[redis 源码走读] - 整数集合(inset)

 
    /* 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))
    
    typedef struct intset {
        uint32_t encoding; // 编码。
        uint32_t length;   // 数组长度。
        int8_t contents[]; // 整数值数值。
    } intset;

根据插入数值大小，决定 contents 数组的 encoding 格式。编码格式分别有 int16_t，int32_t，int64_t。以最大的数值为准，如果最大的数值是 int64_t 那么数组的每个 item 都是 int64_t，优点是统一简单，提高数组查找效率——源码实现是通过二分法查找。如果数组不同数值用不同的编码存储，就很难用二分法查找了。但是这样做一方面提高了查找效率，另一方面也会导致内存浪费，如果所有数据中，只有一个数据是 int64_t，其它数据都是 int16_t，整个数组 item 都以 int64_t 存储，显然会造成内存浪费。当然 redis 的数据结构是丰富的，连续内存上的数据管理有：字符串对象（sds），压缩列表（ziplist），这里有整数集合（intset），都分别针对不同的应用场景进行应用。

2. 接口

2.1. 数值编码

 
    #define INT8_MAX         127
    #define INT16_MAX        32767
    #define INT32_MAX        2147483647
    #define INT64_MAX        9223372036854775807LL
    
    #define INT8_MIN          -128
    #define INT16_MIN         -32768
    
    /* 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;
    }

2.2. 插入数据

检查插入数据是否大于当前编码格式，决定是否需要升级

 
    /* 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. */
        // 如果插入数值大于当前编码，那么需要升级数组
        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. */
            // 如果数值已经存在，就不需要插入数据了。*success = 0;
            if (intsetSearch(is,value,&pos)) {
                if (success) *success = 0;
                return is;
            }
    
            // 数据增长，重新申请内存。
            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;
    }

2.3. 升级

 
    /* Upgrades the intset to a larger encoding and inserts the given integer. */
    static intset *intsetUpgradeAndAdd(intset *is, int64_t value) {
        uint8_t curenc = intrev32ifbe(is->encoding);
        uint8_t newenc = _intsetValueEncoding(value);
        int length = intrev32ifbe(is->length);
    
        // 因为插入新的数据，而且数据超出了当前数组所有数值的范围才会升级。
        // 超出了负数，或者超出了正数。正数在数组末添加，负数在数组前面添加。
        int prepend = value < 0 ? 1 : 0;
    
        /* First set new encoding and resize */
        is->encoding = intrev32ifbe(newenc);
        is = intsetResize(is,intrev32ifbe(is->length)+1);
    
        /* Upgrade back-to-front so we don't overwrite values.
         * Note that the "prepend" variable is used to make sure we have an empty
         * space at either the beginning or the end of the intset. */
        // 扩充内存后，迁移数据。
        while(length--)
            _intsetSet(is,length+prepend,_intsetGetEncoded(is,length,curenc));
    
        /* Set the value at the beginning or the end. */
        if (prepend)
            _intsetSet(is,0,value);
        else
            _intsetSet(is,intrev32ifbe(is->length),value);
        is->length = intrev32ifbe(intrev32ifbe(is->length)+1);
        return is;
    }

2.4. 搜索

二分法搜索数组数据。

 
    /* 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 {
            // 检查数值是否大于或小于数组里的所有数值。
            /* 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;
            }
        }
    
        // 数值如果在存储的数据中间，用二分法查找。
        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;
        }
    }