Not sure about zstd, but in xz the blocks (frames in zstd) are stored across the file and linked by offsets into a linked list, so you can just scan over the co

Not sure about zstd, but in xz the blocks (frames in zstd) are stored across the file and linked by offsets into a linked list, so you can just scan over the compressed file very quickly at the start, and in memory build a map of uncompressed virtual offsets to compressed file positions. Here's the code in nbdkit-xz-filter: https://gitlab.com/nbdkit/nbdkit/-/blob/master/filters/xz/xzfile.c?ref_type=heads#L133 https://gitlab.com/nbdkit/nbdkit/-/blob/master/filters/xz/xz...