, return early. */
    // 比较第1个元素和最后 一个元素，即可确认是否在范围内
    if (!zzlIsInRange(zl,range)) return NULL;

    while (eptr != NULL) {
        sptr = ziplistNext(zl,eptr);
        serverAssert(sptr != NULL);

        score = zzlGetScore(sptr);
        // score >= min
        if (zslValueGteMin(score,range)) {
            /* Check if score <= max. */
            if (zslValueLteMax(score,range))
                return eptr;
            return NULL;
        }

        /* Move to next element. */
        eptr = ziplistNext(zl,sptr);
    }

    return NULL;
}
// 检查zl是否在range范围内
// 检查第1个分数和最后一个数即可
/* Returns if there is a part of the zset is in range. Should only be used
 * internally by zzlFirstInRange and zzlLastInRange. */
int zzlIsInRange(unsigned char *zl, zrangespec *range) {
    unsigned char *p;
    double score;

    /* Test for ranges that will always be empty. */
    if (range->min > range->max ||
            (range->min == range->max && (range->minex || range->maxex)))
        return 0;

    p = ziplistIndex(zl,-1); /* Last score. */
    if (p == NULL) return 0; /* Empty sorted set */
    score = zzlGetScore(p);
    // scoreMax >= min
    if (!zslValueGteMin(score,range))
        return 0;

    p = ziplistIndex(zl,1); /* First score. */
    serverAssert(p != NULL);
    score = zzlGetScore(p);
    // scoreMin <= max
    if (!zslValueLteMax(score,range))
        return 0;

    return 1;
}

// 3.2. 删除 skiplist 中的range元素
/* Delete all the elements with score between min and max from the skiplist.
 * Min and max are inclusive, so a score >= min || score <= max is deleted.
 * Note that this function takes the reference to the hash table view of the
 * sorted set, in order to remove the elements from the hash table too. */
unsigned long zslDeleteRangeByScore(zskiplist *zsl, zrangespec *range, dict *dict) {
    zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x;
    unsigned long removed = 0;
    int i;

    x = zsl->header;
    // 找出每层小于 range->min 的元素
    for (i = zsl->level-1; i >= 0; i--) {
        while (x->level[i].forward && (range->minex ?
            x->level[i].forward->score <= range->min :
            x->level[i].forward->score < range->min))
                x = x->level[i].forward;
        update[i] = x;
    }

    /* Current node is the last with score < or <= min. */
    x = x->level[0].forward;
    // 从第0层开始，依次删除引用，删除元素
    // 同有找到符合条件的元素时，一次循环也不会成立
    /* Delete nodes while in range. */
    while (x &&
           (range->maxex ? x->score < range->max : x->score <= range->max))
    {
        // 保留下一次迭代
        zskiplistNode *next = x->level[0].forward;
        zslDeleteNode(zsl,x,update);
        // 同步删除 dict 数据
        dictDelete(dict,x->ele);
        zslFreeNode(x); /* Here is where x->ele is actually released. */
        removed++;
        x = next;
    }
    return removed;
}