If it doesn't have to be really fast, you can simply binary search fetching no more than 29 lines from the file. Or you can interpolate the expected location of

If it doesn't have to be really fast, you can simply binary search fetching no more than 29 lines from the file. Or you can interpolate the expected location of the hash and read a block around that location. This could get you down to reading only one or two disk blocks if the actual position is never more than half a disk block away from the interpolated position. It could be more but given that we are dealing with cryptographic hashes I would expect the interpolated position to never be too far away from the actual position.