今天我们来看看 Redis 中的set是如何存储数据，然后如何做交集、并集、差集运算的，所以我们一共会探索 SADD, SISMEMBER, SINTER, SUNION, SDIFF 这五个命令的实现。

首先我们从 SADD 开始：

    {"sadd",saddCommand,-3,
     "write use-memory fast @set",
     0,NULL,1,1,1,0,0,0},


void saddCommand(client *c) {
    robj *set;
    int j, added = 0;

    set = lookupKeyWrite(c->db,c->argv[1]);
    if (checkType(c,set,OBJ_SET)) return;
    
    if (set == NULL) {
        set = setTypeCreate(c->argv[2]->ptr);
        dbAdd(c->db,c->argv[1],set);
    }

    for (j = 2; j < c->argc; j++) {
        if (setTypeAdd(set,c->argv[j]->ptr)) added++;
    }
    if (added) {
        signalModifiedKey(c,c->db,c->argv[1]);
        notifyKeyspaceEvent(NOTIFY_SET,"sadd",c->argv[1],c->db->id);
    }
    server.dirty += added;
    addReplyLongLong(c,added);
}

/* Factory method to return a set that *can* hold "value". When the object has
 * an integer-encodable value, an intset will be returned. Otherwise a regular
 * hash table. */
robj *setTypeCreate(sds value) {
    if (isSdsRepresentableAsLongLong(value,NULL) == C_OK)
        return createIntsetObject();
    return createSetObject();
}

/* Add the specified value into a set.
 *
 * If the value was already member of the set, nothing is done and 0 is
 * returned, otherwise the new element is added and 1 is returned. */
int setTypeAdd(robj *subject, sds value) {
    // ...
}

首先我们看到 setTypeCreate，这是一个共长方法，如果 isSdsRepresentableAsLongLong 返回是 C_OK，就会调用 createIntsetObject 并且返回，否则返回 createSetObject。我们分别看看这两个方法：

robj *createIntsetObject(void) {
    intset *is = intsetNew();
    robj *o = createObject(OBJ_SET,is);
    o->encoding = OBJ_ENCODING_INTSET;
    return o;
}

robj *createSetObject(void) {
    dict *d = dictCreate(&setDictType,NULL);
    robj *o = createObject(OBJ_SET,d);
    o->encoding = OBJ_ENCODING_HT;
    return o;
}

可以看到，如果要存储的值，如果可以存成 long long，那么就会用 intset 来存，否则就用 dict。我们先来看后者， 也就是如果放在 dict 里，是怎么实现的：

/* Add the specified value into a set.
 *
 * If the value was already member of the set, nothing is done and 0 is
 * returned, otherwise the new element is added and 1 is returned. */
int setTypeAdd(robj *subject, sds value) {
    long long llval;
    if (subject->encoding == OBJ_ENCODING_HT) {
        dict *ht = subject->ptr;
        dictEntry *de = dictAddRaw(ht,value,NULL);
        if (de) {
            dictSetKey(ht,de,sdsdup(value));
            dictSetVal(ht,de,NULL);
            return 1;
        }

可以看到，其实就是用一个 dict 来存储数据，key为我们要存的数据，value为NULL。接下来我们来看看如果是 intset 将会怎么处理：

/* Add the specified value into a set.
 *
 * If the value was already member of the set, nothing is done and 0 is
 * returned, otherwise the new element is added and 1 is returned. */
int setTypeAdd(robj *subject, sds value) {
    // ...
    } else if (subject->encoding == OBJ_ENCODING_INTSET) {
        if (isSdsRepresentableAsLongLong(value,&llval) == C_OK) { // 如果值可以表示成long long
            uint8_t success = 0;
            subject->ptr = intsetAdd(subject->ptr,llval,&success); // 添加
            if (success) {
                /* Convert to regular set when the intset contains
                 * too many entries. */
                if (intsetLen(subject->ptr) > server.set_max_intset_entries)
                // 如果保存的数据量超过 server.set_max_intset_entries，也转换成 dict 来保存
                    setTypeConvert(subject,OBJ_ENCODING_HT);
                return 1;
            }
        } else {
            /* Failed to get integer from object, convert to regular set. */
            setTypeConvert(subject,OBJ_ENCODING_HT);

