2048: Separate game logic from the view :^)

Look Ali, it's simple:

* The *model* (in many cases, an instance of GUI::Model, but it doesn't have to
  be) should implement the "business logic" (in this case, game logic) and
  should not concern itself with how the data/state is displayed to the user.

* The *view*, conversely, should interact with the user (display data/state,
  accept input) and should not concern itself with the logic. As an example, a
  GUI::Button can display some text and accept clicks -- it doesn't know or care
  what that text *means*, or how that click affects the app state. All it does
  is it gets its text from *somebody* and notifies *somebody* of clicks.

* The *controller* connects the model to the view, and acts as "glue" between
  them.

You could connect *several different* views to one model (see FileManager), or
use identical views with different models (e.g. a table view can display pretty
much anything, depending on what model you connect to it).

In this case, the model is the Game class, which maintains a board and
implements the rules of 2048, including tracking the score. It does not display
anything, and it does not concern itself with undo management. The view is the
BoardView class, which displays a board and accepts keyboard input, but doesn't
know how exactly the tiles move or merge -- all it gets is a board state, ready
to be displayed. The controller is our main(), which connects the two classes
and bridges between their APIs. It also implements undo management, by basically
making straight-up copies of the game.

Isn't this lovely?

Games/2048/Game.cpp

@@ -0,0 +1,212 @@
+/*
+ * Copyright (c) 2020, the SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "Game.h"
+
+Game::Game(size_t rows, size_t columns)
+    : m_rows(rows)
+    , m_columns(columns)
+{
+    m_board.resize(rows);
+    for (auto& row : m_board) {
+        row.ensure_capacity(columns);
+        for (size_t i = 0; i < columns; i++)
+            row.append(0);
+    }
+
+    add_tile(2);
+    add_tile(2);
+}
+
+void Game::add_tile(u32 max_tile_value)
+{
+    int row;
+    int column;
+    do {
+        row = rand() % m_rows;
+        column = rand() % m_columns;
+    } while (m_board[row][column] != 0);
+
+    int value = rand() % max_tile_value;
+    value = round_up_to_power_of_two(value, max_tile_value);
+    m_board[row][column] = max(2, value);
+}
+
+static Game::Board transpose(const Game::Board& board)
+{
+    Vector<Vector<u32>> new_board;
+    auto result_row_count = board[0].size();
+    auto result_column_count = board.size();
+
+    new_board.resize(result_row_count);
+
+    for (size_t i = 0; i < board.size(); ++i) {
+        auto& row = new_board[i];
+        row.clear_with_capacity();
+        row.ensure_capacity(result_column_count);
+        for (auto& entry : board) {
+            row.append(entry[i]);
+        }
+    }
+
+    return new_board;
+}
+
+static Game::Board reverse(const Game::Board& board)
+{
+    auto new_board = board;
+    for (auto& row : new_board) {
+        for (size_t i = 0; i < row.size() / 2; ++i)
+            swap(row[i], row[row.size() - i - 1]);
+    }
+
+    return new_board;
+}
+
+static Vector<u32> slide_row(const Vector<u32>& row, size_t& successful_merge_score)
+{
+    if (row.size() < 2)
+        return row;
+
+    auto x = row[0];
+    auto y = row[1];
+
+    auto result = row;
+    result.take_first();
+
+    if (x == 0) {
+        result = slide_row(result, successful_merge_score);
+        result.append(0);
+        return result;
+    }
+
+    if (y == 0) {
+        result[0] = x;
+        result = slide_row(result, successful_merge_score);
+        result.append(0);
+        return result;
+    }
+
+    if (x == y) {
+        result.take_first();
+        result = slide_row(result, successful_merge_score);
+        result.append(0);
+        result.prepend(x + x);
+        successful_merge_score += x * 2;
+        return result;
+    }
+
+    result = slide_row(result, successful_merge_score);
+    result.prepend(x);
+    return result;
+}
+
+static Game::Board slide_left(const Game::Board& board, size_t& successful_merge_score)
+{
+    Vector<Vector<u32>> new_board;
+    for (auto& row : board)
+        new_board.append(slide_row(row, successful_merge_score));
+
+    return new_board;
+}
+
+static bool is_complete(const Game::Board& board)
+{
+    for (auto& row : board) {
+        if (row.contains_slow(2048))
+            return true;
+    }
+
+    return false;
+}
+
+static bool has_no_neighbors(const Span<const u32>& row)
+{
+    if (row.size() < 2)
+        return true;
+
+    auto x = row[0];
+    auto y = row[1];
+
+    if (x == y)
+        return false;
+
+    return has_no_neighbors(row.slice(1, row.size() - 1));
+};
+
+static bool is_stalled(const Game::Board& board)
+{
+    static auto stalled = [](auto& row) {
+        return !row.contains_slow(0) && has_no_neighbors(row.span());
+    };
+
+    for (auto& row : board)
+        if (!stalled(row))
+            return false;
+
+    for (auto& row : transpose(board))
+        if (!stalled(row))
+            return false;
+
+    return true;
+}
+
+Game::MoveOutcome Game::attempt_move(Direction direction)
+{
+    size_t successful_merge_score = 0;
+    Board new_board;
+
+    switch (direction) {
+    case Direction::Left:
+        new_board = slide_left(m_board, successful_merge_score);
+        break;
+    case Direction::Right:
+        new_board = reverse(slide_left(reverse(m_board), successful_merge_score));
+        break;
+    case Direction::Up:
+        new_board = transpose(slide_left(transpose(m_board), successful_merge_score));
+        break;
+    case Direction::Down:
+        new_board = transpose(reverse(slide_left(reverse(transpose(m_board)), successful_merge_score)));
+        break;
+    }
+
+    bool moved = new_board != m_board;
+    if (moved) {
+        m_board = new_board;
+        m_turns++;
+        add_tile(4);
+        m_score += successful_merge_score;
+    }
+
+    if (is_complete(m_board))
+        return MoveOutcome::Won;
+    if (is_stalled(m_board))
+        return MoveOutcome::GameOver;
+    if (moved)
+        return MoveOutcome::OK;
+    return MoveOutcome::InvalidMove;
+}