Merkle树设置数据和比较结果

Question

此代码用于Merkle树，完整代码可在此链接https://github.com/weichaoguo/merkle-tree/blob/master/merkle_tree.c上访问

让我感到困惑的部分在下面这段代码中：

所以我想知道为什么会有(1 << a->tree_height)-1和所有其他包含<<的行。<<在这里用来做什么？

//compare two merkle trees from node i
//make sure the two trees in same height
//return different data block number
//if no differnece return 0
int tree_cmp(merkle_tree *a, merkle_tree *b, size_t i) {

    int cmp;
    if (i > (1<<a->tree_height)-1)
        return -1;
    if (memcmp(a->nodes[i].hash, b->nodes[i].hash, a->hash_size) != 0) {
        if (i<<1 > (1<<a->tree_height)-1)
            return i - (1 << (a->tree_height - 1)) + 1;
        else {
            cmp = tree_cmp(a, b, i<<1);
            if (cmp == 0)
                return tree_cmp(a, b, (i<<1)+1);
            else
                return cmp;
        }
    }
    else
        return 0;
}

// set tree data with specific block number
//
int set_tree_data(merkle_tree *mt, size_t block_num, char *data) {

    if (block_num > mt->data_blocks)
        return -1;
    size_t i = (1 << (mt->tree_height - 1)) + block_num - 1;
    if (mt->nodes[i].data)
        free(mt->nodes[i].data);
    mt->nodes[i].data = data;
    if (hash_node(mt, i) == -1)
        return -1;
    for (i>>=1; i>0; i>>=1)
        if (hash_node(mt, i) == -1)
            return -1;
    return 0;
}

Answer 1

这是一个位移位。(1 << x) - 1设置了x位。