/*
 * 将键 key 的过期时间设为 when
 */
void setExpire(redisDb *db, robj *key, long long when) {

    dictEntry *kde, *de;

    /* Reuse the sds from the main dict in the expire dict */
    // 取出键
    kde = dictFind(db->dict,key->ptr);

    redisAssertWithInfo(NULL,key,kde != NULL);

    // 根据键取出键的过期时间
    de = dictReplaceRaw(db->expires,dictGetKey(kde));

    // 设置键的过期时间
    // 这里是直接使用整数值来保存过期时间，不是用 INT 编码的 String 对象
    dictSetSignedIntegerVal(de,when);
}

int freeMemoryIfNeeded(void) {
    /* Compute how much memory we need to free. */
    // 计算需要释放多少字节的内存
    mem_tofree = mem_used - server.maxmemory;

    // 初始化已释放内存的字节数为 0
    mem_freed = 0;

    // 根据 maxmemory 策略，
    // 遍历字典，释放内存并记录被释放内存的字节数
    while (mem_freed < mem_tofree) {
        int j, k, keys_freed = 0;

        // 遍历所有字典
        for (j = 0; j < server.dbnum; j++) {
            long bestval = 0; /* just to prevent warning */
            sds bestkey = NULL;
            dictEntry *de;
            redisDb *db = server.db+j;
            dict *dict;

            if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_LRU ||
                server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_RANDOM)
            {
                // 如果策略是 allkeys-lru 或者 allkeys-random 
                // 那么淘汰的目标为所有数据库键
                dict = server.db[j].dict;
            } else {
                // 如果策略是 volatile-lru 、 volatile-random 或者 volatile-ttl 
                // 那么淘汰的目标为带过期时间的数据库键
                dict = server.db[j].expires;
            }

            // 跳过空字典
            if (dictSize(dict) == 0) continue;

            /* volatile-random and allkeys-random policy */
            // 如果使用的是随机策略，那么从目标字典中随机选出键
            if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_RANDOM ||
                server.maxmemory_policy == REDIS_MAXMEMORY_VOLATILE_RANDOM)
            {
                de = dictGetRandomKey(dict);
                bestkey = dictGetKey(de);
            }

            /* volatile-lru and allkeys-lru policy */
            // 如果使用的是 LRU 策略，
            // 那么从一集 sample 键中选出 IDLE 时间最长的那个键
            else if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_LRU ||
                server.maxmemory_policy == REDIS_MAXMEMORY_VOLATILE_LRU)
            {
                struct evictionPoolEntry *pool = db->eviction_pool;

                while(bestkey == NULL) {
                    evictionPoolPopulate(dict, db->dict, db->eviction_pool);
                    /* Go backward from best to worst element to evict. */
                    for (k = REDIS_EVICTION_POOL_SIZE-1; k >= 0; k--) {
                        if (pool[k].key == NULL) continue;
                        de = dictFind(dict,pool[k].key);

                        /* Remove the entry from the pool. */
                        sdsfree(pool[k].key);
                        /* Shift all elements on its right to left. */
                        memmove(pool+k,pool+k+1,
                            sizeof(pool[0])*(REDIS_EVICTION_POOL_SIZE-k-1));
                        /* Clear the element on the right which is empty
                         * since we shifted one position to the left.  */
                        pool[REDIS_EVICTION_POOL_SIZE-1].key = NULL;
                        pool[REDIS_EVICTION_POOL_SIZE-1].idle = 0;

                        /* If the key exists, is our pick. Otherwise it is
                         * a ghost and we need to try the next element. */
                        if (de) {
                            bestkey = dictGetKey(de);
                            break;
                        } else {
                            /* Ghost... */
                            continue;
                        }
                    }
                }
            }

            /* volatile-ttl */
            // 策略为 volatile-ttl ，从一集 sample 键中选出过期时间距离当前时间最接近的键
            else if (server.maxmemory_policy == REDIS_MAXMEMORY_VOLATILE_TTL) {
                for (k = 0; k < server.maxmemory_samples; k++) {
                    sds thiskey;
                    long thisval;

                    de = dictGetRandomKey(dict);
                    thiskey = dictGetKey(de);
                    thisval = (long) dictGetVal(de);

