feat(search): Weigh entry age against depth in transposition table replacement

arche-core/src/engine.rs

@@ -2968,10 +2968,10 @@ impl AlphaBeta {
         SearchOutcome::Complete(self.result_for(play, score), bound)
     }
 
-    /// The root's answer to the table, past the taint policy and the depth
-    /// contest, because the reported line is read back from this slot.
-    /// Under `Skip` a tainted root answer is stored too, and the rare
-    /// tainted cutoff it then offers is refused as under `Refuse`. A
+    /// The root's answer to the table, past the taint policy and the
+    /// replacement contest, because the reported line is read back from
+    /// this slot. Under `Skip` a tainted root answer is stored too, and the
+    /// rare tainted cutoff it then offers is refused as under `Refuse`. A
     /// ceiling is not stored, so the closest move is never promoted over a
     /// move it was not shown to beat.
     fn store_root_answer(&mut self, play: Play, value: Value, depth: u8, bound: ScoreBound) {

arche-core/src/engine/tests.rs

@@ -292,12 +292,13 @@ mod search {
     #[test]
     fn the_reported_line_opens_with_the_move_actually_answered() {
         // A deeper entry for the root position, left by an earlier search of
-        // it, used to win the depth contest against the root's own store:
-        // the table then told a line opening with the leftover's move while
-        // bestmove answered the fresh one, and the two disagreed in front of
-        // whatever was relaying the search. Here the queen hangs, so a fresh
-        // search must answer with the capture, while the planted leftover
-        // claims a quiet king move from a depth no shallow search can beat.
+        // it, used to win the replacement contest against the root's own
+        // store: the table then told a line opening with the leftover's move
+        // while bestmove answered the fresh one, and the two disagreed in
+        // front of whatever was relaying the search. Here the queen hangs, so
+        // a fresh search must answer with the capture, while the planted
+        // leftover claims a quiet king move from a depth no shallow search
+        // can beat.
         let game = Board::from_fen("k7/8/8/3q4/8/8/3R4/K7 w - - 0 1").unwrap();
         let mut e = engine(game);
         let quiet = play_named(&e.board, "a1b1");

arche-core/src/transposition.rs

@@ -96,8 +96,8 @@ pub struct SignatureCounters {
     pub narrow_accepts: [u64; NARROW_WIDTHS.len()],
     /// Stores whose slice matched a foreign full key, so the store replaced
     /// another position's entry as this position's. Landed stores only: a
-    /// store the depth contest turns away after comparing against a foreign
-    /// entry's depth is a related cost, but nothing was evicted.
+    /// store the contest in `set` turns away after comparing against a
+    /// foreign entry's worth is a related cost, but nothing was evicted.
     pub aliased_evictions: u64,
 }
 
@@ -294,10 +294,18 @@ const STALE_AFTER_SEARCHES: u8 = 12;
 /// The generations run from one to this and round again, so generation
 /// zero reads as empty. Ages are taken modulo this, so an entry from
 /// thirty one searches ago or more reads as recent again and holds its slot
-/// by depth until it ages out again. Nothing wrong is read, since a hit is
-/// keyed; a slot is only held longer than it should be.
+/// by its worth until it ages out again. Nothing wrong is read, since a hit
+/// is keyed; a slot is only held longer than it should be.
 const GENERATIONS: u8 = 31;
 
+/// The plies of depth an entry is worth less for each search since it was
+/// stored. A search on, the game has moved two plies and has usually left
+/// the line the entry was searched for, so an entry from this search
+/// outranks an older one unless the older is deeper by eight plies a
+/// search. Stockfish 16 and its current master choose their victim with
+/// the same weight (Stockfish 17 used sixteen).
+const AGE_WEIGHT: i16 = 8;
+
 impl Entry {
     const EMPTY: Entry = Entry {
         key: 0,
@@ -782,6 +790,14 @@ impl TranspositionTable {
         self.replaceable = replaceable_under(self.generation);
     }
 
+    /// What an entry is worth keeping: its depth, less `AGE_WEIGHT` plies
+    /// for each search since it was stored. An entry from this search is
+    /// worth its depth.
+    #[inline(always)]
+    fn worth(&self, entry: Entry) -> i16 {
+        i16::from(entry.depth) - AGE_WEIGHT * i16::from(self.age(entry))
+    }
+
     /// How many searches ago an entry was stored.
     #[inline]
     fn age(&self, entry: Entry) -> u8 {
@@ -908,8 +924,8 @@ impl TranspositionTable {
     }
 
