receive_packet_headers

Previously...

• I did an analysis of the event sequence that should take place in order to use a single process.
• I made some plans about how to create megaphone_tcp.
• Determine if segment contains a complete header
• If not, then keep reading until you do have a complete header
• Parse out the connection ID and packet length, 
• If you have a complete header, then determine if you have a complete megaphone packet
• If not, then keep reading until you have a complete megaphone packet
• Send the megaphone process the completed packet.
• Wait for another packet to arrive.
• I created receive_loop, which takes care of controlling the process.

In today's report, I created receive_header

%

%% extract a complete megaphone header from the supplied data and from the 

%% supplied socket if the data is not enough.

%%

%% This function attempts to combine any leftover data with any newly received

%% data.  It then determines if it has enough data to form a complete megaphone

%% header.  If it does then it extracts out the fields from the data according

%% the standard locations and returns them.

%%

%% If the function does not have enough data to form a complete megaphone 

%% header, it will try to read more data from the supplied socket until it 

%% receives a complete packet.

%%

%% Socket --- The TCP socket that should be used to get more data.

%% LeftOvers --- Any data that should appear before the Data parameter.

%%     This value may be undefined if there was no left over data from the 

%%     previous packet.

%% Data --- Some newly received data from the socket.  This should appear after 

%%     the leftovers from the previous packet, if any exist.

%%

%% Returns { Length, CID, Content } where Length is an integer that is the 

%% number of bytes in the complete message, including the megaphone header.

%% CID is the connection ID of the connection, as a string (list).  Content

%% is the content, if any is left over from the header.  If there is no 

%% data left over from the header, then his value is returned as undefined.

%% 

receive_header(Socket, LeftOvers, Data) ->

    Data2 = case LeftOvers of

        undefined -> Data;

        true -> LeftOvers ++ Data

    end,

    case byte_size(Data2) of

        Length < ?PREFIX_LENGTH ->

            case gen_tcp:recv(Socket, 0) ->

                {ok, NewData},

                receive_header(Socket, Data2, NewData)

            end;

        true ->

            extract_CID_length_body(Data2)

    end.

This function takes create of detecting when the system has not received a complete packet and therefore needs to wait for more data from the client.  It uses the extract_CID_length_body function to extract out the data from the packet:

%%

%% extract out the packet length, the connection ID and the packet contents

%% from a binary block of data.

%%

%% Binary --- this should be the binary representation of a megaphone packet

%%    header.  The binary may also contain the partial or complete contents

%%    of a megaphone packet content.

%%

%% The function returns the tuple {Length, CID, Content} where Length is the 

%% length of the complete packet, include the megaphone header.  CID is the 

%% connection ID as an integer value.  Content is the megaphone packet contents.

%%

%% The input packet may contain no content or only part of the content of the 

%% packet and may therefore have a value of undefine or it may be a binary 

%% whose length is less than that of the complete content.

%%

extract_CID_length_body(Binary) ->

    BinaryPacketLength = binary_part(Binary, ?PACKET_LENGTH_START, ?PACKET_LENGTH_LENGTH),

    StringPacketLength = binary_to_list(BinaryPacketLength),

    { PacketLength, _ } = to_integer(StringPacketLength),

    BinaryConnectionID = binary_part(Binary, ?PACKET_CID_START, ?PACKET_CID_LENGTH),

    StringConnectionID = binary_to_list(BinaryConnectionID),

    BinaryContent = binary_part(Binary, ?PACKET_CONTENT_START, byte_size(Binary) - ?PACKET_HEADER_LENGTH),

    {PacketLength, StringConnectionID, BinaryContent}.

Next time, more functions that handle the extraction of packets.