                    /* Expire sooner (minor expire unix timestamp) is better
                     * candidate for deletion */
                    if (bestkey == NULL || thisval < bestval) {
                        bestkey = thiskey;
                        bestval = thisval;
                    }
                }
            }

            /* Finally remove the selected key. */
            // 删除被选中的键
            if (bestkey) {
                long long delta;

                robj *keyobj = createStringObject(bestkey,sdslen(bestkey));
                propagateExpire(db,keyobj);
                /* We compute the amount of memory freed by dbDelete() alone.
                 * It is possible that actually the memory needed to propagate
                 * the DEL in AOF and replication link is greater than the one
                 * we are freeing removing the key, but we can't account for
                 * that otherwise we would never exit the loop.
                 *
                 * AOF and Output buffer memory will be freed eventually so
                 * we only care about memory used by the key space. */
                // 计算删除键所释放的内存数量
                delta = (long long) zmalloc_used_memory();
                dbDelete(db,keyobj);
                delta -= (long long) zmalloc_used_memory();
                mem_freed += delta;
                
                // 对淘汰键的计数器增一
                server.stat_evictedkeys++;

                notifyKeyspaceEvent(REDIS_NOTIFY_EVICTED, "evicted",
                    keyobj, db->id);
                decrRefCount(keyobj);
                keys_freed++;

                /* When the memory to free starts to be big enough, we may
                 * start spending so much time here that is impossible to
                 * deliver data to the slaves fast enough, so we force the
                 * transmission here inside the loop. */
                if (slaves) flushSlavesOutputBuffers();
            }
        }

        if (!keys_freed) return REDIS_ERR; /* nothing to free... */
    }

    return REDIS_OK;
}

/*
 * 随机返回字典中任意一个节点。
 *
 * 可用于实现随机化算法。
 *
 * 如果字典为空，返回 NULL 。
 *
 * T = O(N)
*/
dictEntry *dictGetRandomKey(dict *d)
{
    dictEntry *he, *orighe;
    unsigned int h;
    int listlen, listele;

    // 字典为空
    if (dictSize(d) == 0) return NULL;

    // 进行单步 rehash
    if (dictIsRehashing(d)) _dictRehashStep(d);

    // 如果正在 rehash ，那么将 1 号哈希表也作为随机查找的目标
    if (dictIsRehashing(d)) {
        // T = O(N)
        do {
            h = random() % (d->ht[0].size+d->ht[1].size);
            he = (h >= d->ht[0].size) ? d->ht[1].table[h - d->ht[0].size] :
                                      d->ht[0].table[h];
        } while(he == NULL);
    // 否则，只从 0 号哈希表中查找节点
    } else {
        // T = O(N)
        do {
            h = random() & d->ht[0].sizemask;
            he = d->ht[0].table[h];
        } while(he == NULL);
    }

    /* Now we found a non empty bucket, but it is a linked
     * list and we need to get a random element from the list.
     * The only sane way to do so is counting the elements and
     * select a random index. */
    // 目前 he 已经指向一个非空的节点链表
    // 程序将从这个链表随机返回一个节点
    listlen = 0;
    orighe = he;
    // 计算节点数量, T = O(1)
    while(he) {
        he = he->next;
        listlen++;
    }
    // 取模，得出随机节点的索引
    listele = random() % listlen;
    he = orighe;
    // 按索引查找节点
    // T = O(1)
    while(listele--) he = he->next;

    // 返回随机节点
    return he;
}

void evictionPoolPopulate(dict *sampledict, dict *keydict, struct evictionPoolEntry *pool) {
    int j, k, count;
    dictEntry *_samples[EVICTION_SAMPLES_ARRAY_SIZE];
    dictEntry **samples;