            /* The set *was* an intset and this value is not integer
             * encodable, so dictAdd should always work. */
            serverAssert(dictAdd(subject->ptr,sdsdup(value),NULL) == DICT_OK);
            return 1;
        }

到底为止，我们了解到了，Redis保存set有两种形式，当数据都是整数，而且保存的数据量小于一定量时，用的是 intset，否则 用 dict 来保存。接下来我们来看看 intset 是怎么实现的。

intset 实现

我们从 setTypeCreate 开始入手：

robj *setTypeCreate(sds value) {
    if (isSdsRepresentableAsLongLong(value,NULL) == C_OK)
        return createIntsetObject();
    return createSetObject();
}

robj *createIntsetObject(void) {
    intset *is = intsetNew();
    robj *o = createObject(OBJ_SET,is);
    o->encoding = OBJ_ENCODING_INTSET;
    return o;
}

/* Create an empty intset. */
intset *intsetNew(void) {
    intset *is = zmalloc(sizeof(intset));
    is->encoding = intrev32ifbe(INTSET_ENC_INT16);
    is->length = 0;
    return is;
}

typedef struct intset {
    uint32_t encoding;
    uint32_t length;
    int8_t contents[];
} intset;

看样子，该不会又是 ziplist 那一套吧？

/* Insert an integer in the intset */
intset *intsetAdd(intset *is, int64_t value, uint8_t *success) {
    uint8_t valenc = _intsetValueEncoding(value);
    uint32_t pos;
    if (success) *success = 1;

    /* Upgrade encoding if necessary. If we need to upgrade, we know that
     * this value should be either appended (if > 0) or prepended (if < 0),
     * because it lies outside the range of existing values. */
    if (valenc > intrev32ifbe(is->encoding)) {
        /* This always succeeds, so we don't need to curry *success. */
        return intsetUpgradeAndAdd(is,value);
    } else {
        /* Abort if the value is already present in the set.
         * This call will populate "pos" with the right position to insert
         * the value when it cannot be found. */
        if (intsetSearch(is,value,&pos)) {
            if (success) *success = 0;
            return is;
        }

        is = intsetResize(is,intrev32ifbe(is->length)+1);
        if (pos < intrev32ifbe(is->length)) intsetMoveTail(is,pos,pos+1);
    }

    _intsetSet(is,pos,value);
    is->length = intrev32ifbe(intrev32ifbe(is->length)+1);
    return is;
}

/* Note that these encodings are ordered, so:
 * INTSET_ENC_INT16 < INTSET_ENC_INT32 < INTSET_ENC_INT64. */
#define INTSET_ENC_INT16 (sizeof(int16_t))
#define INTSET_ENC_INT32 (sizeof(int32_t))
#define INTSET_ENC_INT64 (sizeof(int64_t))

/* Return the required encoding for the provided value. */
static uint8_t _intsetValueEncoding(int64_t v) {
    if (v < INT32_MIN || v > INT32_MAX)
        return INTSET_ENC_INT64;
    else if (v < INT16_MIN || v > INT16_MAX)
        return INTSET_ENC_INT32;
    else
        return INTSET_ENC_INT16;
}

/* Search for the position of "value". Return 1 when the value was found and
 * sets "pos" to the position of the value within the intset. Return 0 when
 * the value is not present in the intset and sets "pos" to the position
 * where "value" can be inserted. */
static uint8_t intsetSearch(intset *is, int64_t value, uint32_t *pos) {
    int min = 0, max = intrev32ifbe(is->length)-1, mid = -1;
    int64_t cur = -1;

