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88 lines
2.4 KiB
Python
88 lines
2.4 KiB
Python
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"""Base or helper classes used a lot for dealing with file formats.
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"""
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import io
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import struct
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class Substream:
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"""Wraps a stream and pretends it starts at an offset other than 0.
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Partly implements the file interface.
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This type always seeks before reading, but doesn't do so afterwards, so
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interleaving reads with the underlying stream may not do what you want.
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"""
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def __init__(self, stream, offset=0, length=-1):
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if isinstance(stream, Substream):
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self.stream = stream.stream
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self.offset = offset + stream.offset
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else:
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self.stream = stream
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self.offset = offset
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self.length = length
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self.pos = 0
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def __repr__(self):
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return "<{} of {} at {}>".format(
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type(self).__name__, self.stream, self.offset)
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def read(self, n=-1):
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self.stream.seek(self.offset + self.pos)
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if n < 0:
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n = self.length
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elif self.length >= 0 and n > self.length:
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n = self.length
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data = self.stream.read(n)
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self.pos += len(data)
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return data
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def seek(self, offset):
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offset = max(offset, 0)
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if self.length >= 0:
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offset = min(offset, self.length)
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self.stream.seek(self.offset + offset)
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self.pos = self.tell()
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def tell(self):
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return self.stream.tell() - self.offset
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def __len__(self):
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if self.length < 0:
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pos = self.stream.tell()
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self.stream.seek(0, io.SEEK_END)
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parent_length = self.stream.tell()
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self.stream.seek(pos)
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return parent_length - self.offset
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else:
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return self.length
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def peek(self, n):
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pos = self.stream.tell()
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self.stream.seek(self.offset + self.pos)
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data = self.stream.read(n)
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self.stream.seek(pos)
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return data
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def unpack(self, fmt):
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"""Unpacks a struct format from the current position in the stream."""
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data = self.read(struct.calcsize(fmt))
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return struct.unpack(fmt, data)
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def slice(self, offset, length=-1):
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# TODO limit or warn if length is too long for this slice?
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return Substream(self, self.offset + offset, length)
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class _ContainerFile:
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slices = ()
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def __len__(self):
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return len(self.slices)
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def __iter__(self):
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return iter(self.slices)
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def __getitem__(self, key):
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return self.slices[key]
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