Initial work on SEE

Chess/nimfish/nimfishpkg/see.nim

@@ -0,0 +1,59 @@
+# Copyright 2024 Mattia Giambirtone & All Contributors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+## Implementation of Static Exchange Evaluation
+import std/algorithm
+
+
+import position
+import pieces
+import movegen
+import board
+import eval
+
+
+proc getSmallestAttacker(position: Position, square: Square, side: PieceColor): Square =
+    ## Returns the square containing the lowest value piece of the
+    ## given side attacking the given square in the given position.
+    ## If the given square is not attacked by the given side, a null 
+    ## piece is returned
+    var attackers: seq[tuple[score: Score, square: Square]] = @[]
+    for attacker in position.getAttacksTo(square, side):
+        attackers.add((position.getPieceScore(attacker), attacker))
+
+    if attackers.len() == 0:
+        return nullSquare()
+
+    proc orderer(a, b: tuple[score: Score, square: Square]): int {.closure.} =
+        return cmp(a.score, b.score)
+
+
+    attackers.sort(orderer)
+    return attackers[0].square
+
+
+proc SEE(board: var Chessboard, square: Square, side: PieceColor): Score =
+    ## Recursive implementation of static exchange evaluation
+    let source = board.position.getSmallestAttacker(square, side)
+    if source == nullSquare():
+        return Score(0)
+    
+    let captured = board.position.getPieceScore(square)
+
+    board.doMove(createMove(source, square, Capture))
+    result = max(0, captured - SEE(board, square, side.opposite()))
+    board.unmakeMove()
+
+
+