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()
+}