access: read once what rules read at the same heads

bench_screening shows judging at about 17 µs of a change's 25: some
two and a half reads at its heads (the policy, bans, its node's member
entry, whether there is a registry, the sealed declarations), each
costing as much as working out what the heads cover. Screening judges
most of a batch at the same heads, and those reads are the same for
every change judged there.

Keep them (rules::Reads), by node and by user where they depend on
one, while the heads stay the same; screening keeps one for a batch.
What the heads hold doesn't change as the document takes changes, so
nothing goes stale but by other heads. The benchmark breakdown shows
judging so and afresh.

1 files changed, 23 insertions, 19 deletions

diff --git a/src/document/bench.rs b/src/document/bench.rs
index 026cf7b..2b82036 100644
--- a/src/document/bench.rs
+++ b/src/document/bench.rs
@@ -684,9 +684,10 @@ fn screening_costs(workload: &str, doc: &Document) {
 }
 
 /// Where screening `changes` spends its time, per change judged: decoding them, their
-/// operations placed (`inspect::Inspector`), and judging them (`rules::judge`), of which
-/// reading the policy (`policy::of`) and, for scale, one read (the committer's member
-/// entry). As screening judges most: the first ones in (the maps they make, which flush
+/// operations placed (`inspect::Inspector`), and judging them, as screening does (what
+/// is read at the same heads read once, `rules::Reads`) and afresh (`rules::judge`); of
+/// that, reading the policy (`policy::of`) and, for scale, one read (the committer's
+/// member entry). As screening judges most: the first ones in (the maps they make, which flush
 /// the batch), the rest judged at the heads after them (`screen`'s frontier).
 fn screening_parts(changes: &[automerge::Change]) {
     use crate::access::{policy, rules};
@@ -736,26 +737,28 @@ fn screening_parts(changes: &[automerge::Change]) {
         .iter()
         .map(|ops| ops.iter().map(|op| op.path.clone()).collect())
         .collect();
-    let judge = median(3, || {
+    let judge_all = |reads: &mut Option<rules::Reads>| {
         let started = Instant::now();
         for (index, change) in rest.iter().enumerate() {
-            let judged = rules::judge(
-                &base,
-                Epoch::GENESIS,
-                &rules::Change {
-                    hash: change.hash(),
-                    deps: change.deps(),
-                    seen: &seen,
-                    committer: committers[index].as_deref(),
-                    user: None,
-                    paths: &paths[index],
-                    ops: &ops[index],
-                },
-            );
+            let change = rules::Change {
+                hash: change.hash(),
+                deps: change.deps(),
+                seen: &seen,
+                committer: committers[index].as_deref(),
+                user: None,
+                paths: &paths[index],
+                ops: &ops[index],
+            };
+            let judged = match reads {
+                Some(reads) => rules::judge_reading(&base, Epoch::GENESIS, &change, reads),
+                None => rules::judge(&base, Epoch::GENESIS, &change),
+            };
             assert_eq!(judged, Ok(()), "every change taken");
         }
         started.elapsed()
-    });
+    };
+    let judge = median(3, || judge_all(&mut Some(rules::Reads::default())));
+    let afresh = median(3, || judge_all(&mut None));
     let read_policy = median(3, || {
         let started = Instant::now();
         for _ in rest {
@@ -775,11 +778,12 @@ fn screening_parts(changes: &[automerge::Change]) {
     });
     let per = |d: Duration| d.as_secs_f64() * 1e6 / rest.len().max(1) as f64;
     println!(
-        "{:<40} per change: decode {:.2} µs, place {:.2} µs, judge {:.2} µs (policy::of {:.2} µs, one read {:.2} µs)",
+        "{:<40} per change: decode {:.2} µs, place {:.2} µs, judge {:.2} µs (afresh {:.2} µs: policy::of {:.2} µs, one read {:.2} µs)",
         "",
         per(decode),
         per(place),
         per(judge),
+        per(afresh),
         per(read_policy),
         per(one_read),
     );