     /// Where in its bucket a position goes: its own entry if it has one,
-    /// else an empty one, else a stale one, else the shallowest. This and
-    /// the depth contest in `set` are the whole replacement policy. The
+    /// else an empty one, else a stale one, else the one worth least. This
+    /// and the contest in `set` are the whole replacement policy. The
     /// third field says the slot was found replaceable, so the contest need
     /// not ask again.
     #[inline(always)]
@@ -922,30 +938,35 @@ impl TranspositionTable {
         }
         let bucket = self.bucket(index);
         let mut victim = 0;
+        let mut least = i16::MAX;
         for i in 0..BUCKET {
             let entry = bucket.entry(i);
             if self.replaceable(entry) {
                 return (index, i, true);
             }
-            if entry.depth < bucket.rest[victim].depth {
+            let worth = self.worth(entry);
+            if worth < least {
+                least = worth;
                 victim = i;
             }
         }
         (index, victim, false)
     }
 
-    /// Store unless the slot holds something worth more. Reports whether
-    /// the entry landed.
+    /// Store unless the slot holds something worth more: an entry whose
+    /// worth is above the new store's depth, or an exact entry of the same
+    /// position whose worth equals that depth where the new store is not
+    /// exact. Reports whether the entry landed.
     #[inline(always)]
     fn set(&mut self, key: u64, pv: Pv) -> bool {
         let (index, i, free) = self.slot_for(key);
         let old = self.bucket(index).entry(i);
         debug_assert!(!free || self.replaceable(old));
         if !free && !self.replaceable(old) {
-            if pv.depth < old.depth {
+            if i16::from(pv.depth) < self.worth(old) {
                 return false;
             }
-            if pv.depth == old.depth
+            if i16::from(pv.depth) == self.worth(old)
                 && old.key == Entry::slice(key)
                 && matches!(old.bound(), Bound::Exact)
                 && !matches!(pv.bound, Bound::Exact)
@@ -977,7 +998,7 @@ impl TranspositionTable {
         audit.keys[at] = key;
     }
 
-    /// Store without the depth contest. For the root's end-of-iteration
+    /// Store without the contest. For the root's end-of-iteration
     /// entry: it names the move about to be answered with, and the reported
     /// line is read back from its slot, so an entry a deeper search left
     /// there earlier in the game must not outrank it. When one did, the
@@ -1033,7 +1054,7 @@ impl TranspositionTable {
     }
 
     /// What the three `record_` methods share: the entry, offered to the
-    /// depth contest.
+    /// replacement contest.
     #[inline(always)]
     fn record(
         &mut self,
@@ -1050,8 +1071,9 @@ impl TranspositionTable {
         )
     }
 
-    /// The move the engine is about to answer with, stored past the depth
-    /// contest for the reason `set_always` gives, so it always lands.
+    /// The move the engine is about to answer with, stored past the
+    /// replacement contest for the reason `set_always` gives, so it always
+    /// lands.
     #[must_use]
     pub fn record_answer(&mut self, board: &Board, play: Play, score: Value, depth: u8) -> bool {
         self.set_always(
@@ -1061,9 +1083,9 @@ impl TranspositionTable {
         true
     }
 
-    /// The move a root iteration failed high on, stored past the depth
-    /// contest as the floor it is, so the wider re-search orders it first.
-    /// It always lands.
+    /// The move a root iteration failed high on, stored past the
+    /// replacement contest as the floor it is, so the wider re-search orders
+    /// it first. It always lands.
     #[must_use]
     pub fn record_floor_answer(
         &mut self,
@@ -1374,8 +1396,8 @@ mod tests {
 
     #[test]
     fn a_quiescence_entry_does_not_evict_a_searched_entry() {
-        // quiescence writes at depth zero, so the depth contest is what
-        // keeps its entries from displacing a searched position's
+        // quiescence writes at depth zero, so within a search the contest is
+        // what keeps its entries from displacing a searched position's
         let mut table = full_bucket(5);
         table.set(5, new_pv(Bound::Lower, 0));
         assert!(table.get(5).is_none());
@@ -1534,7 +1556,8 @@ mod tests {
 
