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.

2 files changed, 4 insertions, 4 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
         };
diff --git a/src/access/screen.rs b/src/access/screen.rs
index 1c6c8c7..793e8bb 100644
--- a/src/access/screen.rs
+++ b/src/access/screen.rs
@@ -40,7 +40,7 @@ use crate::document::inspect::{Action, Inspector, Op, Path};
 use crate::document::meta;
 use crate::document::replica::{Epoch, user_of};
 use crate::document::signatures::Signatures;
-use crate::document::{Change, ChangeHash, Document, ReadDoc};
+use crate::document::{Change, ChangeHash, Document};
 
 /// Changes waiting for their dependencies, kept at most: the oldest go.
 const MAX_WAITING: usize = 10_000;
@@ -289,7 +289,7 @@ pub(crate) fn apply(
         .into_iter()
         .chain(received)
         .filter(|change| seen.insert(change.hash()))
-        .filter(|change| doc.get_change_by_hash(&change.hash()).is_none())
+        .filter(|change| !crate::document::changes::holds(doc, &change.hash()))
         .filter(|change| match refusals.root(&change.hash()) {
             Some(root) => {
                 screened.refused_again += 1;
@@ -780,7 +780,7 @@ fn in_order(doc: &Document, candidates: Vec<Change>) -> (Vec<Change>, Vec<Change
             if by_hash.contains_key(dep) {
                 among += 1;
                 building.entry(*dep).or_default().push(*hash);
-            } else if doc.get_change_by_hash(dep).is_none() {
+            } else if !crate::document::changes::holds(doc, dep) {
                 missing = true;
             }
         }