Nice implementation. I am using your AI opponent to learn how to play 66. Thanks for sharing it!
I have encountered a behavior which I think is a bug. My mouse button has an issue where a single click sometimes registers as two quick clicks in succession.
When this happens as I choose a card to play and there is another card near enough below, then both cards are played. After that even though the stock is still open, only one card is replenished in each of our hands.
Here is a snip from the game log:
16:57:06 AI Opponent (Medium) plays Q♥
16:57:07 AI Opponent (Medium) wins the trick for 2 points.
16:57:09 AI Opponent (Medium) plays Q♠
16:57:13 Human plays K♠
16:57:13 Human plays K♣
16:57:15 AI Opponent (Medium) plays Q♣
16:57:16 Human wins the trick for 7 points.
I'm also able to duplicate the issue by purposefully clicking the mouse butten quickly, twice.