loopd: compaction threshold and the fallback summary

Co-Authored-By: ornith-1.5-35b-a3b <agent@wntrmute.dev>

crates/loopd/src/compact.rs

@@ -0,0 +1,32 @@
+//! Compaction: when a session starts a new epoch, the summary call, and writing the next files
+//! (M5d spec, section 3). Later tasks fill the call and the write.
+
+use bxcore::{LogRecord, Usage};
+
+/// True when `cache_n + prompt_n` is at or above `threshold * n_ctx`.
+pub fn due(usage: &Usage, threshold: f64, n_ctx: u64) -> bool {
+    let total = usage.cache_n.saturating_add(usage.prompt_n) as f64;
+    total >= threshold * (n_ctx as f64)
+}
+
+/// The epoch's `User` texts, newest last, joined with `"\n\n"`, then `cap_chars`.
+pub fn fallback_summary(records: &[LogRecord], summary_chars: usize) -> String {
+    let contents: Vec<&str> = records
+        .iter()
+        .filter_map(|r| match r {
+            LogRecord::User { content, .. } => Some(content.as_str()),
+            _ => None,
+        })
+        .collect();
+    let joined = contents
+        .iter()
+        .rev()
+        .cloned()
+        .collect::<Vec<_>>()
+        .join("\n\n");
+    cap_chars(&joined, summary_chars)
+}
+
+pub(crate) fn cap_chars(s: &str, n: usize) -> String {
+    s.chars().take(n).collect()
+}

crates/loopd/src/lib.rs

@@ -3,6 +3,7 @@
 pub mod baseline;
 pub mod broker_port;
 pub mod channel;
+pub mod compact;
 pub mod config;
 pub mod http;
 pub mod llama;

crates/loopd/tests/compact_due.rs

@@ -0,0 +1,79 @@
+//! Tests for when compaction is due and for the fallback summary. Do not edit.
+
+use bxcore::{Epoch, Hash32, LogRecord, SessionId, Timestamp, Usage};
+use loopd::compact::{due, fallback_summary};
+
+fn usage(cache_n: u64, prompt_n: u64) -> Usage {
+    Usage {
+        cache_n,
+        prompt_n,
+        predicted_n: 0,
+        reasoning_tokens: 0,
+        thinking_capped: false,
+    }
+}
+
+fn user(content: &str) -> LogRecord {
+    LogRecord::User {
+        time: Timestamp::parse("2026-09-18T08:00:00.000Z").unwrap(),
+        content: content.to_string(),
+        speaker: None,
+    }
+}
+
+#[test]
+fn due_at_the_threshold_and_not_below() {
+    assert!(!due(&usage(400, 449), 0.85, 1_000), "849 < 850");
+    assert!(due(&usage(400, 450), 0.85, 1_000), "850 == 0.85 * 1000");
+    assert!(due(&usage(0, 851), 0.85, 1_000));
+    assert!(!due(&usage(0, 0), 0.85, 1_000));
+}
+
+#[test]
+fn due_uses_this_agents_n_ctx() {
+    // 0.85 * 262144 = 222822.4, so 222823 is the first integer at or above.
+    assert!(!due(&usage(0, 222_822), 0.85, 262_144));
+    assert!(due(&usage(0, 222_823), 0.85, 262_144));
+    assert!(due(&usage(200_000, 22_823), 0.85, 262_144));
+}
+
+#[test]
+fn a_threshold_of_one_is_the_full_window() {
+    assert!(!due(&usage(99, 0), 1.0, 100));
+    assert!(due(&usage(100, 0), 1.0, 100));
+}
+
+#[test]
+fn fallback_is_user_texts_newest_last() {
+    let records = vec![
+        LogRecord::SessionStart {
+            time: Timestamp::parse("2026-09-18T08:00:00.000Z").unwrap(),
+            session: SessionId::new("a").unwrap(),
+            epoch: Epoch(0),
+            slot: 0,
+            baseline: Hash32::ZERO,
+        },
+        user("first"),
+        user("second"),
+        user("third"),
+    ];
+    assert_eq!(
+        fallback_summary(&records, 4_000),
+        "third\n\nsecond\n\nfirst"
+    );
+}
+
+#[test]
+fn fallback_is_cut_to_summary_chars_on_a_character_boundary() {
+    let records = vec![user("café"), user("ab")];
+    assert_eq!(fallback_summary(&records, 3), "ab\n");
+    assert_eq!(fallback_summary(&records, 1), "a");
+    assert_eq!(fallback_summary(&records, 0), "");
+    let long = user("ååå");
+    assert_eq!(fallback_summary(&[long], 2), "åå");
+}
+
+#[test]
+fn fallback_with_no_users_is_empty() {
+    assert_eq!(fallback_summary(&[], 100), "");
+}