#!/usr/bin/env python3
"""Grenade 靶场 /var/backups/creds.b92 的 base92 解码器（零依赖）。

用法：
    python3 decode_creds_b92.py creds.b92
    cat creds.b92 | python3 decode_creds_b92.py
"""
import sys

ALPHABET = (
    "!"
    + "".join(chr(ord("#") + i) for i in range(61))
    + "".join(chr(ord("a") + i) for i in range(29))
)
VALUES = {char: index for index, char in enumerate(ALPHABET)}


def base92_decode(data: bytes) -> bytes:
    if data == b"~":
        return b""
    if len(data) == 1:
        raise ValueError("invalid base92 string (single character)")

    bit_buffer = 0
    bit_count = 0
    result = bytearray()

    index = 0
    while index < len(data) - 1:
        chunk = VALUES[chr(data[index])] * 91 + VALUES[chr(data[index + 1])]
        if chunk >= 8192:
            raise ValueError("invalid base92 chunk")
        bit_buffer = (bit_buffer << 13) | chunk
        bit_count += 13
        while bit_count >= 8:
            result.append(bit_buffer >> (bit_count - 8))
            bit_buffer &= (1 << (bit_count - 8)) - 1
            bit_count -= 8
        index += 2

    if index < len(data):
        bit_buffer = (bit_buffer << 6) | VALUES[chr(data[index])]
        bit_count += 6
        while bit_count >= 8:
            result.append(bit_buffer >> (bit_count - 8))
            bit_buffer &= (1 << (bit_count - 8)) - 1
            bit_count -= 8

    return bytes(result)


def main() -> None:
    if len(sys.argv) > 1:
        with open(sys.argv[1], "rb") as handle:
            raw = handle.read()
    else:
        raw = sys.stdin.buffer.read()
    print(base92_decode(raw.strip()).decode("utf-8"))


if __name__ == "__main__":
    main()