From baf4c19f1dd28a51bf474d9cd652868b80cb781d Mon Sep 17 00:00:00 2001
From: Jesse McDonald <nybble41@gmail.com>
Date: Sun, 15 Mar 2020 12:58:14 -0500
Subject: [PATCH] clean up the logic around en passant and pawn promotion

---
 js/chess.js | 31 ++++++++++++++++++++-----------
 1 file changed, 20 insertions(+), 11 deletions(-)

diff --git a/js/chess.js b/js/chess.js
index 98d06f1..23e0a2a 100644
--- a/js/chess.js
+++ b/js/chess.js
@@ -280,35 +280,32 @@ function movePiece(priorBoard, side, from, to){
       from: from,
       to: to
    };
+
    if (took !== ' ') {
       board.move.took = took;
    }
+
    if (replaced !== ' ') {
       board.move.replaced = replaced;
    }
+
    if (alongside !== ' ') {
       board.move.alongside = alongside;
    }
+
    if (type === 'k' && actuallyFrom[0] === 'e' && to[0] === 'g') {
       board.move.castle = true;
       boardPut(board, 'h' + actuallyFrom[1], side, ' ');
       boardPut(board, 'f' + actuallyFrom[1], side, 'r');
    }
-   if (type === 'k' && actuallyFrom[0] === 'e' && to[0] === 'c') {
+   else if (type === 'k' && actuallyFrom[0] === 'e' && to[0] === 'c') {
       board.move.queen_castle = true;
       boardPut(board, 'a' + actuallyFrom[1], side, ' ');
       boardPut(board, 'd' + actuallyFrom[1], side, 'r');
    }
-   if (type === 'p' && (isDark(side) ? (to[1] === '1') : (to[1] === '8'))) {
-      board.move.promotion = 'q';  /* TODO: allow other choices */
-      type = 'q';
-   }
-   if (alongside === 'p' && (isDark(other) ? (to[1] === '1') : (to[1] === '8'))) {
-      board.move.promotion = 'q';  /* TODO: allow other choices */
-      alongside = 'q';
-   }
-   if (type === 'p' && alongside === ' ' && replaced === ' ' && to[0] !== actuallyFrom[0]) {
+   else if (type === 'p' && alongside === ' ' && replaced === ' ' && to[0] !== actuallyFrom[0]) {
       let otherBoard = priorBoard;
+      /* scan for the opponent's last move, since this could be part of a chain */
       while (otherBoard && otherBoard.move && otherBoard.move.side[0] === side[0]) {
          otherBoard = otherBoard.prior;
       }
@@ -318,9 +315,11 @@ function movePiece(priorBoard, side, from, to){
             const moveFrom = (move.from === 'phantom') ? otherBoard.prior.phantom.from : move.from;
             if (move.side[0] === other[0] && moveFrom[1] != to[1]) {
                board.move.en_passant = true;
-               alongside = 'p';
                board.move.took = 'p';
+               /* move the opponent's pawn back one space */
                boardPut(board, move.to, other, ' ');
+               boardPut(board, to, other, 'p');
+               /* see if we replaced a piece from the other square */
                replacedFrom = move.to;
                replaced = boardGet(board, replacedFrom, side);
                boardPut(board, replacedFrom, side, ' ');
@@ -329,6 +328,16 @@ function movePiece(priorBoard, side, from, to){
       }
    }
 
+   if (type === 'p' && to[1] === (isDark(side) ? '1' : '8')) {
+      board.move.promotion = 'q';  /* TODO: allow other choices */
+      type = 'q';
+   }
+
+   if (alongside === 'p' && to[1] === (isDark(other) ? '1' : '8')) {
+      board.move.promotion = 'q';  /* TODO: allow other choices */
+      alongside = 'q';
+   }
+
    if (from === 'phantom') {
       delete board.phantom;
    } else {