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- """
- An implementation of a bitwise prefix tree specially built for decoding
- Huffman-coded content where we already know the Huffman table.
- """
- from __future__ import annotations
- class HuffmanEncoder:
- """
- Encodes a string according to the Huffman encoding table defined in the
- HPACK specification.
- """
- def __init__(self, huffman_code_list: list[int], huffman_code_list_lengths: list[int]) -> None:
- self.huffman_code_list = huffman_code_list
- self.huffman_code_list_lengths = huffman_code_list_lengths
- def encode(self, bytes_to_encode: bytes | None) -> bytes:
- """
- Given a string of bytes, encodes them according to the HPACK Huffman
- specification.
- """
- # If handed the empty string, just immediately return.
- if not bytes_to_encode:
- return b""
- final_num = 0
- final_int_len = 0
- # Turn each byte into its huffman code. These codes aren't necessarily
- # octet aligned, so keep track of how far through an octet we are. To
- # handle this cleanly, just use a single giant integer.
- for byte in bytes_to_encode:
- bin_int_len = self.huffman_code_list_lengths[byte]
- bin_int = self.huffman_code_list[byte] & (
- 2 ** (bin_int_len + 1) - 1
- )
- final_num <<= bin_int_len
- final_num |= bin_int
- final_int_len += bin_int_len
- # Pad out to an octet with ones.
- bits_to_be_padded = (8 - (final_int_len % 8)) % 8
- final_num <<= bits_to_be_padded
- final_num |= (1 << bits_to_be_padded) - 1
- # Convert the number to hex and strip off the leading '0x' and the
- # trailing 'L', if present.
- s = hex(final_num)[2:].rstrip("L")
- # If this is odd, prepend a zero.
- s = "0" + s if len(s) % 2 != 0 else s
- # This number should have twice as many digits as bytes. If not, we're
- # missing some leading zeroes. Work out how many bytes we want and how
- # many digits we have, then add the missing zero digits to the front.
- total_bytes = (final_int_len + bits_to_be_padded) // 8
- expected_digits = total_bytes * 2
- if len(s) != expected_digits:
- missing_digits = expected_digits - len(s)
- s = ("0" * missing_digits) + s
- return bytes.fromhex(s)
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