From 52e307d7b95dda735d1afbc4fe2dcffaa87512a4 Mon Sep 17 00:00:00 2001
From: tv <tv@krebsco.de>
Date: Thu, 5 Nov 2026 00:29:56 +0100
Subject: delta from held heads

---
 src/document/changes.rs | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

(limited to 'src/document/changes.rs')

diff --git a/src/document/changes.rs b/src/document/changes.rs
index 345f151..0409c50 100644
--- a/src/document/changes.rs
+++ b/src/document/changes.rs
@@ -67,24 +67,20 @@ pub(crate) fn change_from_bytes(bytes: Vec<u8>, hash: ChangeHash) -> Result<Chan
 
 /// The bytes of the delta from `from` to `to` in `doc`: the changes in the history of
 /// `to` but not of `from`, each as encoded, sorted by hash and concatenated
-/// (`load_delta` reads that). The same on every node holding `to`, whatever else it
-/// has. None if `doc` lacks part of `to`.
+/// (`load_delta` reads that). The same on every node holding `to` and `from`, whatever
+/// else it has. None if `doc` lacks part of either: without one of `from`, the delta
+/// would carry that one's history too, where `to` shares it, and differ from another
+/// node's (a download takes chunks from several, checked against one's manifest).
 pub(crate) fn delta_bytes(
     doc: &Document,
     from: &[ChangeHash],
     to: &[ChangeHash],
 ) -> Option<Vec<u8>> {
-    if !missing(doc, to).is_empty() {
+    if !missing(doc, to).is_empty() || !missing(doc, from).is_empty() {
         return None;
     }
-    // Heads of `from` we don't have can't hide anything we have.
-    let from: Vec<ChangeHash> = from
-        .iter()
-        .copied()
-        .filter(|head| doc.get_change_by_hash(head).is_some())
-        .collect();
     let since: HashMap<ChangeHash, (Vec<ChangeHash>, Vec<u8>)> = doc
-        .get_changes(&from)
+        .get_changes(from)
         .iter()
         .map(|change| {
             (
@@ -181,7 +177,12 @@ mod tests {
     fn no_delta_without_its_heads() {
         let mut doc = Document::new();
         change(&mut doc, |doc| doc.put(ROOT, "k", 1_i64).unwrap());
+        let heads = doc.get_heads();
         assert!(delta_bytes(&doc, &[], &[ChangeHash([7; 32])]).is_none());
+        assert!(
+            delta_bytes(&doc, &[ChangeHash([7; 32])], &heads).is_none(),
+            "nor without where it starts"
+        );
     }
 
     /// A change loads from its bytes only as the change it was asked for.
-- 
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