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 + zskiplisttypedef 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.cvoid 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zsetrobj *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.crobj *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eleunsigned 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scoredouble 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-scoreunsigned 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.cvoid 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.cunsigned 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;}