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73 lines
1.7 KiB
Lua
73 lines
1.7 KiB
Lua
-- This is released into the public domain.
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-- No warranty is provided, implied or otherwise.
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-- BDIVIDE r5 edition
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-- Algorithm simplified for smaller implementation and potentially better compression
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-- format:
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-- 0-127 for constants
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-- <128 + (length - 4)>, <position high>, <position low>
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-- Position is where in the window it was found, minus 1.
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-- windowSize must be the same between the encoder and decoder,
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-- and is the amount of data preserved after cropping.
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local bdivCore = {}
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function bdivCore.bdivide(blk, p)
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local out = ""
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local windowSize = 0x10000
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local windowData = ("\x00"):rep(windowSize)
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while blk ~= "" do
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p(blk)
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local bestData = blk:sub(1, 1)
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assert(blk:byte() < 128, "BDIVIDE does not handle 8-bit data")
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local bestRes = bestData
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for lm = 0, 127 do
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local al = lm + 4
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local pfx = blk:sub(1, al)
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if #pfx ~= al then
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break
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end
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local p = windowData:find(pfx, 1, true)
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if not p then
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break
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end
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local pm = p - 1
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local thirdByte = pm % 256
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bestData = string.char(128 + lm, math.floor(pm / 256), thirdByte)
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bestRes = pfx
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end
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-- ok, encode!
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out = out .. bestData
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-- crop window
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windowData = (windowData .. bestRes):sub(-windowSize)
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blk = blk:sub(#bestRes + 1)
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end
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return out
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end
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-- Adds padding if required
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function bdivCore.bdividePad(data)
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local i = 1
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-- Basically, if it ends on a literal,
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-- then the literal won't get read without two padding bytes.
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-- Otherwise (including if no data) it's fine.
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local needsPadding = false
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while i <= #data do
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if data:byte(i) > 127 then
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i = i + 3
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needsPadding = false
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else
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i = i + 1
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needsPadding = true
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end
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end
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if needsPadding then
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return data .. "\x00\x00"
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end
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return data
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end
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return bdivCore
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