# M4b task 08: the leading mention, and approval commands

**Branch:** `m4b` (run `git switch m4b`; `git status --short` must be empty, otherwise stop)
**Commit subject:** `gatewayd: strip a leading mention of this bot; recognise approval commands`

## Goal

Two changes to routing (M4b spec, section 7):

1. **The leading mention.** After M4a has decided a post is for this Boxmaker, a leading
   `@<this bot>` is stripped before anything reads the message: `@boxmaker0 !approve 42` is a
   command, and `@boxmaker0 summarise the log` reaches the model as `summarise the log`. Where two
   Boxmakers share a thread, that is how the owner addresses one of them.
2. **Approval commands.** `!approve <id>`, `!deny <id>` and `!deny <id> <reason>` (exactly) become
   a new `Route::Approval`, carrying the command, who wrote it, which post, and when. Task 15 hands
   it to the approvals module; until then `serve` answers it with M4a's notice, so nothing else
   changes yet. Any other `!…` is still answered with `UNKNOWN_COMMAND`.

## Files

- Copy: `crates/gatewayd/tests/sessions_mention.rs`, and **new versions** of two given tests:
  `crates/gatewayd/tests/sessions.rs` (commands now route to `Approval`; the mention is stripped)
  and `crates/gatewayd/tests/serve.rs`, from `serve.rs-task08` (one line: the turn's text is
  `start`, no longer `@boxmaker-straylight start`)
- Modify: `crates/gatewayd/src/sessions.rs`, `crates/gatewayd/src/serve/handle.rs`,
  `docs/implementer-log.md`

## The changes, exactly

In `crates/gatewayd/src/sessions.rs`:

**1.** In `enum Route`, after `Queue(Message),`:

```rust
    /// `!approve` or `!deny`, for the approvals module to judge.
    Approval {
        thread: Thread,
        command: ApprovalCommand,
        /// Who wrote it, which post it is, and when (Mattermost's `create_at`).
        user: String,
        post: String,
        at: i64,
    },
```

**2.** After `enum Route`, the command type and `approval_command`, as given:

```rust
/// An approval command as typed: exactly `!approve <id>`, `!deny <id>` or `!deny <id> <reason>`.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum ApprovalCommand {
    Approve(u64),
    Deny(u64, Option<String>),
    /// `!approve` or `!deny` in any other form.
    Malformed,
}

/// The longest reason a `!deny` may give, in characters.
pub const MAX_REASON: usize = proto::MAX_REASON;

/// `!approve …` or `!deny …` as an approval command; `None` for any other text.
pub fn approval_command(text: &str) -> Option<ApprovalCommand> {
    let after = text.strip_prefix('!')?;
    let (word, rest) = match after.split_once(' ') {
        Some((word, rest)) => (word, Some(rest)),
        None => (after, None),
    };
    if !matches!(word, "approve" | "deny") {
        return None;
    }
    let id = |s: &str| -> Option<u64> {
        if s.is_empty() || s.len() > 20 || !s.bytes().all(|b| b.is_ascii_digit()) {
            return None;
        }
        s.parse().ok()
    };
    let parsed = match (word, rest) {
        ("approve", Some(rest)) => id(rest).map(ApprovalCommand::Approve),
        ("deny", Some(rest)) => match rest.split_once(' ') {
            None => id(rest).map(|n| ApprovalCommand::Deny(n, None)),
            Some((n, reason)) => {
                let fits = !reason.is_empty() && reason.chars().count() <= MAX_REASON;
                id(n)
                    .filter(|_| fits)
                    .map(|n| ApprovalCommand::Deny(n, Some(reason.to_string())))
            }
        },
        _ => None,
    };
    Some(parsed.unwrap_or(ApprovalCommand::Malformed))
}
```

**3.** Write `strip_mention` yourself, next to them:

```rust
/// The text after a leading mention of this bot (`me`, lower-case), and the spaces after it. The
/// mention is matched as `named` matches names; nothing else is removed. A message that would be
/// left empty is kept as it was.
pub fn strip_mention<'a>(message: &'a str, me: &str) -> &'a str
```

   1. `let Some(after) = message.strip_prefix('@') else { return message; };`
   2. The name's length: `after.find(|c: char| !(c.is_ascii_alphanumeric() || matches!(c, '.' | '-' | '_'))).unwrap_or(after.len())`;
      `let raw = after.get(..len).unwrap_or_default();` and
      `let name = raw.trim_end_matches('.');` (a trailing dot is not part of a name).
   3. `name` empty, or not `name.eq_ignore_ascii_case(me)`: return `message`.
   4. `let rest = after.get(name.len()..).unwrap_or_default().trim_start_matches(' ');` (spaces
      only, not newlines).
   5. `if rest.is_empty() { message } else { rest }`.

**4.** In `Router::route`:
   - `let message = &post.message;` becomes `let message = strip_mention(&post.message, &self.me_name);`
   - The block `if let Some(after) = message.strip_prefix('!') { … }` (which answered
     `M4B_COMMAND`) becomes:

```rust
        if let Some(command) = approval_command(message) {
            return Route::Approval {
                thread,
                command,
                user: post.user_id.clone(),
                post: post.id.clone(),
                at: post.create_at,
            };
        }
        if message.starts_with('!') {
            return Route::Reply {
                thread,
                text: UNKNOWN_COMMAND.to_string(),
            };
        }
```

   - In the final `Route::Queue`, `text: message.clone(),` becomes `text: message.to_string(),`.
   - In `route`'s step comment, the part about commands now reads: "Then a leading mention of this
     bot is stripped (M4b spec, section 7), and: "!!..." drops one "!" and goes on as a message;
     `approval_command` -> Approval; any other "!" -> Reply UNKNOWN_COMMAND."

In `crates/gatewayd/src/serve/handle.rs`, `handle_post`'s `match route` gains, after the
`Route::Reply` arm:

```rust
            // Until the approvals module is wired in (task 15), M4a's notice.
            Route::Approval { thread, .. } => {
                self.post(&thread.channel, &thread.root, M4B_COMMAND)
            }
```

and its `use crate::sessions::{…}` gains `M4B_COMMAND`.

## Steps

- [ ] **1. Copy.** `git switch m4b`, then
  `cp docs/plans/M4b/files/crates/gatewayd/tests/sessions_mention.rs docs/plans/M4b/files/crates/gatewayd/tests/sessions.rs crates/gatewayd/tests/ && cp docs/plans/M4b/files/crates/gatewayd/tests/serve.rs-task08 crates/gatewayd/tests/serve.rs`
- [ ] **2. See it fail.** `cargo test -p gatewayd --test sessions_mention`. Expected: it does not
  compile.
- [ ] **3. Make the changes**, `strip_mention` last. Run `cargo fmt --all`.
- [ ] **4. See it pass.** `cargo test -p gatewayd --test sessions_mention --test sessions --test serve`.
  Expected: 4, 8 and 6 passed.
- [ ] **5. Run the gate.** `make gate`. Expected last line: `gate: ok`.
- [ ] **6. Log and commit.** `git add crates/gatewayd docs/implementer-log.md Cargo.lock && git commit`

## Done when

- The three suites pass; `make gate` prints `gate: ok`.

## Stop and report if

- A test wants the mention kept, or stripped somewhere other than at the very start.