    /* The value can never be found when the set is empty */
    if (intrev32ifbe(is->length) == 0) {
        if (pos) *pos = 0;
        return 0;
    } else {
        /* Check for the case where we know we cannot find the value,
         * but do know the insert position. */
        if (value > _intsetGet(is,max)) {
            if (pos) *pos = intrev32ifbe(is->length);
            return 0;
        } else if (value < _intsetGet(is,0)) {
            if (pos) *pos = 0;
            return 0;
        }
    }

    // 二分查找
    while(max >= min) {
        mid = ((unsigned int)min + (unsigned int)max) >> 1;
        cur = _intsetGet(is,mid);
        if (value > cur) {
            min = mid+1;
        } else if (value < cur) {
            max = mid-1;
        } else {
            break;
        }
    }

    if (value == cur) {
        if (pos) *pos = mid;
        return 1;
    } else {
        if (pos) *pos = min;
        return 0;
    }
}

/* Resize the intset */
static intset *intsetResize(intset *is, uint32_t len) {
    uint32_t size = len*intrev32ifbe(is->encoding);
    is = zrealloc(is,sizeof(intset)+size);
    return is;
}

从上面可以看出来，intset又是 ziplist 那一套，is->encoding 记录当前保存的数据，都是什么类型，比如可以选择的 类型为 int16_t, int32_t，int64_t，然后在 is->content 处保存数据，但是是以有序的形式保存。

所以我们现在了解到，intset 有两个特点：

• intset 和 ziplist 一样，也是以连续内存块的形式保存数据，然后保存了数据的编码（其实也就是大小）
• intset 的数据是有序保存的，因此查找时可以用二分查找，插入时也可以二分查找出要插入的位置

集合运算(交集并集差集)是怎么实现的

最后我们来看一眼，交集并集差集分别是怎么实现的。

交集：

    {"sinter",sinterCommand,-2,
     "read-only to-sort @set",
     0,NULL,1,-1,1,0,0,0},

void sinterCommand(client *c) {
    sinterGenericCommand(c,c->argv+1,c->argc-1,NULL);
}

void sinterGenericCommand(client *c, robj **setkeys,
                          unsigned long setnum, robj *dstkey) {
                          // ...
    /* Sort sets from the smallest to largest, this will improve our
     * algorithm's performance */
    qsort(sets,setnum,sizeof(robj*),qsortCompareSetsByCardinality);
                          // ...
    /* Iterate all the elements of the first (smallest) set, and test
     * the element against all the other sets, if at least one set does
     * not include the element it is discarded */
    si = setTypeInitIterator(sets[0]);
    while((encoding = setTypeNext(si,&elesds,&intobj)) != -1) {
        for (j = 1; j < setnum; j++) {
            if (sets[j] == sets[0]) continue;
            if (encoding == OBJ_ENCODING_INTSET) {
                /* intset with intset is simple... and fast */
                if (sets[j]->encoding == OBJ_ENCODING_INTSET &&
                    !intsetFind((intset*)sets[j]->ptr,intobj))
                {
                    break;
                /* in order to compare an integer with an object we
                 * have to use the generic function, creating an object
                 * for this */
                } else if (sets[j]->encoding == OBJ_ENCODING_HT) {
                    elesds = sdsfromlonglong(intobj);
                    if (!setTypeIsMember(sets[j],elesds)) {
                        sdsfree(elesds);
                        break;
                    }
                    sdsfree(elesds);
                }
            } else if (encoding == OBJ_ENCODING_HT) {
                if (!setTypeIsMember(sets[j],elesds)) {
                    break;
                }
            }
        }
    }
    // ...
}

可以看到，做交集的逻辑，其实就是先根据集合大小把集合排序。然后以最小的集合为基准，遍历每一个元素，然后去遍历其它set， 只要有一个不是子元素，就可以跳过这个元素了。

