tune: reduce board history size

seemingly avoiding repetition performs worse as of last commit.

src/lib.rs

@@ -496,19 +496,6 @@ impl<const N: usize> Default for RingPtr<N> {
 
 impl<const N: usize> RingPtr<N> {}
 
-/// Ring-buffer of previously seen hashes, used to avoid draw by repetition.
-///
-/// Only stores at most `HISTORY_SIZE` plies, since most cases of repetition happen recently.
-/// Technically, it should be 100 plies because of the 50-move rule.
-#[derive(Default, Clone, Copy, Debug)]
-struct BoardHistory {
-    hashes: [Zobrist; HISTORY_SIZE],
-    /// Index of the start of the history in the buffer
-    ptr_start: RingPtr<HISTORY_SIZE>,
-    /// Index one-past-the-end of the history in the buffer
-    ptr_end: RingPtr<HISTORY_SIZE>,
-}
-
 #[cfg(test)]
 mod ringptr_tests {
     use super::*;
@@ -527,6 +514,19 @@ mod ringptr_tests {
     }
 }
 
+/// Ring-buffer of previously seen hashes, used to avoid draw by repetition.
+///
+/// Only stores at most `HISTORY_SIZE` plies, since most cases of repetition happen recently.
+/// Technically, it should be 100 plies because of the 50-move rule.
+#[derive(Default, Clone, Copy, Debug)]
+struct BoardHistory {
+    hashes: [Zobrist; HISTORY_SIZE],
+    /// Index of the start of the history in the buffer
+    ptr_start: RingPtr<HISTORY_SIZE>,
+    /// Index one-past-the-end of the history in the buffer
+    ptr_end: RingPtr<HISTORY_SIZE>,
+}
+
 impl PartialEq for BoardHistory {
     /// Always equal, since comparing two boards with different histories shouldn't matter.
     fn eq(&self, _other: &Self) -> bool {
@@ -539,7 +539,7 @@ impl Eq for BoardHistory {}
 /// Size in plies of the board history.
 ///
 /// Actual capacity is one less than this.
-const HISTORY_SIZE: usize = 30;
+const HISTORY_SIZE: usize = 10;
 
 impl BoardHistory {
     /// Counts occurences of this hash in the history.

src/search.rs

@@ -208,8 +208,8 @@ fn minmax(board: &mut Board, state: &mut EngineState, mm: MinmaxState) -> (Vec<M
         }
     }
 
-    if board.history.count(board.zobrist) == 2 {
+    if !mm.quiesce && board.history.count(board.zobrist) == 1 {
-        // draw by repetition
+        // avoid draw by repetition
         return (Vec::new(), SearchEval::Exact(0));
     }