From 6c2d0dea91d054b627714d223d7ffd78683704c9 Mon Sep 17 00:00:00 2001
From: Peter Elliott <pelliott@ualberta.ca>
Date: Mon, 17 Aug 2020 15:40:55 -0600
Subject: [PATCH] Chess: Fix generation of promotion moves

---
 Games/Chess/Chess.cpp | 17 +++++++++++++++--
 Games/Chess/Chess.h   | 22 ++++++++++++----------
 2 files changed, 27 insertions(+), 12 deletions(-)

diff --git a/Games/Chess/Chess.cpp b/Games/Chess/Chess.cpp
index 88a8c220f2..c0a3f55807 100644
--- a/Games/Chess/Chess.cpp
+++ b/Games/Chess/Chess.cpp
@@ -153,7 +153,10 @@ bool Chess::is_legal_no_check(const Move& move, Colour colour) const
     if (move.to.rank > 7 || move.to.file > 7)
         return false;
 
-    if (move.promote_to == Type::Pawn || move.promote_to == Type::King || move.promote_to == Type::None)
+    if (piece.type != Type::Pawn && move.promote_to != Type::None)
+        return false;
+
+    if (move.promote_to == Type::Pawn || move.promote_to == Type::King)
         return false;
 
     if (piece.type == Type::Pawn) {
@@ -161,6 +164,14 @@ bool Chess::is_legal_no_check(const Move& move, Colour colour) const
         unsigned start_rank = (colour == Colour::White) ? 1 : 6;
         unsigned other_start_rank = (colour == Colour::White) ? 6 : 1;
         unsigned en_passant_rank = (colour == Colour::White) ? 4 : 3;
+        unsigned promotion_rank = (colour == Colour::White) ? 7 : 0;
+
+        if (move.to.rank == promotion_rank) {
+            if (move.promote_to == Type::Pawn || move.promote_to == Type::King || move.promote_to == Type::None)
+                return false;
+        } else if (move.promote_to != Type::None) {
+            return false;
+        }
 
         if (move.to.rank == move.from.rank + dir && move.to.file == move.from.file && get_piece(move.to).type == Type::None) {
             // Regular pawn move.
@@ -457,7 +468,9 @@ bool Chess::is_promotion_move(const Move& move, Colour colour) const
     if (colour == Colour::None)
         colour = turn();
 
-    if (!is_legal(move, colour))
+    Move queen_move = move;
+    queen_move.promote_to = Type::Queen;
+    if (!is_legal(queen_move, colour))
         return false;
 
     if (get_piece(move.from).type == Type::Pawn && ((colour == Colour::Black && move.to.rank == 0) || (colour == Colour::White && move.to.rank == 7)))
diff --git a/Games/Chess/Chess.h b/Games/Chess/Chess.h
index 60d49b4da1..920b767aae 100644
--- a/Games/Chess/Chess.h
+++ b/Games/Chess/Chess.h
@@ -91,7 +91,7 @@ public:
         Square to;
         Type promote_to;
         Move(const StringView& algebraic);
-        Move(const Square& from, const Square& to, const Type& promote_to = Type::Queen)
+        Move(const Square& from, const Square& to, const Type& promote_to = Type::None)
             : from(from)
             , to(to)
             , promote_to(promote_to)
@@ -200,15 +200,17 @@ void Chess::generate_moves(Callback callback, Colour colour) const
 
         bool keep_going = true;
         if (piece.type == Type::Pawn) {
-            keep_going =
-                try_move({sq, {sq.rank+1, sq.file}}) &&
-                try_move({sq, {sq.rank+2, sq.file}}) &&
-                try_move({sq, {sq.rank-1, sq.file}}) &&
-                try_move({sq, {sq.rank-2, sq.file}}) &&
-                try_move({sq, {sq.rank+1, sq.file+1}}) &&
-                try_move({sq, {sq.rank+1, sq.file-1}}) &&
-                try_move({sq, {sq.rank-1, sq.file+1}}) &&
-                try_move({sq, {sq.rank-1, sq.file-1}});
+            for (auto& piece : Vector({Type::None, Type::Knight, Type::Bishop, Type::Rook, Type::Queen})) {
+                keep_going =
+                    try_move({sq, {sq.rank+1, sq.file}, piece}) &&
+                    try_move({sq, {sq.rank+2, sq.file}, piece}) &&
+                    try_move({sq, {sq.rank-1, sq.file}, piece}) &&
+                    try_move({sq, {sq.rank-2, sq.file}, piece}) &&
+                    try_move({sq, {sq.rank+1, sq.file+1}}) &&
+                    try_move({sq, {sq.rank+1, sq.file-1}}) &&
+                    try_move({sq, {sq.rank-1, sq.file+1}}) &&
+                    try_move({sq, {sq.rank-1, sq.file-1}});
+            }
         } else if (piece.type == Type::Knight) {
             keep_going =
                 try_move({sq, {sq.rank+2, sq.file+1}}) &&