omit source location in history when unambiguous

js/chess.js

@@ -350,12 +350,26 @@ function movePiece(priorBoard, side, from, to){
    return board;
 }
 
-function renderHistory(board) {
+function findPieces(board, side, type){
+   let pieces = [];
+
+   for (const row of "12345678") {
+      for (const column of "abcdefgh") {
+         const here = column + row;
+         if (boardGet(board, here, side) === type) {
+            pieces.push(here);
+         }
+      }
+   }
+
+   return pieces;
+}
+
+function renderHistory(currentBoard) {
    let list = [];
 
-   while (board && board.move) {
+   for (let board = currentBoard; board && board.move; board = board.prior) {
-      list.push(board.move);
+      list.push(board);
-      board = board.prior;
    }
 
    const NBSP = '\u00a0';  /* non-breaking space */
@@ -366,11 +380,11 @@ function renderHistory(board) {
    let n = 0;
 
    while (list.length > 0) {
-      const move = list.pop();
+      const board = list.pop();
+      const move = board.move;
+
       if (move.from === 'phantom') {
-         const piece = move.type === 'p' ? '' : move.type.toUpperCase();
+         result += SHY + '*';
-         const took = move.took ? 'x' : '';
-         result += SHY + '*' + piece + took + move.to;
       } else {
          if (n > 0 || isDark(move.side)) {
             result += ' ';
@@ -380,24 +394,61 @@ function renderHistory(board) {
             ++n;
             result += String(n) + '.' + NBSP;
          }
+      }
 
-         if (move.pass) {
+      if (move.pass) {
-            result += '...';
+         result += '...';
-         } else if (move.alongside) {
+      } else if (move.castle) {
+         result += 'O-O';
+      } else if (move.queen_castle) {
+         result += 'O-O-O';
+      } else {
+         let piece = '';
+
+         if (move.alongside) {
             if (isLight(move.side)) {
-               result += move.type.toUpperCase() + move.alongside.toUpperCase() + move.from + move.to;
+               piece = move.type.toUpperCase() + move.alongside.toUpperCase();
             } else {
-               result += move.alongside.toUpperCase() + move.type.toUpperCase() + move.from + move.to;
+               piece = move.alongside.toUpperCase() + move.type.toUpperCase();
             }
-         } else if (move.castle) {
-            result += 'O-O';
-         } else if (move.queen_castle) {
-            result += 'O-O-O';
          } else {
-            const piece = move.type === 'p' ? '' : move.type.toUpperCase();
+            piece = move.type === 'p' ? '' : move.type.toUpperCase();
-            const took = move.took ? 'x' : '';
-            result += piece + move.from + took + move.to;
          }
+
+         if (move.from !== 'phantom') {
+            const sameKind = findPieces(board.prior, move.side, move.type);
+            const legalFrom = [];
+            let sameFile = 0;  /* column / letter */
+            let sameRank = 0;  /* row / number */
+
+            for (const where of sameKind) {
+               if (legalMoves(board.prior, move.side, move.type, where, true).includes(move.to)) {
+                  legalFrom.push(where);
+
+                  if (where[0] === move.from[0]) {
+                     sameFile += 1;
+                  }
+
+                  if (where[1] === move.from[1]) {
+                     sameRank += 1;
+                  }
+               }
+            }
+
+            if (legalFrom.length !== 1) {
+               /* append file, rank, or both to disambiguate */
+               if (sameFile === 1) {
+                  piece += move.from[0];
+               } else if (sameRank === 1) {
+                  piece += move.from[1];
+               } else {
+                  piece += move.from;
+               }
+            }
+         }
+
+         const took = move.took ? 'x' : '';
+         result += piece + took + move.to;
       }
 
       if (move.en_passant) {