From bd8dcc6077ab449023d5435b0dfc84ec6da0f06e Mon Sep 17 00:00:00 2001
From: Jesse McDonald <nybble41@gmail.com>
Date: Sat, 14 Mar 2020 17:21:43 -0500
Subject: [PATCH] fix implementation of en passant to match official rules

---
 js/chess.js | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/js/chess.js b/js/chess.js
index 5e6881a..dec8dd5 100644
--- a/js/chess.js
+++ b/js/chess.js
@@ -253,7 +253,8 @@ function movePiece(priorBoard, side, from, to){
    var other = otherSide(side);
    var type = boardGet(priorBoard, from, side);
    var took = boardGet(priorBoard, to, other);
-   var replaced = boardGet(priorBoard, to, side);
+   var replacedFrom = to;
+   var replaced = boardGet(priorBoard, replacedFrom, side);
    var alongside = boardGet(priorBoard, from, other);
    var actuallyFrom = (from === 'phantom') ? priorBoard.phantom.from : from;
 
@@ -306,7 +307,7 @@ function movePiece(priorBoard, side, from, to){
       board.move.promotion = 'q';  /* TODO: allow other choices */
       alongside = 'q';
    }
-   if (type === 'p' && alongside === ' ' && to[0] !== actuallyFrom[0]) {
+   if (type === 'p' && alongside === ' ' && replaced === ' ' && to[0] !== actuallyFrom[0]) {
       var otherBoard = priorBoard;
       while (otherBoard && otherBoard.move && otherBoard.move.side[0] === side[0]) {
          otherBoard = otherBoard.prior;
@@ -319,6 +320,9 @@ function movePiece(priorBoard, side, from, to){
                board.move.en_passant = true;
                alongside = 'p';
                boardPut(board, move.to, other, ' ');
+               replacedFrom = move.to;
+               replaced = boardGet(board, replacedFrom, side);
+               boardPut(board, replacedFrom, side, ' ');
             }
          }
       }
@@ -340,7 +344,7 @@ function movePiece(priorBoard, side, from, to){
    if (replaced === ' ') {
       board.player = otherSide(board.player);
    } else {
-      board.phantom = { from: to, type: replaced + side[0] };
+      board.phantom = { from: replacedFrom, type: replaced + side[0] };
    }
 
    return board;