Extract logic to check if move to column is legal

gamestate.cpp

@@ -85,29 +85,7 @@ bool GameState::moveCardToColumn(int columnId)
     // We'll be moving the last card in the throwaway pile (maybe)
     PlayingCard *cardToMove = m_throwawayPile.last();
 
-    if (m_columns[columnId].isEmpty()) {
+    if (!isMoveToColumnLegal(cardToMove, columnId))
-        // If the column is empty, we can only place a king
-        if (cardToMove->value() != PlayingCard::Value::King)
-            return false;
-
-        ColumnSlot *col = new ColumnSlot(cardToMove, true);
-        m_columns[columnId].append(col);
-        m_throwawayPile.removeLast();
-
-        emit throwawayPileChanged();
-        emit columnsChanged();
-        return true;
-    }
-
-    // We'll be comparing this card against the last card in the column
-    PlayingCard* columnCard = m_columns[columnId].last()->card();
-
-    // This card's value must be one less than the card in the column
-    if (cardToMove->value() != columnCard->value() - 1)
-        return false;
-
-    // This card must be of opposite color
-    if (!PlayingCard::areOppositeColors(*cardToMove, *columnCard))
         return false;
 
     ColumnSlot *col = new ColumnSlot(cardToMove, true);
@@ -221,6 +199,26 @@ bool GameState::tryMoveCardToFoundation(PlayingCard::Suit foundationId, PlayingC
     return true;
 }
 
+bool GameState::isMoveToColumnLegal(PlayingCard *cardToMove, int columnId)
+{
+    assert(columnId >= 0 && columnId < 7);
+
+    if (m_columns[columnId].isEmpty()) {
+        // Column is empty: only a King can be placed in an empty column
+        return cardToMove->value() == PlayingCard::Value::King;
+    }
+
+    // Compare against the last card in the column
+    PlayingCard* columnCard = m_columns[columnId].last()->card();
+
+    // The card's value must be one less than the card in the column
+    if (cardToMove->value() != columnCard->value() - 1)
+        return false;
+
+    // The card must be of opposite color
+    return PlayingCard::areOppositeColors(*cardToMove, *columnCard);
+}
+
 void GameState::ensureColumnRevealed(int columnId)
 {
     assert(columnId >= 0 && columnId < 7);

gamestate.h

@@ -47,6 +47,7 @@ private:
     bool m_gameWon;
 
     bool tryMoveCardToFoundation(PlayingCard::Suit foundationId, PlayingCard* cardToMove);
+    bool isMoveToColumnLegal(PlayingCard* cardToMove, int columnId);
     void ensureColumnRevealed(int columnId);
 };