do
local websocket = {}
_G.websocket = websocket
local band = bit.band
local bor = bit.bor
local rshift = bit.rshift
local lshift = bit.lshift
local char = string.char
    local byte = string.byte
local sub = string.sub
local applyMask = crypto.mask
local toBase64 = crypto.toBase64
local hash = crypto.hash

local function decode(chunk)
  if #chunk < 2 then return end
  local second = byte(chunk, 2)
  local len = band(second, 0x7f)
  local offset
  if len == 126 then
    if #chunk < 4 then return end
    len = bor(
      lshift(byte(chunk, 3), 8),
      byte(chunk, 4))
    offset = 4
  elseif len == 127 then
    if #chunk < 10 then return end
    len = bor(
      -- Ignore lengths longer than 32bit
      lshift(byte(chunk, 7), 24),
      lshift(byte(chunk, 8), 16),
      lshift(byte(chunk, 9), 8),
      byte(chunk, 10))
    offset = 10
  else
    offset = 2
  end
  local mask = band(second, 0x80) > 0
  if mask then
    offset = offset + 4
  end
  if #chunk < offset + len then return end
  local first = byte(chunk, 1)
  local payload = sub(chunk, offset + 1, offset + len)
  assert(#payload == len, "Length mismatch")
  if mask then
    payload = applyMask(payload, sub(chunk, offset - 3, offset))
  end
  local extra = sub(chunk, offset + len + 1)
  local opcode = band(first, 0xf)
  return extra, payload, opcode
end

local function encode(payload, opcode)
  opcode = opcode or 2
  assert(type(opcode) == "number", "opcode must be number")
  assert(type(payload) == "string", "payload must be string")
  local len = #payload
  local head = char(
    bor(0x80, opcode),
    bor(len < 126 and len or len < 0x10000 and 126 or 127)
  )
  if len >= 0x10000 then
    head = head .. char(
    0,0,0,0, -- 32 bit length is plenty, assume zero for rest
    band(rshift(len, 24), 0xff),
    band(rshift(len, 16), 0xff),
    band(rshift(len, 8), 0xff),
    band(len, 0xff)
  )
  elseif len >= 126 then
    head = head .. char(band(rshift(len, 8), 0xff), band(len, 0xff))
  end
  return head .. payload
end

local guid = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11"
local function acceptKey(key)
  return toBase64(hash("sha1", key .. guid))
end

function websocket.createServer(port, callback)
  net.createServer(net.TCP):listen(port, function(conn)
    local buffer = false
    local socket = {}
    local queue = {}
    local waiting = false
    local function onSend()
      if queue[1] then
        local data = table.remove(queue, 1)
        return conn:send(data, onSend)
      end
      waiting = false
    end
    function socket.send(...)
      local data = encode(...)
      if not waiting then
        waiting = true
        conn:send(data, onSend)
      else
        queue[#queue + 1] = data
      end
    end

    conn:on("receive", function(_, chunk)
      if buffer then
        buffer = buffer .. chunk
        while true do
          local extra, payload, opcode = decode(buffer)
          if not extra then return end
          buffer = extra
          socket.onmessage(payload, opcode)
        end
      end
      local _, e, method = string.find(chunk, "([A-Z]+) /[^\r]* HTTP/%d%.%d\r\n")
      local key, name, value
      while true do
        _, e, name, value = string.find(chunk, "([^ ]+): *([^\r]+)\r\n", e + 1)
        if not e then break end
        if string.lower(name) == "sec-websocket-key" then
          key = value
        end
      end

      if method == "GET" and key then
        conn:send(
          "HTTP/1.1 101 Switching Protocols\r\n" ..
          "Upgrade: websocket\r\nConnection: Upgrade\r\n" ..
          "Sec-WebSocket-Accept: " .. acceptKey(key) .. "\r\n\r\n",
          function () callback(socket) end)
        buffer = ""
      else
        conn:send(
          "HTTP/1.1 404 Not Found\r\nConnection: Close\r\n\r\n",
          conn.close)
      end
    end)
  end)
end

end