Add a module (plugin) system for bot features

Introduce a `Module` trait (src/bot/module.rs): a feature declares the
commands it provides and turns each invocation into a `Vec<Action>`, without
touching the connection. Modules are pure — context in, actions out — so they
are unit-testable in isolation and avoid borrowing the `Bot` that owns them.

Bot::new builds the module set and a command -> module route table; the
`commands` dispatcher routes to the owning module and executes its actions,
and `help` is auto-generated from every module's CommandSpec metadata.

The existing ping/echo/hello move into a `builtins` module, and a stateful
`dice` module (`roll [NdM]`, with its own dependency-free xorshift PRNG) is
added as a worked example. Each network gets its own module set, so stateful
modules keep independent per-network state with no locking. Adding a feature
is now a new file in bot/modules/ plus one line in modules::all().

Tests: new tests/modules.rs drives modules through the trait with no network;
end-to-end dispatch and auto-help verified against a fake server. No new deps.

src/bot/commands.rs

@@ -6,6 +6,7 @@
 
 use std::io;
 
+use super::module::{Action, Command};
 use super::Bot;
 use crate::irc::{now_unix, parse_server_time, Message};
 
@@ -83,7 +84,8 @@ impl Bot {
         self.handle_command(&reply_to, sender, &command, &args)
     }
 
-    /// The command table. Add new bot commands here.
+    /// Route a command to the module that declares it and execute the actions
+    /// it returns. `help` is handled here, aggregating every module's commands.
     fn handle_command(
         &mut self,
         reply_to: &str,
@@ -91,22 +93,54 @@ impl Bot {
         command: &str,
         args: &[&str],
     ) -> io::Result<()> {
-        match command {
-            "ping" => self.conn.privmsg(reply_to, "pong")?,
-            "echo" => self.conn.privmsg(reply_to, &args.join(" "))?,
-            "hello" => {
-                let text = format!("hello, {sender}!");
-                self.conn.privmsg(reply_to, &text)?;
+        if command == "help" {
+            let text = self.help_text(args.first().copied());
+            return self.conn.privmsg(reply_to, &text);
+        }
+
+        let Some(&idx) = self.routes.get(command) else {
+            return Ok(()); // Unknown command: stay silent rather than spam channels.
+        };
+
+        let cmd = Command {
+            sender,
+            reply_to,
+            name: command,
+            args,
+            prefix: &self.config.command_prefix,
+            network: &self.config.name,
+        };
+        let actions = self.modules[idx].on_command(&cmd);
+        for action in actions {
+            match action {
+                Action::Reply(text) => self.conn.privmsg(reply_to, &text)?,
+                Action::Raw(line) => self.conn.send_raw(&line)?,
             }
-            "help" => {
-                let p = &self.config.command_prefix;
-                let text = format!("commands: {p}ping, {p}echo <text>, {p}hello, {p}help");
-                self.conn.privmsg(reply_to, &text)?;
-            }
-            _ => {} // Unknown command: stay silent rather than spam channels.
         }
         Ok(())
     }
+
+    /// Build the `help` reply: with no argument, list every module's commands;
+    /// with a command name, show that command's description.
+    fn help_text(&self, arg: Option<&str>) -> String {
+        let p = &self.config.command_prefix;
+        if let Some(name) = arg {
+            for m in &self.modules {
+                if let Some(spec) = m.commands().iter().find(|s| s.name == name) {
+                    return format!("{p}{}: {}", spec.usage, spec.about);
+                }
+            }
+            return format!("no such command '{name}'; try {p}help");
+        }
+        let mut items: Vec<String> = Vec::new();
+        for m in &self.modules {
+            for spec in m.commands() {
+                items.push(format!("{p}{}", spec.usage));
+            }
+        }
+        items.push(format!("{p}help"));
+        format!("commands: {}", items.join(", "))
+    }
 }
 
 /// Channels start with one of the standard channel-type prefixes.

src/bot/mod.rs

@@ -1,10 +1,10 @@
 //! Bot behavior: the [`Bot`] state, registration, and the event loop.
 //!
-//! This layer is transport-agnostic in spirit: it drives an [`crate::irc::Connection`]
-//! and turns incoming [`Message`]s into actions. The behaviour is split across
-//! sibling modules — capability negotiation and SASL live in [`caps`], and
-//! message routing plus the command table live in [`commands`]. To add a
-//! feature you usually only touch [`commands`].
+//! Behaviour is split across sibling modules: capability negotiation and SASL
+//! live in [`caps`], and message routing in [`commands`]. Features live behind
+//! the [`Module`] plugin trait ([`module`]) and are registered in [`modules`],
+//! so adding a command is a new file in `bot/modules/` plus one line in
+//! [`modules::all`] — no changes to the core dispatch.
 
 use std::collections::{HashMap, HashSet};
 use std::io;
@@ -14,8 +14,13 @@ use crate::irc::{Connection, Message};
 
 mod caps;
 mod commands;
+pub mod module;
+pub mod modules;
 
