From 11d098477cfc76da0bc4385635e4c4b7124a9259 Mon Sep 17 00:00:00 2001
From: tv <tv@krebsco.de>
Date: Thu, 5 Nov 2026 03:40:09 +0100
Subject: announce only to those behind

---
 src/document/changes.rs | 43 +++++++++++++++++++++++++++++++++++++++++++
 src/document/sync.rs    |  5 +++++
 2 files changed, 48 insertions(+)

(limited to 'src/document')

diff --git a/src/document/changes.rs b/src/document/changes.rs
index 19d4db6..1091261 100644
--- a/src/document/changes.rs
+++ b/src/document/changes.rs
@@ -8,6 +8,25 @@ use anyhow::{Context, Result, anyhow, bail};
 
 use super::{Change, ChangeHash, Document, ReadDoc};
 
+/// Whether the history of `theirs` (of them, those `doc` has) holds all of `ours`
+/// (heads of `doc`): a peer at `theirs` has every change we have.
+pub(crate) fn covers(doc: &Document, theirs: &[ChangeHash], ours: &[ChangeHash]) -> bool {
+    let known: Vec<ChangeHash> = theirs
+        .iter()
+        .copied()
+        .filter(|head| doc.get_change_by_hash(head).is_some())
+        .collect();
+    if known.is_empty() {
+        return ours.is_empty();
+    }
+    let beyond: std::collections::HashSet<ChangeHash> = doc
+        .get_changes_meta(&known)
+        .into_iter()
+        .map(|change| change.hash)
+        .collect();
+    ours.iter().all(|head| !beyond.contains(head))
+}
+
 /// Those of `heads` that `doc` doesn't have.
 pub(crate) fn missing(doc: &impl ReadDoc, heads: &[ChangeHash]) -> Vec<ChangeHash> {
     heads
@@ -176,6 +195,30 @@ mod tests {
         });
     }
 
+    /// Heads cover ours when their history holds all of ours: as far as ours, or
+    /// further; not when behind or beside; heads we don't know count for nothing.
+    #[test]
+    fn heads_that_cover_ours() {
+        let mut doc = Document::new();
+        change(&mut doc, |doc| doc.put(ROOT, "a", 1_i64).unwrap());
+        let early = doc.get_heads();
+        change(&mut doc, |doc| doc.put(ROOT, "b", 2_i64).unwrap());
+        let ours = doc.get_heads();
+        let mut further = doc.fork();
+        change(&mut further, |doc| doc.put(ROOT, "c", 3_i64).unwrap());
+        doc.merge(&mut further.clone()).unwrap();
+        let unknown = ChangeHash([7; 32]);
+
+        assert!(covers(&doc, &ours, &ours));
+        assert!(covers(&doc, &further.get_heads(), &ours), "further");
+        assert!(!covers(&doc, &early, &ours), "behind");
+        assert!(!covers(&doc, &[unknown], &ours), "unknown");
+        assert!(
+            covers(&doc, &[unknown, ours[0]], &ours),
+            "the known one does"
+        );
+    }
+
     #[test]
     fn no_delta_without_its_heads() {
         let mut doc = Document::new();
diff --git a/src/document/sync.rs b/src/document/sync.rs
index 8c04632..e3ff157 100644
--- a/src/document/sync.rs
+++ b/src/document/sync.rs
@@ -28,6 +28,11 @@ impl Received {
         !self.0.changes.is_empty()
     }
 
+    /// The sender's heads, as it says.
+    pub(crate) fn heads(&self) -> &[crate::document::ChangeHash] {
+        &self.0.heads
+    }
+
     /// The bytes of the changes it carries, and of its bloom filters (`have`).
     pub(crate) fn sizes(&self) -> (usize, usize) {
         let changes = self.0.changes.iter().map(<[u8]>::len).sum();
-- 
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