Sparse index lookup

An SSTable uses a sparse index: only the FIRST key of each data block is indexed. You are given block_first_keys, a list of the first integer key of each block in ascending block order (so the list is sorted ascending and all blocks are non-empty and key ranges don't overlap). For a list of point-lookup queries (integer keys), return for each query the 0-based index of the block that MUST be scanned to find it: the block whose first key is the largest first-key <= the query. If the query is smaller than the first block's first key, the key cannot exist, so return -1 for that query. Return the list of block indices, one per query in order. <= 10^5 blocks and queries.