Design Connect Four

Game design scenarios in OOAD interviews provide a well-rounded assessment of a candidate's object-oriented programming skills, problem-solving abilities, and design thinking. They allow companies to identify well-rounded software engineers who can effectively translate conceptual ideas into functioning systems.**

Assessment of OO Principles:

• Classes and Objects: Games naturally lend themselves to object-oriented representation. Candidates need to identify classes like Player, Card, GameBoard, etc., demonstrating their understanding of object creation and interaction.
• Inheritance: Games often have objects with similar attributes and behaviors. Inheritance allows for code reuse by creating base classes like GamePiece and subclasses like ChessPiece or PuzzlePiece. The interviewer can assess your ability to identify appropriate inheritance hierarchies.
• Polymorphism: Games might have objects that respond differently to the same action. For example, different chess pieces move differently. The interview can gauge your understanding of polymorphism through methods like move() that behave differently based on the subclass.
• Encapsulation: Games should encapsulate data and logic within objects. The interviewer can see if you design classes that hide internal implementation details and provide appropriate access methods.

Problem-Solving and Design Skills:

• Breaking Down Complexity: Games have rules and mechanics. The interview assesses your ability to analyze the problem, identify core components, and design a system that fulfills the game's requirements.
• Algorithmic Thinking: Games often involve calculations, decision-making, and managing game state. The interview can see if you can implement core game logic using appropriate algorithms and data structures.

Background
Connect Four is a popular game played on a 7x6 grid. Two players take turns dropping colored discs into the grid. The first player to get four discs in a row (vertically, horizontally or diagonally) wins.

Requirements
Some possible questions to ask:

What are the rules of the game?
What size is the grid?
How many players are there? Player vs Computer? Player vs Player?
Are we keeping track of the score?
Basics
The game will be played by only two players, player vs player
The game board should be of variable dimensions
The target is to connect N discs in a row (vertically, horizontally or diagonally)
N is a variable (e.g. connect 4, 5, 6, etc)
There should be a score tracking system
After a player reaches the target score, they are the winner

Design
High-level
We will need a Grid class to maintain the state of the 2-D board
The board cell can be empty, yellow (occupied by Player 1) or red (occupied by Player 2)
The grid will also be responsible for checking for a win condition
We can have a Player class to represent the player's piece color
This isn't super important, but encapsulating information is generally a good practice
The Game class will be composed of the Grid and Players
The Game class will be responsible for the game loop and keeping track of the score
Code

import java.util.*;

enum GridPosition {
    EMPTY, YELLOW, RED
}

class Grid {
    private int rows;
    private int columns;
    private int[][] grid;

    public Grid(int rows, int columns) {
        this.rows = rows;
        this.columns = columns;
        initGrid();
    }

    public void initGrid() {
        this.grid = new int[rows][columns];
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < columns; j++) {
                grid[i][j] = GridPosition.EMPTY.ordinal();
            }
        }
    }

    public int[][] getGrid() {
        return this.grid;
    }

    public int getColumnCount() {
        return this.columns;
    }

    public int placePiece(int column, GridPosition piece) {
        if (column < 0 || column >= this.columns) {
            throw new Error("Invalid column");
        }
        if (piece == GridPosition.EMPTY) {
            throw new Error("Invalid piece");
        }
        // Place piece in the lowest empty row
        for (int row = this.rows - 1; row >= 0; row--) {
            if (this.grid[row][column] == GridPosition.EMPTY.ordinal()) {
                this.grid[row][column] = piece.ordinal();
                return row;
            }
        }
        return -1;
    }

    public boolean checkWin(int connectN, int row, int col, GridPosition piece) {
        // Check horizontal
        int count = 0;
        for (int c = 0; c < this.columns; c++) {
            if (this.grid[row][c] == piece.ordinal()) {
                count++;
            } else {
                count = 0;
            }
            if (count == connectN) {
                return true;
            }
        }

