From 2dfced2501d5635c3da3904409190e90364982d5 Mon Sep 17 00:00:00 2001
From: Gunnar Beutner <gbeutner@serenityos.org>
Date: Sun, 23 May 2021 14:49:58 +0200
Subject: [PATCH] Hearts: Let the AI prefer lead cards for which other cards
 are in play

When picking a lead card the AI should avoid playing cards where it
knows that no other player has a lower value card of the same type.
---
 Userland/Games/Hearts/Game.cpp   |  5 ++++-
 Userland/Games/Hearts/Player.cpp | 17 ++++++++++++++---
 Userland/Games/Hearts/Player.h   |  2 +-
 3 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/Userland/Games/Hearts/Game.cpp b/Userland/Games/Hearts/Game.cpp
index d17fa9097b..2b661630dd 100644
--- a/Userland/Games/Hearts/Game.cpp
+++ b/Userland/Games/Hearts/Game.cpp
@@ -210,7 +210,10 @@ size_t Game::pick_card(Player& player)
             auto prefer_card = [this, &player](Card& card) {
                 return !other_player_has_lower_value_card(player, card) && other_player_has_higher_value_card(player, card);
             };
-            return player.pick_lead_card(move(valid_card), move(prefer_card));
+            auto lower_value_card_in_play = [this, &player](Card& card) {
+                return other_player_has_lower_value_card(player, card);
+            };
+            return player.pick_lead_card(move(valid_card), move(prefer_card), move(lower_value_card_in_play));
         }
     }
     auto* high_card = &m_trick[0];
diff --git a/Userland/Games/Hearts/Player.cpp b/Userland/Games/Hearts/Player.cpp
index 4e3bd7839f..c36ee8e3cf 100644
--- a/Userland/Games/Hearts/Player.cpp
+++ b/Userland/Games/Hearts/Player.cpp
@@ -11,7 +11,8 @@
 
 namespace Hearts {
 
-size_t Player::pick_lead_card(Function<bool(Card&)> valid_play, Function<bool(Card&)> prefer_card)
+size_t Player::pick_lead_card(Function<bool(Card&)> valid_play, Function<bool(Card&)> prefer_card,
+    Function<bool(Card&)> lower_value_card_in_play)
 {
     struct CardWithIndex {
         RefPtr<Card> card;
@@ -38,17 +39,27 @@ size_t Player::pick_lead_card(Function<bool(Card&)> valid_play, Function<bool(Ca
         dbgln("----");
     }
 
-    size_t last_index = -1;
+    ssize_t fallback_index = -1;
+    ssize_t last_index = -1;
     for (auto& cwi : sorted_hand) {
         if (!valid_play(*cwi.card))
             continue;
+        if (lower_value_card_in_play(*cwi.card)) {
+            // Allow falling back to this card if no other suitable card is in play.
+            fallback_index = cwi.index;
+            continue;
+        }
         if (prefer_card(*cwi.card)) {
             dbgln_if(HEARTS_DEBUG, "Preferring card {}", *cwi.card);
             return cwi.index;
         }
         last_index = cwi.index;
     }
-    return last_index;
+    if (last_index == -1) {
+        dbgln_if(HEARTS_DEBUG, "Falling back to card {}", *hand[fallback_index]);
+        return fallback_index;
+    } else
+        return last_index;
 }
 
 Optional<size_t> Player::pick_low_points_high_value_card(Optional<Card::Type> type)
diff --git a/Userland/Games/Hearts/Player.h b/Userland/Games/Hearts/Player.h
index d7737951fe..6f143f34a9 100644
--- a/Userland/Games/Hearts/Player.h
+++ b/Userland/Games/Hearts/Player.h
@@ -21,7 +21,7 @@ public:
     {
     }
 
-    size_t pick_lead_card(Function<bool(Card&)>, Function<bool(Card&)>);
+    size_t pick_lead_card(Function<bool(Card&)>, Function<bool(Card&)>, Function<bool(Card&)>);
     Optional<size_t> pick_low_points_high_value_card(Optional<Card::Type> type = {});
     Optional<size_t> pick_lower_value_card(Card& other_card);
     Optional<size_t> pick_slightly_higher_value_card(Card& other_card);