41 lines
927 B
Python
Executable File
41 lines
927 B
Python
Executable File
#!python3
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import math
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BLOCK_SIZE = 16
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UMAX = int(math.pow(256, BLOCK_SIZE))
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def remove_line(s):
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# returns the header line, and the rest of the file
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return s[:s.index(b'\n') + 1], s[s.index(b'\n')+1:]
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def parse_header_ppm(f):
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data = f.read()
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header = b""
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for i in range(3):
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header_i, data = remove_line(data)
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header += header_i
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return header, data
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with open("body.enc.ppm", 'rb') as f:
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header, data = parse_header_ppm(f)
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blocks = [data[i * BLOCK_SIZE:(i+1) * BLOCK_SIZE] for i in range(len(data) // BLOCK_SIZE)]
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for i in reversed(range(len(blocks) - 1)):
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prev_blk = int.from_bytes(blocks[i], 'big')
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curr_blk = int.from_bytes(blocks[i+1], 'big')
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n_curr_blk = (curr_blk - prev_blk) % UMAX
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blocks[i+1] = n_curr_blk.to_bytes(16, 'big')
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blocks = blocks[1:]
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with open("body.ppm", 'wb') as f:
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f.write(header)
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for block in blocks:
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f.write(block)
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