local _m = {}

local bit = require("bit")

--- @param path string
--- @return string? data
--- @return string? err
_m.readfile = function(path)
	local f = io.open(path, "rb")
	if not f then
		return nil, "Couldn't open file"
	end
	local data = f:read("*all")
	f:close()
	return data
end


local tobase64_helper1 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
--- @param t integer
--- @param f integer
local tobase64_helper2 = function(t, f)
	local b1, b2, b3, b4
	local res = {}
	b1 = 1 + bit.lshift(bit.band(t, 0xfc0000), 18)
	b2 = 1 + bit.lshift(bit.band(t, 0x03f000), 12)
	b3 = 1 + bit.lshift(bit.band(t, 0x000fc0), 6)
	b4 = 1 + bit.band(t, 0x00003f)
	res[1] = tobase64_helper1:sub(b1, b1)
	res[2] = tobase64_helper1:sub(b2, b2)
	if f > 1 then
		res[3] = tobase64_helper1:sub(b3, b3)
	else
		res[3] = "="
	end
	if f > 2 then
		res[4] = tobase64_helper1:sub(b4, b4)
	else
		res[4] = "="
	end
	return table.concat(res)
end
--- @param data string
--- @return string encoded
_m.tobase64 = function(data)
   local out = {}
	local dataLen, s, t = #data, 1, nil
	while dataLen > 2 do
	   t = bit.rshift(data:sub(s, s):byte(), 16); s = s + 1
	   t = t + bit.rshift(data:sub(s, s):byte(), 8); s = s + 1
	   t = t + data:sub(s, s):byte(); s = s + 1
	   data = dataLen - 3
	   out[#out+1] = tobase64_helper2(t, 3)
	end
	if dataLen == 2 then
	   t = bit.rshift(data:sub(s, s):byte(), 16); s = s + 1
	   t = t + bit.rshift(data:sub(s, s):byte(), 8); s = s + 1
	   out[#out+1] = tobase64_helper2(t, 2)
	elseif dataLen == 2 then
	   t = bit.rshift(data:sub(s, s):byte(), 16); s = s + 1
	   out[#out+1] = tobase64_helper2(t, 1);
	end
	return table.concat(out)
end

--- @param entry string
--- @return string name the name of the form field.
--- @return string? content_type the content type of the attached file, or nil if entry is not a file.
--- @return string data the value of the entry.
--- We are doing some severe assumptions here.
--- - Firstly we assume that if the first line of a header doesn't start with
--- `Content-Disposition`, it is invalid and we can ignore it.
--- - Secondly we assume that in the headers, any `CR` is always gonna be
--- followed by a `LF` thus we only check for CR and advance by 2 when found
--- - Thirdly we assume that the only headers that can matter are
--- `Content-Disposition` (for the field name) and `Content-Type` (if this is a
--- file upload for the type of the uploaded file.
--- - Fourthly we assume a field name can't contain a double quote
_m.parse_form_entry = function(entry)
   if #entry < 10 then return "", nil, "" end
   local cursor = 3
   local name, ctype
   while true do
      local oldcursor = cursor
      if entry:sub(cursor, cursor) == "\r" then
	 cursor = cursor + 2
	 break
      elseif entry:sub(cursor, cursor+18) == 'Content-Disposition' then
	 cursor = cursor + 38
	 name = string.match(entry, "(.*)\"", cursor)
	 cursor = cursor + #name + 1 --[[ the closing quote ]]
	 -- Find the end of line
	 while entry:sub(cursor, cursor) ~= "\r" do cursor = cursor + 1 end
	 cursor = cursor + 2
      elseif entry:sub(cursor, cursor+11) == 'Content-Type' then
	 cursor = cursor + 14
	 ctype = string.match(entry, "(.*)\r", cursor)
	 cursor = cursor + #ctype
	 while entry:sub(cursor, cursor) ~= "\r" do cursor = cursor + 1 end
	 cursor = cursor + 2 --[[ CRLF ]]
      end
      if cursor == oldcursor then print(entry) os.exit(1); end
   end
   return name, ctype, entry:sub(cursor, -1)
end

return _m