base64

-- Make some functions easier to write
local floor = math.floor
local sub = string.sub
local gsub = string.gsub
local rem = table.remove

-- Our base64 value table
local base64 = { ['A']=0,['B']=1,['C']=2,['D']=3,['E']=4,['F']=5,['G']=6,['H']=7,['I']=8,
                ['J']=9,['K']=10,['L']=11,['M']=12,['N']=13,['O']=14,['P']=15,['Q']=16,
                ['R']=17,['S']=18,['T']=19,['U']=20,['V']=21,['W']=22,['X']=23,['Y']=24,
                ['Z']=25,['a']=26,['b']=27,['c']=28,['d']=29,['e']=30,['f']=31,['g']=32,
                ['h']=33,['i']=34,['j']=35,['k']=36,['l']=37,['m']=38,['n']=39,['o']=40,
                ['p']=41,['q']=42,['r']=43,['s']=44,['t']=45,['u']=46,['v']=47,['w']=48,
                ['x']=49,['y']=50,['z']=51,['0']=52,['1']=53,['2']=54,['3']=55,['4']=56,
                ['5']=57,['6']=58,['7']=59,['8']=60,['9']=61,['+']=62,['/']=63,['=']=nil}

-- Decimal values for binary digits
local bin ={}
local mult = 1
for i = 1,40 do
    bin[i] = mult
    mult = mult*2
end

-- A buffer we will use to process the bits
local buffer = 0
local pos = 0
local function clearBuffer()
    buffer = 0
    pos = 1
end

-- Shift all of the bits up in the buffer and put the base64 number on the bottom
local function pushBase64(n)
    if base64[n] == nil then return end
    buffer = buffer * bin[7] + base64[n]
    pos = pos + 6
end

-- Get an int out of the buffer. This is tricky. The byte order is in little endian so we're going
-- to have to isolate and cut the bytes out and then move them around.
local function getInt()
    -- If our buffer isn't filled all the way then fill it with zeros
    while pos < 33 do
        buffer = buffer * bin[2]
        pos = pos + 1
    end
    -- Move the buffer position to just below the integer.
    pos = pos - 32

    -- Swap the first and forth byte and then the second and third.
    local tmp = floor((buffer%bin[33+pos-1])/bin[25+pos-1]) +
                floor((buffer%bin[25+pos-1])/bin[17+pos-1])*bin[9] +
                floor((buffer%bin[17+pos-1])/bin[9+pos-1])*bin[17] +
                floor((buffer%bin[9+pos-1])/bin[pos])*bin[25]

    -- We've got our integer so let's cut that portion out of the buffer
    buffer = buffer % bin[pos]
    -- Return the int
    return  tmp
end

-- Get a byte out of the buffer
local function getByte()
    -- If our buffer isn't filled all the way then fill it with zeros
    while pos < 9 do
        buffer = buffer * bin[2]
        pos = pos + 1
    end
    -- Move the buffer position to just below the byte.
    pos = pos - 8
    -- Cut out the byte
    local tmp = floor((buffer%bin[9+pos-1])/bin[pos])
    -- Delete the byte from the buffer
    buffer = buffer % bin[pos]
    -- Return the byte
    return tmp
end

-- Glues together an integer from four bytes. Little endian
local function glueInt(b1, b2, b3, b4)
    return b1%bin[9] + b2%bin[9]*bin[9] + b3%bin[9]*bin[17] + b4%bin[9]*bin[25]
end

-- A Lua set that will filter out characters that aren't in the base64 table
local set = "[^%a%d%+%/%=]"

-- Decodes a base64 string into the given type
local function decode(mode, raw)

    -- Make sure the mode is supported
    assert(mode=="string" or mode=="int" or mode=="byte", "Base64 decode - Invalid mode: " .. mode)

    -- Clear the buffer
    clearBuffer()

    -- Filters undefined characters out of the string
    raw = gsub(raw, set, "")

    local size = 0          -- Size of the returned type in bits
    local val = {}          -- A table containing the data to be returned
    local raw_pos = 1       -- The position of the progress through the raw base64 string
    local raw_size = #raw   -- The size of the base64 string
    local char = ""         -- The current base64 character to be processed

    -- If we're expected to return an int then the bit size is 32, otherwise it's 8
    if mode == "int" then size = 32 else size = 8 end

    -- While we still have input
    while raw_pos <= raw_size do
        -- Fill the buffer until we have enough bits
        while pos <= size and raw_pos <= raw_size do
            char = sub(raw,raw_pos,raw_pos)
            pushBase64( char )
            raw_pos = raw_pos + 1
        end
        -- If a nil character is encountered the end the loop
        if char == "=" then break end
        -- Get data from the buffer depending on the type
        if mode == "string" then val[#val+1] = string.char( getByte() ) end
        if mode == "byte" then val[#val+1] = getByte() end
        if mode == "int" then val[#val+1] = getInt() end
    end

    if mode == "string" then return table.concat(val) end
    return val
end

-- Returns the functions
return {decode = decode, glueInt = glueInt, base64 = base64}