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/access/rules.rs b/src/access/rules.rs
index 3aba0ba..bef9a13 100644
--- a/src/access/rules.rs
+++ b/src/access/rules.rs
@@ -125,7 +125,7 @@ pub(crate) fn judge_reading(
         // The first epoch's: the first change a document holds, as it comes to an empty
         // one, or as it is in one (judged again, `swarm::refusals`).
         let root = if epoch.generation == 0 {
-            doc.get_heads().is_empty() || doc.get_change_by_hash(&change.hash).is_some()
+            doc.get_heads().is_empty() || crate::document::changes::holds(doc, &change.hash)
         } else {
             change.hash.0 == epoch.id
         };