67 lines
2.4 KiB
Python
67 lines
2.4 KiB
Python
# -*- coding: utf-8 -*-
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"""
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hpack/huffman_decoder
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~~~~~~~~~~~~~~~~~~~~~
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An implementation of a bitwise prefix tree specially built for decoding
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Huffman-coded content where we already know the Huffman table.
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"""
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class HuffmanEncoder:
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"""
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Encodes a string according to the Huffman encoding table defined in the
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HPACK specification.
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"""
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def __init__(self, huffman_code_list, huffman_code_list_lengths):
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self.huffman_code_list = huffman_code_list
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self.huffman_code_list_lengths = huffman_code_list_lengths
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def encode(self, bytes_to_encode):
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"""
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Given a string of bytes, encodes them according to the HPACK Huffman
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specification.
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"""
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# If handed the empty string, just immediately return.
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if not bytes_to_encode:
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return b''
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final_num = 0
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final_int_len = 0
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# Turn each byte into its huffman code. These codes aren't necessarily
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# octet aligned, so keep track of how far through an octet we are. To
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# handle this cleanly, just use a single giant integer.
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for byte in bytes_to_encode:
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bin_int_len = self.huffman_code_list_lengths[byte]
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bin_int = self.huffman_code_list[byte] & (
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2 ** (bin_int_len + 1) - 1
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)
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final_num <<= bin_int_len
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final_num |= bin_int
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final_int_len += bin_int_len
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# Pad out to an octet with ones.
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bits_to_be_padded = (8 - (final_int_len % 8)) % 8
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final_num <<= bits_to_be_padded
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final_num |= (1 << bits_to_be_padded) - 1
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# Convert the number to hex and strip off the leading '0x' and the
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# trailing 'L', if present.
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final_num = hex(final_num)[2:].rstrip('L')
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# If this is odd, prepend a zero.
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final_num = '0' + final_num if len(final_num) % 2 != 0 else final_num
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# This number should have twice as many digits as bytes. If not, we're
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# missing some leading zeroes. Work out how many bytes we want and how
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# many digits we have, then add the missing zero digits to the front.
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total_bytes = (final_int_len + bits_to_be_padded) // 8
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expected_digits = total_bytes * 2
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if len(final_num) != expected_digits:
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missing_digits = expected_digits - len(final_num)
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final_num = ('0' * missing_digits) + final_num
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return bytes.fromhex(final_num)
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