Split DiceServ command out of lib.rs

The one command handler (ROLL/EXROLL/CALC/EXCALC all route to it) and
its formatting helpers move to roll.rs; lib.rs keeps the dispatcher, the
expression-evaluator module, and its tests. No behaviour change.

modules/diceserv/src/roll.rs

@@ -0,0 +1,70 @@
+use fedserv_api::{Sender, ServiceCtx};
+
+use super::expr;
+
+// `N~expr` may repeat a roll at most this many times.
+const MAX_REPEATS: u32 = 25;
+
+// ROLL/EXROLL/CALC/EXCALC <expr>: evaluate a dice/math expression and reply.
+// `round_result` gives a whole number (ROLL); `extended` also shows each die.
+pub fn handle(me: &str, from: &Sender, rest: &[&str], ctx: &mut ServiceCtx, extended: bool, round_result: bool) {
+    let input = rest.join(" ");
+    let input = input.trim();
+    if input.is_empty() {
+        ctx.notice(me, from.uid, "Give me something to roll, e.g. \x022d6+3\x02.");
+        return;
+    }
+    // `N~expr` repeats the roll N times.
+    let (times, body) = match input.split_once('~') {
+        Some((n, body)) => match n.trim().parse::<u32>() {
+            Ok(t) if (1..=MAX_REPEATS).contains(&t) => (t, body.trim()),
+            _ => {
+                ctx.notice(me, from.uid, format!("The repeat count before \x02~\x02 must be a number from 1 to {MAX_REPEATS}."));
+                return;
+            }
+        },
+        None => (1, input),
+    };
+
+    let mut rng = rand::thread_rng();
+    for i in 0..times {
+        match expr::evaluate(body, &mut rng) {
+            Ok(ev) => {
+                let result = format_value(ev.value, round_result);
+                let prefix = if times > 1 { format!("[{}/{}] ", i + 1, times) } else { String::new() };
+                let detail = if extended && !ev.rolls.is_empty() {
+                    format!(" — {}", ev.rolls.iter().map(render_roll).collect::<Vec<_>>().join(", "))
+                } else {
+                    String::new()
+                };
+                ctx.notice(me, from.uid, format!("{prefix}\x02{body}\x02 = \x02{result}\x02{detail}"));
+            }
+            Err(why) => {
+                ctx.notice(me, from.uid, format!("I couldn't roll \x02{body}\x02: {why}."));
+                return; // the whole batch shares the expression, so it'll fail the same way
+            }
+        }
+    }
+}
+
+// "2d6: 4+5=9" for the extended output.
+fn render_roll(r: &expr::Roll) -> String {
+    if r.values.len() == 1 {
+        format!("{}: {}", r.spec, r.total)
+    } else {
+        let each = r.values.iter().map(u64::to_string).collect::<Vec<_>>().join("+");
+        format!("{}: {}={}", r.spec, each, r.total)
+    }
+}
+
+// ROLL rounds to a whole number; CALC keeps up to four decimals, trimmed.
+fn format_value(v: f64, round_result: bool) -> String {
+    if round_result {
+        return format!("{}", v.round() as i64);
+    }
+    if v.fract() == 0.0 && v.abs() < 1e15 {
+        return format!("{}", v as i64);
+    }
+    let s = format!("{v:.4}");
+    s.trim_end_matches('0').trim_end_matches('.').to_string()
+}