relax eager check for too many pieces

src/errors.rs

@@ -12,8 +12,8 @@ pub enum SyzygyError {
     /// Position has castling rights, but Syzygy tables do not contain
     /// positions with castling rights.
     Castling,
-    /// Position has too many pieces. Syzygy tables only support up to
-    /// 6 or 7 pieces.
+    /// Position has too many pieces, i.e., more pieces than any opened table.
+    /// Syzygy tables for standard chess support up to 7 pieces.
     TooManyPieces,
     /// Missing table.
     MissingTable {

src/tablebase.rs

@@ -376,9 +376,14 @@ impl<S: Position + Clone + Syzygy> Tablebase<S> {
     }
 
     fn probe<'a>(&'a self, pos: &'a S) -> SyzygyResult<WdlEntry<'a, S>> {
-        if pos.board().occupied().count() > S::MAX_PIECES {
+        // Probing resolves captures, so sometimes we can obtain results
+        // for positions that have more pieces than the maximum amount of
+        // supported pieces. We artificially limit this to one additional
+        // level, to make sure search remains somewhat bounded.
+        if pos.board().occupied().count() > S::MAX_PIECES + 1 {
             return Err(SyzygyError::TooManyPieces);
         }
+
         if pos.castles().any() {
             return Err(SyzygyError::Castling);
         }
@@ -629,6 +634,11 @@ impl<S: Position + Clone + Syzygy> Tablebase<S> {
             return Ok(Wdl::Draw);
         }
 
+        // More pieces than any opened table.
+        if pos.board().occupied().count() > self.max_pieces {
+            return Err(SyzygyError::TooManyPieces);
+        }
+
         // Get raw WDL value from the appropriate table.
         let key = Material::from_board(pos.board());
         self.wdl_table(&key)