document: tell whether a document holds a change without building it

With the policy's reads kept, judging a change still costs about one
read (some 3.5 µs), and so does judging without a policy: what is left
is At::new, which asks get_change_by_hash for each head. Automerge
builds the change again from the document's operations to answer, at
a cost growing with its history. Screening asks it again for every
candidate and every dependency, and sync for heads.

Ask by the missing dependencies instead (changes::holds), which look
the hash up: one the document holds is not among them. A change queued
in the document, waiting for its dependencies, isn't either; but then
some are missing, and only then is it asked the slow way. Every check
of whether a document holds a change goes through it.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/document/at.rs b/src/document/at.rs
index d318f17..2d35736 100644
--- a/src/document/at.rs
+++ b/src/document/at.rs
@@ -23,7 +23,7 @@ impl<'a> At<'a> {
     pub(crate) fn new(doc: &'a Document, heads: &[ChangeHash]) -> Option<Self> {
         heads
             .iter()
-            .all(|head| doc.get_change_by_hash(head).is_some())
+            .all(|head| crate::document::changes::holds(doc, head))
             .then(|| Self {
                 doc,
                 heads: heads.to_vec(),