Usage: chunksum <dir> [<alg_name>] [<prev_chunksums_file>]

alg_name:
  Format "fc[k|m|g][0-9][sha2|blake2b|blake2s][32]".

  For example, "fck4sha2", means using FastCDC("fc") with an
  average chunk size of 2**8=256KB("k8") and using sha256("sha2")
  to calculate the checksum.

  "fcm4blake2b32" means using FastCDC with an average chunk size
  of 2**4=16MB("m4") and using "blake2b" to calculate and output
  a checksum of length "32" bytes(save storage).

  For large files, you may using large chunk size, such as "m4",
  to reduce the number of chunks.

  (default: fck4sha2)

prev_chunksums_file:
  You can specify the previous chunksums file if you want to
  resume a previous check, or if you want to find the incremental
  updates (new files) of the directory.


Examples:

  $ chunksum /etc > ~/etc.chunksums

  $ chunksum ~/Videos fcm4blake2b32 > ~/Videos/chunksums

  $ chunksum ~/Videos fcm4blake2b32 ~/chunksums > ~/chunksums.incr