#!/usr/bin/env python3

"""
Get full hashes.  Remove end hashes.

Each record in our input corresponds to one file.

These records in our input have 2 columsn: the end hash, and the pathname.
To form our output, we remove the first column, then prepend the "full hash", which is the blake2b hash of the last Kilobyte of
the file.

There's not a lot of opportunity for reuse here, so we forgo most of the usual parameterization.
"""

import hashlib
import os
import sys

import readline0


def get_full_hash(filename: bytes) -> bytes:
    """Get a cryptographic digest of `filename` in its entirety."""
    hasher = hashlib.blake2b(digest_size=32)  # Can produce digests between 1 and 64 bytes
    with open(filename, "rb") as file_:
        while True:
            buffer = file_.read(2**20)
            if not buffer:
                break
            hasher.update(buffer)
        return hasher.hexdigest().encode("UTF-8")


def usage(retval: int) -> None:
    """Output a usage message."""
    if retval == 0:
        file_ = sys.stdout
    else:
        file_ = sys.stderr

    print(f"Usage: {sys.argv[0]} --write-file-to /path/name.txt --help", file=file_)

    sys.exit(retval)


def main() -> None:
    """Start the ball rolling."""
    write_count_to = ""
    while sys.argv[1:]:
        match sys.argv[1]:
            case "--write-count-to":
                write_count_to = sys.argv[2]
                del sys.argv[1]
            case "--help" | "-h":
                usage(0)
            case _:
                print(f"{sys.argv[0]}: unrecognized option: {sys.argv[1]}", file=sys.stderr)
                usage(1)
        del sys.argv[1]

    for index, record in enumerate(readline0.readline0(file_=0, separator=b"\0")):
        if not record:
            break
        input_fields = record.split(b",", 2)
        full_hash = get_full_hash(input_fields[1])
        output_fields = [full_hash, input_fields[1]]
        os.write(1, b",".join(output_fields) + b"\0")

    if write_count_to:
        with open(write_count_to, "w") as file_:
            file_.write(str(index) + "\n")


main()