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- # -*- coding: utf-8 -*-
- """
- hpack/huffman_decoder
- ~~~~~~~~~~~~~~~~~~~~~
- An implementation of a bitwise prefix tree specially built for decoding
- Huffman-coded content where we already know the Huffman table.
- """
- from .compat import to_byte, decode_hex
- class HuffmanEncoder(object):
- """
- Encodes a string according to the Huffman encoding table defined in the
- HPACK specification.
- """
- def __init__(self, huffman_code_list, huffman_code_list_lengths):
- self.huffman_code_list = huffman_code_list
- self.huffman_code_list_lengths = huffman_code_list_lengths
- def encode(self, bytes_to_encode):
- """
- 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 char in bytes_to_encode:
- byte = to_byte(char)
- 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.
- final_num = hex(final_num)[2:].rstrip('L')
- # If this is odd, prepend a zero.
- final_num = '0' + final_num if len(final_num) % 2 != 0 else final_num
- # 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(final_num) != expected_digits:
- missing_digits = expected_digits - len(final_num)
- final_num = ('0' * missing_digits) + final_num
- return decode_hex(final_num)
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