    /* Try to use a static buffer: this function is a big hit...
     * Note: it was actually measured that this helps. */
    if (server.maxmemory_samples <= EVICTION_SAMPLES_ARRAY_SIZE) {
        samples = _samples;
    } else {
        samples = zmalloc(sizeof(samples[0])*server.maxmemory_samples);
    }

#if 1 /* Use bulk get by default. */
    count = dictGetRandomKeys(sampledict,samples,server.maxmemory_samples);
#else
    count = server.maxmemory_samples;
    for (j = 0; j < count; j++) samples[j] = dictGetRandomKey(sampledict);
#endif

    for (j = 0; j < count; j++) {
        unsigned long long idle;
        sds key;
        robj *o;
        dictEntry *de;

        de = samples[j];
        key = dictGetKey(de);
        /* If the dictionary we are sampling from is not the main
         * dictionary (but the expires one) we need to lookup the key
         * again in the key dictionary to obtain the value object. */
        if (sampledict != keydict) de = dictFind(keydict, key);
        o = dictGetVal(de);
        idle = estimateObjectIdleTime(o);

        /* Insert the element inside the pool.
         * First, find the first empty bucket or the first populated
         * bucket that has an idle time smaller than our idle time. */
        k = 0;
        while (k < REDIS_EVICTION_POOL_SIZE &&
               pool[k].key &&
               pool[k].idle < idle) k++;
        if (k == 0 && pool[REDIS_EVICTION_POOL_SIZE-1].key != NULL) {
            /* Can't insert if the element is < the worst element we have
             * and there are no empty buckets. */
            continue;
        } else if (k < REDIS_EVICTION_POOL_SIZE && pool[k].key == NULL) {
            /* Inserting into empty position. No setup needed before insert. */
        } else {
            /* Inserting in the middle. Now k points to the first element
             * greater than the element to insert.  */
            if (pool[REDIS_EVICTION_POOL_SIZE-1].key == NULL) {
                /* Free space on the right? Insert at k shifting
                 * all the elements from k to end to the right. */
                memmove(pool+k+1,pool+k,
                    sizeof(pool[0])*(REDIS_EVICTION_POOL_SIZE-k-1));
            } else {
                /* No free space on right? Insert at k-1 */
                k--;
                /* Shift all elements on the left of k (included) to the
                 * left, so we discard the element with smaller idle time. */
                sdsfree(pool[0].key);
                memmove(pool,pool+1,sizeof(pool[0])*k);
            }
        }
        pool[k].key = sdsdup(key);
        pool[k].idle = idle;
    }
    if (samples != _samples) zfree(samples);
}

int dictGetRandomKeys(dict *d, dictEntry **des, int count) {
    int j; /* internal hash table id, 0 or 1. */
    int stored = 0;

    if (dictSize(d) < count) count = dictSize(d);
    while(stored < count) {
        for (j = 0; j < 2; j++) {
            /* Pick a random point inside the hash table 0 or 1. */
            unsigned int i = random() & d->ht[j].sizemask;
            int size = d->ht[j].size;

            /* Make sure to visit every bucket by iterating 'size' times. */
            while(size--) {
                dictEntry *he = d->ht[j].table[i];
                while (he) {
                    /* Collect all the elements of the buckets found non
                     * empty while iterating. */
                    *des = he;
                    des++;
                    he = he->next;
                    stored++;
                    if (stored == count) return stored;
                }
                i = (i+1) & d->ht[j].sizemask;
            }
            /* If there is only one table and we iterated it all, we should
             * already have 'count' elements. Assert this condition. */
            assert(dictIsRehashing(d) != 0);
        }
    }
    return stored; /* Never reached. */
}

for (k = 0; k < server.maxmemory_samples; k++) {
                    sds thiskey;
                    long thisval;

                    de = dictGetRandomKey(dict);
                    thiskey = dictGetKey(de);
                    thisval = (long) dictGetVal(de);

                    /* Expire sooner (minor expire unix timestamp) is better
                     * candidate for deletion */
                    if (bestkey == NULL || thisval < bestval) {
                        bestkey = thiskey;
                        bestval = thisval;
                    }
                }