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packages/draughts/computer/evaluate-board.js

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+import {
+  eachCell,
+  pieceIsPlayer,
+  pieceIsQueen,
+  BOARD_SIZE,
+  Players,
+  GameStates,
+} from '@draughts/core';
+
+const winnerMap = {
+  [GameStates.WHITE_WON]: Players.WHITE,
+  [GameStates.BLACK_WON]: Players.BLACK,
+};
+
+export function evaluateBoard(board) {
+  if (board.state === GameStates.DRAW) {
+    return 0;
+  }
+  if (board.state !== GameStates.PLAYING) {
+    const winner = winnerMap[board.state];
+    return board.playerToMove === winner
+      ? Number.POSITIVE_INFINITY
+      : Number.NEGATIVE_INFINITY;
+  }
+  if (isEndgame(board)) {
+    return evaluateEndgamePosition(board);
+  }
+  return evaluateRegularPosition(board);
+}
+
+function isEndgame(board) {
+  for (const { piece } of eachCell(board.position)) {
+    if (!pieceIsPlayer(piece, Players.NONE) && !pieceIsQueen(piece)) {
+      return false;
+    }
+  }
+  return true;
+}
+
+function evaluateEndgamePosition(board) {
+  let playerPieces = 0;
+  let opponentPieces = 0;
+  let distances = 0;
+  for (const { cell, piece } of eachCell(board.position)) {
+    if (pieceIsPlayer(piece, board.playerToMove)) {
+      playerPieces += 1;
+      distances += calculateDistances(board, cell);
+    } else if (!pieceIsPlayer(piece, Players.NONE)) {
+      opponentPieces += 1;
+    }
+  }
+  if (playerPieces >= opponentPieces) {
+    return -distances;
+  }
+  return distances;
+}
+
+function evaluateRegularPosition(board) {
+  let e = 0;
+  for (const { cell, piece } of eachCell(board.position)) {
+    const pieceEvaluation = evaluatePiece(cell, piece);
+    if (pieceIsPlayer(piece, board.playerToMove)) {
+      e += pieceEvaluation;
+    } else if (!pieceIsPlayer(piece, Players.NONE)) {
+      e -= pieceEvaluation;
+    }
+  }
+  return e;
+}
+
+function evaluatePiece(cell, piece) {
+  let e = 20;
+  if (pieceIsQueen(piece)) {
+    e += 40;
+  } else if (pieceIsPlayer(piece, Players.WHITE)) {
+    e += BOARD_SIZE - 1 - cell.row;
+  } else if (pieceIsPlayer(piece, Players.BLACK)) {
+    e += cell.row;
+  }
+  return e;
+}
+
+function calculateDistances(board, baseCell) {
+  let distances = 0;
+  for (const { cell, piece } of eachCell(board.position)) {
+    if (
+      !pieceIsPlayer(piece, board.playerToMove) &&
+      !pieceIsPlayer(piece, Players.NONE)
+    ) {
+      distances += euclideanDistance(baseCell, cell);
+    }
+  }
+  return distances;
+}
+
+function euclideanDistance(a, b) {
+  const rowSquare = Math.pow(a.row - b.row, 2);
+  const colSquare = Math.pow(a.col - b.col, 2);
+  const distanceFloat = Math.sqrt(rowSquare + colSquare);
+  return Math.ceil(distanceFloat);
+}