import hashlib
import zlib
from typing import Tuple


class GSZ3DecompressionError(Exception):
    pass


class GSZ3Decompressor:
    MAGIC = b'GSZ3'
    VERSION = 1

    @staticmethod
    def decompress(data: bytes) -> str:
        if len(data) < 12:
            raise GSZ3DecompressionError("Data too short for GSZ3 header")

        if data[:4] != GSZ3Decompressor.MAGIC:
            raise GSZ3DecompressionError("Invalid GSZ3 magic number")

        version = int.from_bytes(data[4:5], 'big')
        if version != GSZ3Decompressor.VERSION:
            raise GSZ3DecompressionError(f"Unsupported GSZ3 version: {version}")

        payload_len = int.from_bytes(data[5:9], 'big')
        stored_checksum = data[9:12]

        if len(data) < 12 + payload_len:
            raise GSZ3DecompressionError("Incomplete payload")

        payload = data[12:12 + payload_len]
        computed_checksum = GSZ3Decompressor._compute_checksum(payload)

        if computed_checksum != stored_checksum:
            raise GSZ3DecompressionError("Checksum mismatch")

        try:
            decompressed = zlib.decompress(payload)
            return decompressed.decode('utf-8')
        except (zlib.error, UnicodeDecodeError) as e:
            raise GSZ3DecompressionError(f"Decompression failed: {e}")

    @staticmethod
    def compress(text: str) -> bytes:
        data = text.encode('utf-8')
        compressed = zlib.compress(data, level=9)
        checksum = GSZ3Decompressor._compute_checksum(compressed)
        payload_len = len(compressed)

        header = GSZ3Decompressor.MAGIC
        header += bytes([GSZ3Decompressor.VERSION])
        header += payload_len.to_bytes(4, 'big')
        header += checksum

        return header + compressed

    @staticmethod
    def _compute_checksum(data: bytes) -> bytes:
        h = hashlib.sha256(data).digest()
        return h[:3]