深入剖析 redis 数据结构 intset

typedef struct intset {
    // 每个整数的类型
    uint32_t encoding;
    // intset 长度
    uint32_t length;
    // 整数数组
    int8_t contents[];
} intset;

/* 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))

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 */
    // 集合为空
    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. */
        // value 比最大元素还大
        if (value > _intsetGet(is,intrev32ifbe(is->length)-1)) {
            if (pos) *pos = intrev32ifbe(is->length);
            return 0;
        // value 比最小元素还小
        } else if (value < _intsetGet(is,0)) {
            if (pos) *pos = 0;
            return 0;
        }
    }
    // 二分查找
    while(max >= min) {
        mid = (min+max)/2;
        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;
    }
}

/* 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. */
    // 需要插入整数的所需内存超出了原有集合整数的范围，即内存类型不同，
    // 则升级整数类型
    if (valenc > intrev32ifbe(is->encoding)) {
        /* This always succeeds, so we don&#039;t need to curry *success. */
        return intsetUpgradeAndAdd(is,value);
    // 正常，分配内存，插入
    } else {
        // intset 内部不允许重复
        /* 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;
        }
        // realloc
        is = intsetResize(is,intrev32ifbe(is->length)+1);
        // 迁移内存，腾出空间给新的数据。intsetMoveTail() 完成内存迁移工作
        if (pos < intrev32ifbe(is->length)) intsetMoveTail(is,pos,pos+1);
    }
    // 在腾出的空间中设置新的数据
    _intsetSet(is,pos,value);
    // 更新 intset size
    is->length = intrev32ifbe(intrev32ifbe(is->length)+1);
    return is;
}
// 升级整数类型，譬如从 short->int。当插入数据的内存占用比原有数据大
// 的时候，会被调用
/* 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);
    // value<0 头插，value>0 尾插
    int prepend = value < 0 ? 1 : 0;
    // realloc
    /* First set new encoding and resize */
    is->encoding = intrev32ifbe(newenc);
    is = intsetResize(is,intrev32ifbe(is->length)+1);
    // 逆向处理，防止数据被覆盖，一般的插入排序步骤
    /* Upgrade back-to-front so we don&#039;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. */
    while(length--)
        _intsetSet(is,length+prepend,_intsetGetEncoded(is,length,curenc));
    // value<0 放在集合开头，否则放在集合末尾。
    // 因为，此函数是对整数所占内存进行升级，意味着 value 不是在集合中最大就是最小！
    /* Set the value at the beginning or the end. */
    if (prepend)
        _intsetSet(is,0,value);
    else
        _intsetSet(is,intrev32ifbe(is->length),value);
    // 更新 set size
    is->length = intrev32ifbe(intrev32ifbe(is->length)+1);
    return is;
}