        // Check vertical
        count = 0;
        for (int r = 0; r < this.rows; r++) {
            if (this.grid[r][col] == piece.ordinal()) {
                count++;
            } else {
                count = 0;
            }
            if (count == connectN) {
                return true;
            }
        }

        // Check diagonal
        count = 0;
        for (int r = 0; r < this.rows; r++) {
            int c = row + col - r; // row + col = r + c, for a diagonal
            if (c >= 0 && c < this.columns && this.grid[r][c] == piece.ordinal()) {
                count++;
            } else {
                count = 0;
            }
            if (count == connectN) {
                return true;
            }
        }

        // Check anti-diagonal
        count = 0;
        for (int r = 0; r < this.rows; r++) {
            int c = col - row + r; // row - col = r - c, for an anti-diagonal
            if (c >= 0 && c < this.columns && this.grid[r][c] == piece.ordinal()) {
                count++;
            } else {
                count = 0;
            }
            if (count == connectN) {
                return true;
            }
        }
        return false;
    }
}

class Player {
    private String name;
    private GridPosition piece;

    public Player(String name, GridPosition piece) {
        this.name = name;
        this.piece = piece;
    }

    public String getName() {
        return this.name;
    }

    public GridPosition getPieceColor() {
        return this.piece;
    }
}

class Game {
    static Scanner input = new Scanner(System.in);
    private Grid grid;
    private int connectN;
    private Player[] players;
    private Map<String, Integer> score;
    private int targetScore;

    public Game(Grid grid, int connectN, int targetScore) {
        this.grid = grid;
        this.connectN = connectN;
        this.targetScore = targetScore;

        this.players = new Player[] {
            new Player("Player 1", GridPosition.YELLOW),
            new Player("Player 2", GridPosition.RED)
        };

        this.score = new HashMap<>();
        for (Player player : this.players) {
            this.score.put(player.getName(), 0);
        }
    }

    private void printBoard() {
        System.out.println("Board:");
        int[][] grid = this.grid.getGrid();
        for (int i = 0; i < grid.length; i++) {
            String row = "";
            for (int piece : grid[i]) {
                if (piece == GridPosition.EMPTY.ordinal()) {
                    row += "0 ";
                } else if (piece == GridPosition.YELLOW.ordinal()) {
                    row += "Y ";
                } else if (piece == GridPosition.RED.ordinal()) {
                    row += "R ";
                }
            }
            System.out.println(row);
        }
        System.out.println();
    }

    private int[] playMove(Player player) {
        printBoard();
        System.out.println(player.getName() + "'s turn");
        int colCnt = this.grid.getColumnCount();

        System.out.print("Enter column between 0 and " + (colCnt - 1) + " to add piece: ");
        int moveColumn = input.nextInt();
        int moveRow = this.grid.placePiece(moveColumn, player.getPieceColor());
        return new int[] { moveRow, moveColumn };
    }

    private Player playRound() {
        while (true) {
            for (Player player : this.players) {
                int[] pos = playMove(player);
                int row = pos[0];
                int col = pos[1];
                GridPosition pieceColor = player.getPieceColor();
                if (this.grid.checkWin(this.connectN, row, col, pieceColor)) {
                    this.score.put(player.getName(), this.score.get(player.getName()) + 1);
                    return player;
                }
            }
        }
    }

    public void play() {
        int maxScore = 0;
        Player winner = null;
        while (maxScore < this.targetScore) {
            winner = playRound();
            System.out.println(winner.getName() + " won the round");
            maxScore = Math.max(this.score.get(winner.getName()), maxScore);

            this.grid.initGrid(); // reset grid
        } 
        System.out.println(winner.getName() + " won the game");
    }
}

class Main {
    public static void main(String[] args) {
        Grid grid = new Grid(6, 7);
        Game game = new Game(grid, 4, 10);
        game.play();
    }
}