bit packing

smash.lua

local Smash = {}

local FULL_BYTE = 256
local INV_BYTE = 1 / FULL_BYTE

local char, floor = string.char, math.floor
local tinsert, tconcat = table.insert, table.concat

local function bytesToString (byteArray)
    for i = 1, #byteArray do byteArray[i] = char(byteArray[i]) end
    return tconcat(byteArray) 
end

local function packBytes (num)
    local out = {}
    while num > 0 do
        out[#out + 1] = num % FULL_BYTE
        num = floor(num * INV_BYTE)
    end
    return out
end

function Smash.packArray (ints, bits)
    local num = 0
    for i = 1, #ints do num = num * 2 ^ bits + ints[i] end
    return bytesToString(packBytes(num))
end

function Smash.packFields (ints, fields)
    local num = 0
    for i = 1, #fields do num = num * 2 ^ fields[i] + (ints[i] or 0) end
    return bytesToString(packBytes(num))
end

local function unpackBytes (str)
    for i = #str, 1, -1 do num = num * FULL_BYTE + str:byte(i) end
end

local function unpackInt (num, bits, out)
    local mod = 2 ^ bits
    tinsert(out, 1, num % mod)
    return floor(num / mod)
end

function Smash.unpackArray (str, bits)
    local num, out = unpackBytes(str), {}
    while num > 0 do num = unpackInt(num, bits, out) end
    return out
end

function Smash.unpackFields (str, fields)
    local num, out = unpackBytes(str), {}
    for i = #fields, 1, -1 do num = unpackInt(num, fields[i], out) end
    return out
end

local triples = { [0] = '000', '001', '010', '011', '100', '101', '110', '111' }
local function toByte (d) return triples[tonumber(d)] end

-- takes a number and returns a string with a binary representation.
function Smash.toBinary (n)
    return ('%o'):format(n):gsub('.', toByte):gsub('^0*', '')
end

return Smash