-/// The bot: owns its configuration and connection, and runs one session.
+pub use module::{Action, Command, CommandSpec, Module};
+
+/// The bot: owns its configuration, connection, and feature modules, and runs
+/// one session.
 pub struct Bot {
     config: Config,
     conn: Connection,
@@ -31,11 +36,31 @@ pub struct Bot {
     batches: HashMap<String, String>,
     /// Whether the SASL exchange has begun this session.
     sasl_started: bool,
+    /// Registered feature modules (a fresh set per network/session).
+    modules: Vec<Box<dyn Module>>,
+    /// Command word -> index into `modules`, built from each module's specs.
+    routes: HashMap<&'static str, usize>,
 }
 
 impl Bot {
     pub fn new(config: Config, conn: Connection) -> Bot {
         let current_nick = config.nick.clone();
+
+        // Build the module set and the command -> module route table.
+        let mods = modules::all();
+        let mut routes: HashMap<&'static str, usize> = HashMap::new();
+        for (i, m) in mods.iter().enumerate() {
+            for spec in m.commands() {
+                if routes.insert(spec.name, i).is_some() {
+                    crate::log(&format!(
+                        "module '{}' overrides command '{}'",
+                        m.name(),
+                        spec.name
+                    ));
+                }
+            }
+        }
+
         Bot {
             config,
             conn,
@@ -45,6 +70,8 @@ impl Bot {
             enabled_caps: HashSet::new(),
             batches: HashMap::new(),
             sasl_started: false,
+            modules: mods,
+            routes,
         }
     }
 

src/bot/module.rs

@@ -0,0 +1,71 @@
+//! The module (plugin) framework: the [`Module`] trait plus the [`Command`]
+//! context it receives and the [`Action`]s it returns.
+//!
+//! Modules are pure in spirit: given a [`Command`], a module returns the
+//! [`Action`]s to perform without ever touching the connection. That keeps them
+//! trivially unit-testable and sidesteps borrow conflicts with the [`super::Bot`]
+//! that owns them (a module never needs `&mut` access to the bot).
+
+/// Metadata describing one command a module provides. Used to route commands to
+/// modules and to auto-generate `help`.
+#[derive(Debug, Clone, Copy)]
+pub struct CommandSpec {
+    /// The command word, lowercase, without the prefix (e.g. `"roll"`).
+    pub name: &'static str,
+    /// Usage shown in help, without the prefix (e.g. `"roll [NdM]"`).
+    pub usage: &'static str,
+    /// One-line description shown by `help <command>`.
+    pub about: &'static str,
+}
+
+/// The context passed to a module for one command invocation. All fields borrow
+/// from the current message and configuration.
+pub struct Command<'a> {
+    /// Nick of the sender.
+    pub sender: &'a str,
+    /// Where a reply should go: the channel for channel messages, or the
+    /// sender's nick for private messages.
+    pub reply_to: &'a str,
+    /// The command word (lowercased, prefix stripped).
+    pub name: &'a str,
+    /// Arguments after the command word (whitespace-split).
+    pub args: &'a [&'a str],
+    /// The configured command prefix (e.g. `">"`), for building help text.
+    pub prefix: &'a str,
+    /// The network's name (`Config::name`), for network-aware modules.
+    pub network: &'a str,
+}
+
+/// Something a module asks the bot to do in response to a command. Modules
+/// return these instead of performing I/O themselves.
+pub enum Action {
+    /// Send a `PRIVMSG` back to [`Command::reply_to`].
+    Reply(String),
+    /// Send an arbitrary raw IRC line (for power modules, e.g. an admin module).
+    Raw(String),
+}
+
+impl Action {
+    /// Convenience constructor for a reply from anything string-like.
+    pub fn reply(text: impl Into<String>) -> Action {
+        Action::Reply(text.into())
+    }
+}
+
+/// A bot feature: a self-contained unit that declares the commands it handles
+/// and turns each invocation into [`Action`]s.
+///
+/// Add one by creating a file in `bot/modules/` that implements this trait and
+/// listing its constructor in [`super::modules::all`].
+pub trait Module {
+    /// Short identifier for logs and diagnostics (e.g. `"dice"`).
+    fn name(&self) -> &'static str;
+
+    /// The commands this module provides. The registry routes matching command
+    /// words here and builds `help` from these specs.
+    fn commands(&self) -> &'static [CommandSpec];
+
+    /// Handle one command (only ever called for a name in [`Module::commands`]).
+    /// Return the actions to perform; an empty vec does nothing.
+    fn on_command(&mut self, cmd: &Command) -> Vec<Action>;
+}

