From a93d0fe8c27f0b03f9bd3243974edd7f248c3f27 Mon Sep 17 00:00:00 2001
From: Dmitrii Ubskii <ubskydm@gmail.com>
Date: Sat, 15 May 2021 17:37:29 +0300
Subject: [PATCH] 2048: Evil AI logic: select new tile in the worst position

Not very smart: the worst position is defined such that after the
player's next move the fewest tiles remain empty.
---
 Userland/Games/2048/Game.cpp | 72 ++++++++++++++++++++++++++++++++++--
 Userland/Games/2048/Game.h   | 14 ++++++-
 Userland/Games/2048/main.cpp |  4 +-
 3 files changed, 83 insertions(+), 7 deletions(-)

diff --git a/Userland/Games/2048/Game.cpp b/Userland/Games/2048/Game.cpp
index c196dc9d8c..ac68441930 100644
--- a/Userland/Games/2048/Game.cpp
+++ b/Userland/Games/2048/Game.cpp
@@ -5,11 +5,14 @@
  */
 
 #include "Game.h"
+#include <AK/Array.h>
+#include <AK/NumericLimits.h>
 #include <AK/String.h>
 #include <stdlib.h>
 
-Game::Game(size_t grid_size, size_t target_tile)
+Game::Game(size_t grid_size, size_t target_tile, bool evil_ai)
     : m_grid_size(grid_size)
+    , m_evil_ai(evil_ai)
 {
     if (target_tile == 0)
         m_target_tile = 2048;
@@ -25,8 +28,8 @@ Game::Game(size_t grid_size, size_t target_tile)
             row.append(0);
     }
 
-    add_random_tile();
-    add_random_tile();
+    add_tile();
+    add_tile();
 }
 
 void Game::add_random_tile()
@@ -161,6 +164,16 @@ static bool is_stalled(const Game::Board& board)
     return true;
 }
 
+static size_t get_number_of_free_cells(const Game::Board& board)
+{
+    size_t accumulator = 0;
+    for (auto& row : board) {
+        for (auto& cell : row)
+            accumulator += cell == 0;
+    }
+    return accumulator;
+}
+
 bool Game::slide_tiles(Direction direction)
 {
     size_t successful_merge_score = 0;
@@ -195,7 +208,7 @@ Game::MoveOutcome Game::attempt_move(Direction direction)
     bool moved = slide_tiles(direction);
     if (moved) {
         m_turns++;
-        add_random_tile();
+        add_tile();
     }
 
     if (is_complete(m_board, m_target_tile))
@@ -207,6 +220,57 @@ Game::MoveOutcome Game::attempt_move(Direction direction)
     return MoveOutcome::InvalidMove;
 }
 
+void Game::add_evil_tile()
+{
+    size_t worst_row = 0;
+    size_t worst_column = 0;
+    u32 worst_value = 2;
+
+    size_t most_free_cells = NumericLimits<size_t>::max();
+
+    for (size_t row = 0; row < m_grid_size; row++) {
+        for (size_t column = 0; column < m_grid_size; column++) {
+            if (m_board[row][column] != 0)
+                continue;
+
+            for (u32 value : Array { 2, 4 }) {
+                Game saved_state = *this;
+                saved_state.m_board[row][column] = value;
+
+                if (is_stalled(saved_state.m_board)) {
+                    // We can stall the board now, instant game over.
+                    worst_row = row;
+                    worst_column = column;
+                    worst_value = value;
+
+                    goto found_worst_tile;
+                }
+
+                // This is the best outcome the player can achieve in one move.
+                // We want this to be as low as possible.
+                size_t best_outcome = 0;
+                for (auto direction : Array { Direction::Down, Direction::Left, Direction::Right, Direction::Up }) {
+                    Game moved_state = saved_state;
+                    bool moved = moved_state.slide_tiles(direction);
+                    if (!moved) // invalid move
+                        continue;
+                    best_outcome = max(best_outcome, get_number_of_free_cells(moved_state.board()));
+                }
+
+                if (best_outcome < most_free_cells) {
+                    worst_row = row;
+                    worst_column = column;
+                    worst_value = value;
+
+                    most_free_cells = best_outcome;
+                }
+            }
+        }
+    }
+found_worst_tile:
+    m_board[worst_row][worst_column] = worst_value;
+}
+
 u32 Game::largest_tile() const
 {
     u32 tile = 0;
diff --git a/Userland/Games/2048/Game.h b/Userland/Games/2048/Game.h
index 1e36afbccb..0c5ee5f039 100644
--- a/Userland/Games/2048/Game.h
+++ b/Userland/Games/2048/Game.h
@@ -10,8 +10,9 @@
 
 class Game final {
 public:
-    Game(size_t board_size, size_t target_tile = 0);
+    Game(size_t grid_size, size_t target_tile, bool evil_ai);
     Game(const Game&) = default;
+    Game& operator=(const Game&) = default;
 
     enum class MoveOutcome {
         OK,
@@ -49,11 +50,22 @@ public:
 private:
     bool slide_tiles(Direction);
 
+    void add_tile()
+    {
+        if (m_evil_ai)
+            add_evil_tile();
+        else
+            add_random_tile();
+    }
+
     void add_random_tile();
+    void add_evil_tile();
 
     size_t m_grid_size { 0 };
     u32 m_target_tile { 0 };
 
+    bool m_evil_ai { false };
+
     Board m_board;
     size_t m_score { 0 };
     size_t m_turns { 0 };
diff --git a/Userland/Games/2048/main.cpp b/Userland/Games/2048/main.cpp
index 97a3440a0e..db75e0aee5 100644
--- a/Userland/Games/2048/main.cpp
+++ b/Userland/Games/2048/main.cpp
@@ -76,7 +76,7 @@ int main(int argc, char** argv)
     main_widget.set_layout<GUI::VerticalBoxLayout>();
     main_widget.set_fill_with_background_color(true);
 
-    Game game { board_size, target_tile };
+    Game game { board_size, target_tile, evil_ai };
 
     auto& board_view = main_widget.add<BoardView>(&game.board());
     board_view.set_focus(true);
@@ -123,7 +123,7 @@ int main(int argc, char** argv)
         undo_stack.clear();
         redo_stack.clear();
 
-        game = Game(board_size, target_tile);
+        game = Game(board_size, target_tile, evil_ai);
 
         // This ensures that the sizes are correct.
         board_view.set_board(nullptr);