server: Parse messages

[tattlekey] / server / src / main.rs

// tattlekey: A one-key UDP keyboard
// Copyright (C) 2023  Scott Worley <scottworley@scottworley.com>
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <https://www.gnu.org/licenses/>.

use std::net::UdpSocket;

const MESSAGE_SIZE: usize = 12;

#[derive(Debug)]
struct MessageKey {
    epoch: u32,
    device: u16,
    seq: u16,
}

#[derive(Debug)]
struct Message {
    key: MessageKey,
    ago: u32,
}

impl From<&[u8; MESSAGE_SIZE]> for Message {
    fn from(value: &[u8; MESSAGE_SIZE]) -> Self {
        Self {
            key: MessageKey {
                epoch: u32::from_be_bytes(value[0..=3].try_into().expect("I can't count")),
                device: u16::from_be_bytes(value[4..=5].try_into().expect("I can't count")),
                seq: u16::from_be_bytes(value[6..=7].try_into().expect("I can't count")),
            },
            ago: u32::from_be_bytes(value[8..=11].try_into().expect("I can't count")),
        }
    }
}
impl TryFrom<&[u8]> for Message {
    type Error = std::array::TryFromSliceError;
    fn try_from(value: &[u8]) -> Result<Self, Self::Error> {
        match <[u8; MESSAGE_SIZE]>::try_from(value) {
            Ok(correct_size) => Ok(Message::from(&correct_size)),
            Err(e) => Err(e),
        }
    }
}

fn main() {
    let socket = UdpSocket::bind("0.0.0.0:29803").expect("couldn't bind to address");
    loop {
        let mut buf = [0; MESSAGE_SIZE];
        match socket.recv_from(&mut buf) {
            Err(e) => eprintln!("Didn't receive data: {e}"),
            Ok((number_of_bytes, src_addr)) => {
                let filled_buf = &buf[..number_of_bytes];
                if number_of_bytes != MESSAGE_SIZE {
                    eprintln!("Ignoring short message ({number_of_bytes}) from {src_addr}");
                    continue;
                }
                let message = Message::try_from(filled_buf).expect("I can't count");
                println!("Got packet from {src_addr}: {message:?}");
            }
        }
    }
}