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71 lines
2.1 KiB
Python
71 lines
2.1 KiB
Python
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# Copyright 2024 Google LLC
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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CRC_POLY = 0x04C11DB7
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def precompute_table(bits):
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lookup_table = []
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for i in xrange(2**bits):
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rr = i << (32 - bits)
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for x in xrange(bits):
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if rr & 0x80000000:
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rr = (rr << 1) ^ CRC_POLY
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else:
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rr <<= 1
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lookup_table.append(rr & 0xffffffff)
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return lookup_table
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lookup_table = precompute_table(8)
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def process_word(data, crc=0xffffffff):
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if (len(data) < 4):
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# The CRC data is "padded" in a very unique and confusing fashion.
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data = data[::-1] + '\0' * (4 - len(data))
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for char in reversed(data):
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b = ord(char)
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crc = ((crc << 8) ^ lookup_table[(crc >> 24) ^ b]) & 0xffffffff
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return crc
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def process_buffer(buf, c=0xffffffff):
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word_count = (len(buf) + 3) / 4
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crc = c
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for i in xrange(word_count):
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crc = process_word(buf[i * 4 : (i + 1) * 4], crc)
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return crc
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def crc32(data):
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return process_buffer(data)
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if __name__ == '__main__':
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import sys
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assert(0x89f3bab2 == process_buffer("123 567 901 34"))
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assert(0xaff19057 == process_buffer("123456789"))
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assert(0x519b130 == process_buffer("\xfe\xff\xfe\xff"))
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assert(0x495e02ca == process_buffer("\xfe\xff\xfe\xff\x88"))
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print "All tests passed!"
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# arg1 == path to file to crc
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# arg2 == only crc first N bytes of file specified in arg 1
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if len(sys.argv) >= 2:
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if len(sys.argv) >= 3:
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b = open(sys.argv[1]).read(int(sys.argv[2]))
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else:
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b = open(sys.argv[1]).read()
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crc = crc32(b)
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print "%u or 0x%x" % (crc, crc)
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