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59 lines
1.8 KiB
Python
59 lines
1.8 KiB
Python
"""Python implementation of Z85 85-bit encoding
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Z85 encoding is a plaintext encoding for a bytestring interpreted as 32bit integers.
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Since the chunks are 32bit, a bytestring must be a multiple of 4 bytes.
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See ZMQ RFC 32 for details.
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"""
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# Copyright (C) PyZMQ Developers
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# Distributed under the terms of the Modified BSD License.
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from __future__ import annotations
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import struct
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# Z85CHARS is the base 85 symbol table
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Z85CHARS = b"0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ.-:+=^!/*?&<>()[]{}@%$#"
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# Z85MAP maps integers in [0,84] to the appropriate character in Z85CHARS
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Z85MAP = {c: idx for idx, c in enumerate(Z85CHARS)}
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_85s = [85**i for i in range(5)][::-1]
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def encode(rawbytes):
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"""encode raw bytes into Z85"""
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# Accepts only byte arrays bounded to 4 bytes
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if len(rawbytes) % 4:
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raise ValueError(f"length must be multiple of 4, not {len(rawbytes)}")
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nvalues = len(rawbytes) // 4
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values = struct.unpack(f'>{nvalues:d}I', rawbytes)
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encoded = []
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for v in values:
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for offset in _85s:
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encoded.append(Z85CHARS[(v // offset) % 85])
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return bytes(encoded)
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def decode(z85bytes):
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"""decode Z85 bytes to raw bytes, accepts ASCII string"""
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if isinstance(z85bytes, str):
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try:
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z85bytes = z85bytes.encode('ascii')
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except UnicodeEncodeError:
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raise ValueError('string argument should contain only ASCII characters')
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if len(z85bytes) % 5:
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raise ValueError(f"Z85 length must be multiple of 5, not {len(z85bytes)}")
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nvalues = len(z85bytes) // 5
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values = []
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for i in range(0, len(z85bytes), 5):
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value = 0
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for j, offset in enumerate(_85s):
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value += Z85MAP[z85bytes[i + j]] * offset
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values.append(value)
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return struct.pack(f'>{nvalues:d}I', *values)
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