src/bot/modules/builtins.rs

@@ -0,0 +1,33 @@
+//! The built-in commands: `ping`, `echo`, `hello`. (`help` is provided by the
+//! dispatcher itself, since it aggregates every module's commands.)
+
+use crate::bot::module::{Action, Command, CommandSpec, Module};
+
+/// Core commands that ship with the bot.
+pub struct Builtins;
+
+const COMMANDS: &[CommandSpec] = &[
+    CommandSpec { name: "ping", usage: "ping", about: "reply with 'pong'" },
+    CommandSpec { name: "echo", usage: "echo <text>", about: "echo the text back" },
+    CommandSpec { name: "hello", usage: "hello", about: "greet the sender" },
+];
+
+impl Module for Builtins {
+    fn name(&self) -> &'static str {
+        "builtins"
+    }
+
+    fn commands(&self) -> &'static [CommandSpec] {
+        COMMANDS
+    }
+
+    fn on_command(&mut self, cmd: &Command) -> Vec<Action> {
+        let reply = match cmd.name {
+            "ping" => "pong".to_string(),
+            "echo" => cmd.args.join(" "),
+            "hello" => format!("hello, {}!", cmd.sender),
+            _ => return Vec::new(),
+        };
+        vec![Action::Reply(reply)]
+    }
+}

src/bot/modules/dice.rs

@@ -0,0 +1,99 @@
+//! A dice roller: `roll 2d6`, `roll d20`, or `roll` (defaults to `1d6`).
+//!
+//! Demonstrates a *stateful* module — it carries its own tiny PRNG, seeded once
+//! at construction — while keeping the crate dependency-free (no `rand`).
+
+use std::time::{SystemTime, UNIX_EPOCH};
+
+use crate::bot::module::{Action, Command, CommandSpec, Module};
+
+const COMMANDS: &[CommandSpec] = &[CommandSpec {
+    name: "roll",
+    usage: "roll [NdM]",
+    about: "roll N dice of M sides (default 1d6), e.g. roll 2d20",
+}];
+
+/// Upper bounds to keep output (and the channel) sane.
+const MAX_DICE: u64 = 100;
+const MAX_SIDES: u64 = 1000;
+
+/// Dice roller with a self-contained xorshift PRNG — fine for dice, not for
+/// anything security-sensitive.
+pub struct Dice {
+    rng: u64,
+}
+
+impl Dice {
+    pub fn new() -> Dice {
+        let seed = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .map(|d| d.as_nanos() as u64)
+            .unwrap_or(0x9E37_79B9_7F4A_7C15);
+        Dice { rng: seed | 1 } // never seed to 0 (xorshift would stay stuck)
+    }
+
+    fn next_u64(&mut self) -> u64 {
+        let mut x = self.rng;
+        x ^= x << 13;
+        x ^= x >> 7;
+        x ^= x << 17;
+        self.rng = x;
+        x
+    }
+
+    /// Roll a single die in `1..=sides`.
+    fn die(&mut self, sides: u64) -> u64 {
+        self.next_u64() % sides + 1
+    }
+}
+
+impl Default for Dice {
+    fn default() -> Dice {
+        Dice::new()
+    }
+}
+
+impl Module for Dice {
+    fn name(&self) -> &'static str {
+        "dice"
+    }
+
+    fn commands(&self) -> &'static [CommandSpec] {
+        COMMANDS
+    }
+
+    fn on_command(&mut self, cmd: &Command) -> Vec<Action> {
+        let spec = cmd.args.first().copied().unwrap_or("1d6");
+        let reply = match parse_spec(spec) {
+            Some((count, sides)) => {
+                let rolls: Vec<u64> = (0..count).map(|_| self.die(sides)).collect();
+                let total: u64 = rolls.iter().sum();
+                if count == 1 {
+                    format!("{count}d{sides}: {total}")
+                } else {
+                    let parts: Vec<String> = rolls.iter().map(u64::to_string).collect();
+                    format!("{count}d{sides}: {} = {total}", parts.join(" + "))
+                }
+            }
+            None => format!("usage: {p}roll [NdM], e.g. {p}roll 2d6", p = cmd.prefix),
+        };
+        vec![Action::Reply(reply)]
+    }
+}
+
+/// Parse `NdM` / `dM` / `M` into `(count, sides)` within sane bounds. A bare
+/// number `M` means `1dM`.
+fn parse_spec(s: &str) -> Option<(u64, u64)> {
+    let (count, sides) = match s.split_once('d') {
+        Some((c, m)) => {
+            let count = if c.is_empty() { 1 } else { c.parse().ok()? };
+            (count, m.parse().ok()?)
+        }
+        None => (1, s.parse().ok()?),
+    };
+    if (1..=MAX_DICE).contains(&count) && (2..=MAX_SIDES).contains(&sides) {
+        Some((count, sides))
+    } else {
+        None
+    }
+}

src/bot/modules/mod.rs

@@ -0,0 +1,17 @@
+//! The registry of feature modules. Add a feature by creating a file here that
+//! implements [`super::module::Module`] and listing its constructor in [`all`].
+
+pub mod builtins;
+pub mod dice;
+
+use super::module::Module;
+
+/// Construct the default set of modules, in priority order (earlier modules win
+/// command-name collisions). Each network's [`super::Bot`] gets its own fresh
+/// set, so stateful modules keep independent per-network state.
+pub fn all() -> Vec<Box<dyn Module>> {
+    vec![
+        Box::new(builtins::Builtins),
+        Box::new(dice::Dice::new()),
+    ]
+}