[Python-Dev] Re: [Distutils] Questions about distutils strategy
James C. Ahlstrom
jim@interet.com
Tue, 14 Dec 1999 10:22:56 -0500
Guido van Rossum wrote:
> I propose to use Gary Brown's code. I'll defend this to CNRI's
> lawyers if need be.
>
> Jim, have you checked that this is the right CRC to use for zip's CRC?
> (This in the light of Tim's assertion that there are many CRCs around.)
The CRC it calculates agrees with the CRC of WinZip for all
files I have tried. The original Gary Brown code was much
longer and included file reading. Here is the shortened version:
JimA
# * Crc - 32 BIT ANSI X3.66 CRC checksum files
#*********************************************************************\
#* *|
#* Demonstration program to compute the 32-bit CRC used as the frame *|
#* check sequence in ADCCP (ANSI X3.66, also known as FIPS PUB 71 *|
#* and FED-STD-1003, the U.S. versions of CCITT's X.25 link-level *|
#* protocol). The 32-bit FCS was added via the Federal Register, *|
#* 1 June 1982, p.23798. I presume but don't know for certain that *|
#* this polynomial is or will be included in CCITT V.41, which *|
#* defines the 16-bit CRC (often called CRC-CCITT) polynomial. FIPS *|
#* PUB 78 says that the 32-bit FCS reduces otherwise undetected *|
#* errors by a factor of 10^-5 over 16-bit FCS. *|
#* *|
#*********************************************************************
#
#* Copyright (C) 1986 Gary S. Brown. You may use this program, or
#* code or tables extracted from it, as desired without restriction.
# First, the polynomial itself and its table of feedback terms. The
# polynomial is
# X^32+X^26+X^23+X^22+X^16+X^12+X^11+X^10+X^8+X^7+X^5+X^4+X^2+X^1+X^0
# Note that we take it "backwards" and put the highest-order term in
# the lowest-order bit. The X^32 term is "implied"; the LSB is the
# X^31 term, etc. The X^0 term (usually shown as "+1") results in
# the MSB being 1.
# Note that the usual hardware shift register implementation, which
# is what we're using (we're merely optimizing it by doing eight-bit
# chunks at a time) shifts bits into the lowest-order term. In our
# implementation, that means shifting towards the right. Why do we
# do it this way? Because the calculated CRC must be transmitted in
# order from highest-order term to lowest-order term. UARTs transmit
# characters in order from LSB to MSB. By storing the CRC this way,
# we hand it to the UART in the order low-byte to high-byte; the UART
# sends each low-bit to hight-bit; and the result is transmission bit
# by bit from highest- to lowest-order term without requiring any bit
# shuffling on our part. Reception works similarly.
# The feedback terms table consists of 256, 32-bit entries. Notes:
#
# 1. The table can be generated at runtime if desired; code to do so
# is shown later. It might not be obvious, but the feedback
# terms simply represent the results of eight shift/xor opera-
# tions for all combinations of data and CRC register values.
#
# 2. The CRC accumulation logic is the same for all CRC polynomials,
# be they sixteen or thirty-two bits wide. You simply choose the
# appropriate table. Alternatively, because the table can be
# generated at runtime, you can start by generating the table for
# the polynomial in question and use exactly the same "updcrc",
# if your application needn't simultaneously handle two CRC
# polynomials. (Note, however, that XMODEM is strange.)
#
# 3. For 16-bit CRCs, the table entries need be only 16 bits wide;
# of course, 32-bit entries work OK if the high 16 bits are zero.
#
# 4. The values must be right-shifted by eight bits by the "updcrc"
# logic; the shift must be unsigned (bring in zeroes). On some
# hardware you could probably optimize the shift in assembler by
# using byte-swap instructions.
# Converted to Python by James C. Ahlstrom
crc_32_tab = [ # CRC polynomial 0xedb88320
0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f,
0xe963a535, 0x9e6495a3,
0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd,
0xe7b82d07, 0x90bf1d91,
0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb,
0xf4d4b551, 0x83d385c7,
0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
0xfa0f3d63, 0x8d080df5,
0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172, 0x3c03e4d1, 0x4b04d447,
0xd20d85fd, 0xa50ab56b,
0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75,
0xdcd60dcf, 0xabd13d59,
0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423,
0xcfba9599, 0xb8bda50f,
0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924, 0x2f6f7c87, 0x58684c11,
0xc1611dab, 0xb6662d3d,
0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
0x9fbfe4a5, 0xe8b8d433,
0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d,
0x91646c97, 0xe6635c01,
0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed, 0x1b01a57b,
0x8208f4c1, 0xf50fc457,
0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49,
0x8cd37cf3, 0xfbd44c65,
0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7,
0xa4d1c46d, 0xd3d6f4fb,
0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
0xaa0a4c5f, 0xdd0d7cc9,
0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3,
0xb966d409, 0xce61e49f,
0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81,
0xb7bd5c3b, 0xc0ba6cad,
0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af,
0x04db2615, 0x73dc1683,
0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d,
0x0a00ae27, 0x7d079eb1,
0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
0x196c3671, 0x6e6b06e7,
0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9,
0x17b7be43, 0x60b08ed5,
0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767,
0x3fb506dd, 0x48b2364b,
0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55,
0x316e8eef, 0x4669be79,
0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236, 0xcc0c7795, 0xbb0b4703,
0x220216b9, 0x5505262f,
0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
0x2cd99e8b, 0x5bdeae1d,
0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a, 0x9c0906a9, 0xeb0e363f,
0x72076785, 0x05005713,
0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b, 0xe5d5be0d,
0x7cdcefb7, 0x0bdbdf21,
0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b,
0x6fb077e1, 0x18b74777,
0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69,
0x616bffd3, 0x166ccf45,
0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
0x4969474d, 0x3e6e77db,
0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5,
0x47b2cf7f, 0x30b5ffe9,
0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693,
0x54de5729, 0x23d967bf,
0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1,
0x5a05df1b, 0x2d02ef8d
]
def crc32(string):
crc = 0xFFFFFFFF
for ch in string:
crc = crc_32_tab[((crc) ^ ord(ch)) & 0xff] ^ (((crc) >> 8) &
0xFFFFFF)
return ~crc