import socket

class BufferOverflow(Exception):
    def __init__(self):
        super().__init__("Buffer overflow")

class LineBuffer():
    __slots__ = 'buf', 'size', 'offset', 'count', 'eof', 'done',

    BUFFER_SIZE = 4096

    def __init__(self, size: int=BUFFER_SIZE):
        self.buf:    bytearray = bytearray(size)
        self.size:   int = size
        self.offset: int = 0
        self.count:  int = 0
        self.eof:    bool = False
        self.done:   bool = False

    def _shift(self):
        count = self.size - self.offset

        self.buf[0:count] = self.buf[self.offset:self.count]

        self.offset = 0
        self.count  = count

    def _is_full(self):
        return self.count == self.size

    def _fill(self, sock: socket.socket):
        if self.eof:
            return

        readlen = self.size - self.count
        data    = sock.recv(readlen)

        if data == b'':
            self.eof = True
            return

        readlen = len(data)

        self.buf[self.offset:self.offset+readlen] = data

        self.count += readlen

    def _flush(self, end: int):
        ret = self.buf[self.offset:end+1]

        self.offset = end + 1

        return str(ret, 'ascii')

    def readline(self, sock: socket.socket) -> str:
        while True:
            index = self.buf.find(b'\n', self.offset, self.count)

            if index < 0:
                if self._is_full():
                    if self.offset == 0:
                        raise BufferOverflow()
                    else:
                        self._shift()
                else:
                    self._fill(sock)
            else:
                return self._flush(index)