BASE64.LUA

#!/usr/bin/env lua

-- b64c = base64 characters
--   dt = decode table
local b64c, dt = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/', {}
for i = 1, #b64c do dt[b64c:sub(i,i)] = i - 1 end

---Encode a file into base64
---@param file file The file to turn into base64
---@return string The base64 string
function base64_encode_file(file)

	-- r = result
	-- l = length
	local r, l = {}, 0

	while true do

		-- c = chunk of 3 bytes
		local c = file:read(3)
		if not c or #c == 0 then break end

		-- Pad chunk to 3 bytes if needed
		-- pa = padded
		-- p  = pad length
		local p = 3 - #c
		for _ = 1, p do
			c = c .. '\0'
		end

		-- Convert 3 bytes to 4 base64 characters
		local b1, b2, b3, c1, c2, c3, c4 = c:byte(1, 3)
		local n = (b1 << 16) + (b2 << 8) + b3

		c1 = b64c:sub(((n >> 18) & 63) + 1, ((n >> 18) & 63) + 1)
		c2 = b64c:sub(((n >> 12) & 63) + 1, ((n >> 12) & 63) + 1)
		c3 = b64c:sub(((n >> 6) & 63) + 1, ((n >> 6) & 63) + 1)
		c4 = b64c:sub((n & 63) + 1, (n & 63) + 1)

		-- Apply padding
		if p == 1 then
			c4 = '='
		elseif p == 2 then
			c3 = '='
			c4 = '='
		end

		local en = c1 .. c2 .. c3 .. c4
		table.insert(r, en)

		l = l + 4
		if l >= 76 then -- Standard base64 line length
			table.insert(r, '\n')
			l = 0
		end
	end

	return table.concat(r)
end

---Decode a file from base64
---@param file file The file containing the base64 string to decode
---@return string The decoded raw data
function base64_decode_file(file)

	-- r = result
	-- b = buffer
	local r, b = {}, ""

	-- Read all data and remove whitespace/newlines
	while true do

		-- l = line
		local l = file:read("l")

		if not l then break end
		b = b .. l:gsub("[^" .. b64c .. "=]", "")
	end

	-- Process 4 characters at a time
	for i = 1, #b, 4 do

		-- c = chunk of 3 bytes
		--b1 = byte 1
		--b2 = byte 2
		--b3 = byte 3
		--b4 = byte 4
		--c1 = chunk 1
		--c2 = chunk 2
		--c3 = chunk 3
		local c, b1, b2, b3, b4, n, c1, c2, c3 = b:sub(i, i + 3)
		if #c < 4 then break end

		b1, b2, b3, b4 = dt[c:sub(1,1)] or 0, dt[c:sub(2,2)] or 0, dt[c:sub(3,3)] or 0, dt[c:sub(4,4)] or 0
		n = (b1 << 18) + (b2 << 12) + (b3 << 6) + b4
		c1, c2, c3 = string.char((n >> 16) & 255), string.char((n >> 8) & 255), string.char(n & 255)

		-- Handle padding
		if c:sub(4,4) == '=' then
			if c:sub(3,3) == '=' then
				table.insert(r, c1) -- Only first byte
			else
				table.insert(r, c1 .. c2) -- First two bytes
			end
		else
			table.insert(r, c1 .. c2 .. c3) -- All three bytes
		end
	end

	return table.concat(r)
end

if #arg < 1 or #arg > 2 then
	print(arg[0] .. [[ [-e|-d] [file]
	-e: encode (default)
	-d: decode"]])
	os.exit(1)
end

-- em = encode mode
-- fn = file name
--  f = file pointer
local em, fn, f = 1

for _, v in ipairs(arg) do
	if v == "-d" then
		em = nil
	else
		fn = v
	end
end

if fn then
	f, e = io.open(fn, "rb")
	if not f then
		print(e)
		os.exit(1)
	end
else
	f, e = io.stdin
end

if not f then print(e) exit(1) end

if em then
	local encoded = base64_encode_file(f)
	io.write(encoded)
	if not encoded:sub(-1):match('\n') then
		io.write('\n')
	end
else
	io.write(base64_decode_file(f))
end

-- Close file if it was opened
if fn then
	f:close()
end