并集：

    {"sunion",sunionCommand,-2,
     "read-only to-sort @set",
     0,NULL,1,-1,1,0,0,0},


void sunionCommand(client *c) {
    sunionDiffGenericCommand(c,c->argv+1,c->argc-1,NULL,SET_OP_UNION);
}


void sunionDiffGenericCommand(client *c, robj **setkeys, int setnum,
                              robj *dstkey, int op) {
                              // ...
    /* We need a temp set object to store our union. If the dstkey
     * is not NULL (that is, we are inside an SUNIONSTORE operation) then
     * this set object will be the resulting object to set into the target key*/
    dstset = createIntsetObject();

    if (op == SET_OP_UNION) {
        /* Union is trivial, just add every element of every set to the
         * temporary set. */
        for (j = 0; j < setnum; j++) {
            if (!sets[j]) continue; /* non existing keys are like empty sets */

            si = setTypeInitIterator(sets[j]);
            while((ele = setTypeNextObject(si)) != NULL) {
                if (setTypeAdd(dstset,ele)) cardinality++;
                sdsfree(ele);
            }
            setTypeReleaseIterator(si);
        }
    // ...
}

并集很简单，遍历所有集合，把元素全都加进去即可。

差集：

    {"sdiff",sdiffCommand,-2,
     "read-only to-sort @set",
     0,NULL,1,-1,1,0,0,0},

void sdiffCommand(client *c) {
    sunionDiffGenericCommand(c,c->argv+1,c->argc-1,NULL,SET_OP_DIFF);
}

void sunionDiffGenericCommand(client *c, robj **setkeys, int setnum,
                              robj *dstkey, int op) {
                              // ...

    } else if (op == SET_OP_DIFF && sets[0] && diff_algo == 1) {
        /* DIFF Algorithm 1:
         *
         * We perform the diff by iterating all the elements of the first set,
         * and only adding it to the target set if the element does not exist
         * into all the other sets.
         *
         * This way we perform at max N*M operations, where N is the size of
         * the first set, and M the number of sets. */
        si = setTypeInitIterator(sets[0]);
        while((ele = setTypeNextObject(si)) != NULL) {
            for (j = 1; j < setnum; j++) {
                if (!sets[j]) continue; /* no key is an empty set. */
                if (sets[j] == sets[0]) break; /* same set! */
                if (setTypeIsMember(sets[j],ele)) break;
            }
            if (j == setnum) {
                /* There is no other set with this element. Add it. */
                setTypeAdd(dstset,ele);
                cardinality++;
            }
            sdsfree(ele);
        }
        setTypeReleaseIterator(si);
    } else if (op == SET_OP_DIFF && sets[0] && diff_algo == 2) {
        /* DIFF Algorithm 2:
         *
         * Add all the elements of the first set to the auxiliary set.
         * Then remove all the elements of all the next sets from it.
         *
         * This is O(N) where N is the sum of all the elements in every
         * set. */
        for (j = 0; j < setnum; j++) {
            if (!sets[j]) continue; /* non existing keys are like empty sets */

            si = setTypeInitIterator(sets[j]);
            while((ele = setTypeNextObject(si)) != NULL) {
                if (j == 0) {
                    if (setTypeAdd(dstset,ele)) cardinality++;
                } else {
                    if (setTypeRemove(dstset,ele)) cardinality--;
                }
                sdsfree(ele);
            }
            setTypeReleaseIterator(si);

            /* Exit if result set is empty as any additional removal
             * of elements will have no effect. */
            if (cardinality == 0) break;
        }
    // ...
}

做差集有两种方式，一种是遍历第一个集合，如果元素不在后面所有集合内，就加入；第二种是把第一个集合所有元素加到结果集合里， 然后再把所有在后面的集合里的元素，从这个结果集合里移除。

总结

这篇文章里，我们看了一下Redis是如何实现set这个数据结构的，Redis会在内容为整数并且数量足够小时，使用一种类似 ziplist 的方式，也就是 intset 来保存数据，其它情况，则会使用一个 dict 来保存数据，此外我们分别看了一下三种常见的集合运算 是怎么实现的。