"""
*Channel* is an abstraction layer over streams that works with *packets of data*,
rather than an endless stream of bytes, and adds support for compression.
"""
from plainbox.vendor.rpyc.lib import safe_import
from plainbox.vendor.rpyc.lib.compat import Struct, BYTES_LITERAL
zlib = safe_import("zlib")

# * 64 bit length field?
# * separate \n into a FlushingChannel subclass?
# * add thread safety as a subclass?

class Channel(object):
    """Channel implementation.

    Note: In order to avoid problems with all sorts of line-buffered transports,
    we deliberately add ``\\n`` at the end of each frame.
    """

    COMPRESSION_THRESHOLD = 3000
    COMPRESSION_LEVEL = 1
    FRAME_HEADER = Struct("!LB")
    FLUSHER = BYTES_LITERAL("\n") # cause any line-buffered layers below us to flush
    __slots__ = ["stream", "compress"]

    def __init__(self, stream, compress = True):
        self.stream = stream
        if not zlib:
            compress = False
        self.compress = compress
    def close(self):
        """closes the channel and underlying stream"""
        self.stream.close()
    @property
    def closed(self):
        """indicates whether the underlying stream has been closed"""
        return self.stream.closed
    def fileno(self):
        """returns the file descriptor of the underlying stream"""
        return self.stream.fileno()
    def poll(self, timeout):
        """polls the underlying steam for data, waiting up to *timeout* seconds"""
        return self.stream.poll(timeout)
    def recv(self):
        """Receives the next packet (or *frame*) from the underlying stream.
        This method will block until the packet has been read completely

        :returns: string of data
        """
        header = self.stream.read(self.FRAME_HEADER.size)
        length, compressed = self.FRAME_HEADER.unpack(header)
        data = self.stream.read(length + len(self.FLUSHER))[:-len(self.FLUSHER)]
        if compressed:
            data = zlib.decompress(data)
        return data
    def send(self, data):
        """Sends the given string of data as a packet over the underlying
        stream. Blocks until the packet has been sent.

        :param data: the byte string to send as a packet
        """
        if self.compress and len(data) > self.COMPRESSION_THRESHOLD:
            compressed = 1
            data = zlib.compress(data, self.COMPRESSION_LEVEL)
        else:
            compressed = 0
        header = self.FRAME_HEADER.pack(len(data), compressed)
        buf = header + data + self.FLUSHER
        self.stream.write(buf)

