Implement direct P2P session scaffolding (#39)

Closes #24

Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/args.rs

@@ -1,5 +1,6 @@
 use bevy::prelude::*;
 use clap::Parser;
+use std::net::SocketAddr;
 use std::path::PathBuf;
 
 #[derive(Parser, Debug, Clone, Resource)]
@@ -15,6 +16,12 @@ pub struct Args {
     pub check_distance: usize,
     #[clap(long, default_value_t = 1)]
     pub players: usize,
+    /// UDP port to bind for a direct peer-to-peer session.
+    #[clap(long, default_value_t = 6969)]
+    pub local_port: u16,
+    /// Player ownership declarations in the form HANDLE=local or HANDLE=ADDRESS.
+    #[clap(long = "net-player", value_name = "HANDLE=local|ADDRESS")]
+    pub net_players: Vec<String>,
     /// Record a replay to this path.
     #[clap(long, conflicts_with = "play_replay")]
     pub record_replay: Option<PathBuf>,
@@ -25,3 +32,41 @@ pub struct Args {
     #[clap(long, value_enum, default_value_t = crate::replay::ReplayFormat::Binary)]
     pub replay_format: crate::replay::ReplayFormat,
 }
+
+pub fn parse_socket_addr(value: &str) -> Result<SocketAddr, String> {
+    let address = value
+        .parse::<SocketAddr>()
+        .map_err(|_| format!("invalid socket address: {value}"))?;
+    if address.port() == 0 {
+        return Err(format!("socket address must use a nonzero port: {value}"));
+    }
+    Ok(address)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parses_network_options() {
+        let args = Args::try_parse_from([
+            "game",
+            "--local-port",
+            "7000",
+            "--net-player",
+            "0=local",
+            "--net-player",
+            "1=127.0.0.1:7001",
+        ])
+        .unwrap();
+
+        assert_eq!(args.local_port, 7000);
+        assert_eq!(args.net_players, ["0=local", "1=127.0.0.1:7001"]);
+    }
+
+    #[test]
+    fn rejects_invalid_socket_addresses() {
+        assert!(parse_socket_addr("127.0.0.1:0").is_err());
+        assert!(parse_socket_addr("not-an-address").is_err());
+    }
+}