83 lines
2.1 KiB
Lua
83 lines
2.1 KiB
Lua
-- Copyright (c) 2015 Phil Leblanc -- see LICENSE file
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------------------------------------------------------------
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-- checksums: adler-32, crc-32
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------------------------------------------------------------
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local byte = string.byte
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------------------------------------------------------------
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-- adler32
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local function adler32(s)
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-- return adler32 checksum (uint32)
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-- adler32 is a checksum defined by Mark Adler for zlib
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-- (based on the Fletcher checksum used in ITU X.224)
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-- implementation based on RFC 1950 (zlib format spec), 1996
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local prime = 65521 --largest prime smaller than 2^16
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local s1, s2 = 1, 0
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-- limit s size to ensure that modulo prime can be done only at end
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-- 2^40 bytes should be enough for pure Lua with 64-bit integers...
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if #s > (1 << 40) then error("adler32: string too large") end
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for i = 1,#s do
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local b = byte(s, i)
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s1 = s1 + b
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s2 = s2 + s1
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-- no need to test or compute mod prime every turn.
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end
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s1 = s1 % prime
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s2 = s2 % prime
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return (s2 << 16) + s1
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end --adler32()
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local function crc32_nt(s)
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-- return crc32 checksum of string s as an integer
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-- uses no lookup table
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-- inspired by crc32b at
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-- http://www.hackersdelight.org/hdcodetxt/crc.c.txt
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local b, crc, mask
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crc = 0xffffffff
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for i = 1, #s do
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b = byte(s, i)
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crc = crc ~ b
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for _ = 1, 8 do --eight times
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mask = -(crc & 1)
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crc = (crc >> 1) ~ (0xedb88320 & mask)
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end
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end--for
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return (~crc) & 0xffffffff
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end --crc32_nt()
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local function crc32(s, lt)
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-- return crc32 checksum of string as an integer
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-- use lookup table lt if provided or create one on the fly
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-- if lt is empty, it is initialized.
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lt = lt or {}
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local b, crc, mask
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if not lt[1] then -- setup table
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for i = 1, 256 do
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crc = i - 1
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for _ = 1, 8 do --eight times
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mask = -(crc & 1)
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crc = (crc >> 1) ~ (0xedb88320 & mask)
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end
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lt[i] = crc
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end--for
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end--if
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-- compute the crc
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crc = 0xffffffff
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for i = 1, #s do
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b = byte(s, i)
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crc = (crc >> 8) ~ lt[((crc ~ b) & 0xFF) + 1]
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end
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return (~crc) & 0xffffffff
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end --crc32()
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return {
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-- module
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adler32 = adler32,
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crc32_nt = crc32_nt,
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crc32 = crc32,
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}
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