local FILLER = 61

local Alphabet = {}
local Indexes = {}

for index = 65, 90 do table.insert(Alphabet, index) end -- A-Z
for index = 97, 122 do table.insert(Alphabet, index) end -- a-z
for index = 48, 57 do table.insert(Alphabet, index) end -- 0-9

table.insert(Alphabet, 43) -- +
table.insert(Alphabet, 47) -- /

for index, character in pairs(Alphabet) do
	Indexes[character] = index
end

local function BuildString(values)
	local output = {}

	for index = 1, #values, 4096 do
		table.insert(output, string.char(
			unpack(values, index, math.min(index + 4096 - 1, #values))
		))
	end

	return table.concat(output, "")
end

local Base64 = {}

function Base64.Encode(input)
	local output = {}

	for index = 1, #input, 3 do
		local C1, C2, C3 = string.byte(input, index, index + 2)

		local A = bit.rshift(C1, 2)
		local B = bit.lshift(bit.band(C1, 3), 4) + bit.rshift(C2 or 0, 4)
		local C = bit.lshift(bit.band(C2 or 0, 15), 2) + bit.rshift(C3 or 0, 6)
		local D = bit.band(C3 or 0, 63)

		output[#output + 1] = Alphabet[A + 1]
		output[#output + 1] = Alphabet[B + 1]
		output[#output + 1] = C2 and Alphabet[C + 1] or Filler
		output[#output + 1] = C3 and Alphabet[D + 1] or Filler
	end

	return BuildString(output)
end

function Base64.Decode(input)
	local output = {}

	for index = 1, #input, 4 do
		local C1, C2, C3, C4 = string.byte(input, index, index + 3)

		local I1 = Indexes[C1] - 1
		local I2 = Indexes[C2] - 1
		local I3 = (Indexes[C3] or 1) - 1
		local I4 = (Indexes[C4] or 1) - 1

		local A = bit.lshift(I1, 2) + bit.rshift(I2, 4)
		local B = bit.lshift(bit.band(I2, 15), 4) + bit.rshift(I3, 2)
		local C = bit.lshift(bit.band(I3, 3), 6) + I4

		output[#output + 1] = A
		if C3 ~= Filler then output[#output + 1] = B end
		if C4 ~= Filler then output[#output + 1] = C end
	end

	return BuildString(output)
end

return Base64