3582893d8b
1) Applied special case from C code for POS_2BYTE_MAX + 1. 2) Handle B/b format chars directly. Up to now these would have been encoded using default int encoding (so values above 63 get errors when encoding.
144 lines
5.2 KiB
Python
144 lines
5.2 KiB
Python
#!/usr/bin/env python
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#
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# Public Domain 2014-2015 MongoDB, Inc.
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# Public Domain 2008-2014 WiredTiger, Inc.
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#
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# This is free and unencumbered software released into the public domain.
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#
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# Anyone is free to copy, modify, publish, use, compile, sell, or
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# distribute this software, either in source code form or as a compiled
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# binary, for any purpose, commercial or non-commercial, and by any
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# means.
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#
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# In jurisdictions that recognize copyright laws, the author or authors
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# of this software dedicate any and all copyright interest in the
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# software to the public domain. We make this dedication for the benefit
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# of the public at large and to the detriment of our heirs and
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# successors. We intend this dedication to be an overt act of
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# relinquishment in perpetuity of all present and future rights to this
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# software under copyright law.
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#
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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
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# IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
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# OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
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# ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
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# OTHER DEALINGS IN THE SOFTWARE.
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#
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import math, struct
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# Variable-length integer packing
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# need: up to 64 bits, both signed and unsigned
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#
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# Try hard for small values (up to ~2 bytes), after that, just encode the
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# length in the first byte.
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#
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# First byte | Next | |
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# byte | bytes| Min Value | Max Value
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# ------------+------+------------------------+--------------------------------
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# [00 00xxxx] | free | N/A | N/A
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# [00 01llll] | 8-l | -2^64 | -2^13 - 2^6
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# [00 1xxxxx] | 1 | -2^13 - 2^6 | -2^6 - 1
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# [01 xxxxxx] | 0 | -2^6 | -1
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# [10 xxxxxx] | 0 | 0 | 2^6 - 1
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# [11 0xxxxx] | 1 | 2^6 | 2^13 + 2^6 - 1
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# [11 10llll] | l | 2^14 + 2^7 | 2^64 - 1
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# [11 11xxxx] | free | N/A | N/A
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NEG_MULTI_MARKER = 0x10
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NEG_2BYTE_MARKER = 0x20
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NEG_1BYTE_MARKER = 0x40
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POS_1BYTE_MARKER = 0x80
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POS_2BYTE_MARKER = 0xc0
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POS_MULTI_MARKER = 0xe0
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NEG_1BYTE_MIN = -2**6
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NEG_2BYTE_MIN = -2**13 + NEG_1BYTE_MIN
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POS_1BYTE_MAX = 2**6 - 1
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POS_2BYTE_MAX = 2**13 + POS_1BYTE_MAX
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MINUS_BIT = -1 << 64
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UINT64_MASK = 0xffffffffffffffff
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def getbits(x, start, end=0):
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'''return the least significant bits of x, from start to end'''
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return (x & ((1 << start) - 1)) >> (end)
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def get_int(b, size):
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r = 0
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for i in xrange(size):
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r = (r << 8) | ord(b[i])
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return r
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def pack_int(x):
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if x < NEG_2BYTE_MIN:
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packed = struct.pack('>Q', x & UINT64_MASK)
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while packed and packed[0] == '\xff':
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packed = packed[1:]
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return chr(NEG_MULTI_MARKER | getbits(8 - len(packed), 4)) + packed
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elif x < NEG_1BYTE_MIN:
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x -= NEG_2BYTE_MIN
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return chr(NEG_2BYTE_MARKER | getbits(x, 13, 8)) + chr(getbits(x, 8))
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elif x < 0:
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x -= NEG_1BYTE_MIN
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return chr(NEG_1BYTE_MARKER | getbits(x, 6))
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elif x <= POS_1BYTE_MAX:
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return chr(POS_1BYTE_MARKER | getbits(x, 6))
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elif x <= POS_2BYTE_MAX:
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x -= (POS_1BYTE_MAX + 1)
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return chr(POS_2BYTE_MARKER | getbits(x, 13, 8)) + chr(getbits(x, 8))
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elif x == POS_2BYTE_MAX + 1:
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# This is a special case where we could store the value with
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# just a single byte, but we append a zero byte so that the
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# encoding doesn't get shorter for this one value.
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return chr(POS_MULTI_MARKER | 0x1) + chr(0)
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else:
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packed = struct.pack('>Q', x - (POS_2BYTE_MAX + 1))
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while packed and packed[0] == '\x00':
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packed = packed[1:]
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return chr(POS_MULTI_MARKER | getbits(len(packed), 4)) + packed
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def unpack_int(b):
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marker = ord(b[0])
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if marker < NEG_2BYTE_MARKER:
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sz = 8 - getbits(marker, 4)
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return ((-1 << (sz << 3)) | get_int(b[1:], sz), b[sz+1:])
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elif marker < NEG_1BYTE_MARKER:
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return (NEG_2BYTE_MIN + ((getbits(marker, 5) << 8) | ord(b[1])), b[2:])
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elif marker < POS_1BYTE_MARKER:
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return (NEG_1BYTE_MIN + getbits(marker, 6), b[1:])
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elif marker < POS_2BYTE_MARKER:
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return (getbits(marker, 6), b[1:])
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elif marker < POS_MULTI_MARKER:
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return (POS_1BYTE_MAX + 1 +
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((getbits(marker, 5) << 8) | ord(b[1])), b[2:])
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else:
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sz = getbits(marker, 4)
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return (POS_2BYTE_MAX + 1 + get_int(b[1:], sz), b[sz+1:])
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# Sanity testing
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if __name__ == '__main__':
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import random
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for big in (100, 10000, 1 << 40, 1 << 64):
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for i in xrange(1000):
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r = random.randint(-big, big)
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print "\rChecking %d" % r,
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if unpack_int(pack_int(r))[0] != r:
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print "\nFound a problem with %d" % r
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break
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print
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for i in xrange(1000):
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r1 = random.randint(-big, big)
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r2 = random.randint(-big, big)
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print "\rChecking %d, %d" % (r1, r2),
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if cmp(r1, r2) != cmp(pack_int(r1), pack_int(r2)):
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print "\nFound a problem with %d, %d" % (r1, r2)
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break
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print
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