     #[test]
     fn an_entry_from_searches_ago_is_replaced_regardless_of_depth() {
-        let mut table = full_bucket(8);
+        // deep enough that its worth holds the slot until the window ends
+        let mut table = full_bucket(MAX_PLY);
         for _ in 0..STALE_AFTER_SEARCHES - 1 {
             table.new_search();
         }
@@ -1546,6 +1569,61 @@ mod tests {
         assert_eq!(kept(&table, 1..=4), 3);
     }
 
+    #[test]
+    fn an_entry_from_the_last_search_is_worth_eight_plies_less() {
+        let mut table = full_bucket(12);
+        table.new_search();
+        table.set(5, new_pv(Bound::Lower, 3));
+        assert!(table.get(5).is_none(), "worth four, deeper than three");
+        table.set(5, new_pv(Bound::Lower, 4));
+        assert!(table.get(5).is_some(), "worth four, no deeper than four");
+        assert_eq!(kept(&table, 1..=4), 3);
+    }
+
+    #[test]
+    fn a_positions_own_entry_from_the_last_search_takes_a_shallower_store() {
+        let mut table = TranspositionTable::with_capacity(4).expect("a table of a few buckets");
+        table.set(1, new_pv(Bound::Lower, 10));
+        table.set(1, new_pv(Bound::Lower, 9));
+        assert_eq!(table.get(1).unwrap().depth, 10, "this search's, kept");
+        table.new_search();
+        table.set(1, new_pv(Bound::Lower, 2));
+        assert_eq!(table.get(1).unwrap().depth, 2, "worth two, replaced");
+    }
+
+    #[test]
+    fn an_exact_entry_from_the_last_search_is_held_at_its_worth() {
+        let mut table = TranspositionTable::with_capacity(4).expect("a table of a few buckets");
+        table.set(1, new_pv(Bound::Exact, 10));
+        table.new_search();
+        table.set(1, new_pv(Bound::Lower, 2));
+        assert!(
+            matches!(table.get(1).unwrap().bound, Bound::Exact),
+            "worth two, exact, holds against a bound of two"
+        );
+        table.set(1, new_pv(Bound::Exact, 2));
+        assert_eq!(table.get(1).unwrap().depth, 2);
+    }
+
+    #[test]
+    fn the_victim_is_the_entry_worth_least_not_the_shallowest() {
+        let mut table = TranspositionTable::with_capacity(4).expect("a table of a few buckets");
+        table.set(1, new_pv(Bound::Exact, 10));
+        table.set(2, new_pv(Bound::Exact, 9));
+        table.new_search();
+        table.set(3, new_pv(Bound::Exact, 3));
+        table.set(4, new_pv(Bound::Exact, 3));
+        // worth 2, 1, 3 and 3: the depth nine entry from the last search goes
+        table.set(5, new_pv(Bound::Lower, 3));
+        assert!(
+            table.get(2).is_none(),
+            "the entry worth least should have gone"
+        );
+        for key in [1, 3, 4, 5] {
+            assert!(table.get(key).is_some(), "key {key} should be there");
+        }
+    }
+
     #[test]
     fn the_root_store_takes_a_slot_whatever_the_bucket_holds() {
         let mut table = full_bucket(20);
@@ -1563,11 +1641,11 @@ mod tests {
             table.new_search();
         }
         for key in 1..=4 {
-            table.set(key, new_pv(Bound::Exact, 8));
+            table.set(key, new_pv(Bound::Exact, MAX_PLY));
         }
         table.new_search();
         table.set(5, new_pv(Bound::Lower, 1));
-        assert!(table.get(5).is_none(), "one search old, kept by depth");
+        assert!(table.get(5).is_none(), "one search old, kept by its worth");
         for _ in 0..STALE_AFTER_SEARCHES - 1 {
             table.new_search();
         }

docs/INSTRUMENTS.md

@@ -792,9 +792,9 @@ evictions count stores. A comparison is one live entry a probe really compared
 its signature against whose full key belonged to another position, so each is
 one chance in two to the signature's width, and every expectation is drawn from
 that total. An aliased eviction is a store that landed in a slot the signature
-said was its own and replaced another position's entry. A store the depth
-contest turned away after comparing itself with a foreign entry is a related
-cost but evicted nothing, so it is not counted.
+said was its own and replaced another position's entry. A store the
+replacement contest turned away after comparing itself with a foreign entry
+is a related cost but evicted nothing, so it is not counted.
 
 The thirty two bit count cannot say anything on its own: a run of this size
 expects a few thousandths of a false accept, so a zero is what a working