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C#で学ぶデザインパターン入門 ⑩Strategy Pattern

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概要

@hyuki 先生著の『Javaで学ぶデザインパターン入門』(2004年、SB Creative)の1章ずつをベースに、サンプルコードをC#で置き換えながら勉強していく記事です。

※著者の @hyuki 先生には適切に書籍への参照を入れれば問題ない旨ご確認いただいています。

本題

Strategyパターン
第9回はStrategy Patternパターンです。Strategy Patternパターンは「アルゴリズムをごっそり切り替え、同じ問題を別の方法で解くのを容易にする」デザインパターンです。

サンプルコード

早速具体的な事例を見てみましょう。『Javaで学ぶデザインパターン入門』(2004年、SB Creative)に掲載されているコードをC#で(大体)書き換えます。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace StrategyPattern
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length != 2)
            {
                Console.WriteLine("Usage: Input  two args as radomeseed1 ane randomeseed2");
                Console.WriteLine("Example: 314 15");
                Environment.Exit(0);
            }
            int seed1 = int.Parse(args[0]);
            int seed2 = int.Parse(args[1]);
            Player player1 = new Player("Taro", new WinningStrategy(seed1));
            Player player2 = new Player("Hanako", new ProbStrategy(seed2));
            for (int i = 0; i < 10000; i++)
            {
                Hand nextHand1 = player1.NextHand();
                Hand nextHand2 = player2.NextHand();
                if (nextHand1.IsStrongerThan(nextHand2))
                {
                    Console.WriteLine($"Winner: {player1}");
                    player1.Win();
                    player2.Lose();
                }
                else if (nextHand2.IsStrongerThan(nextHand1))
                {
                    Console.WriteLine($"Winner: {player2}");
                    player1.Lose();
                    player2.Win();
                }
                else
                {
                    Console.WriteLine("Even...");
                    player1.Even();
                    player2.Even();
                }

                Console.WriteLine("Total result:");
                Console.WriteLine(player1.ToString());
                Console.WriteLine(player2.ToString());
            }
            // 実行が一瞬で終わって確認できないので、キーの入力を待ちます
            Console.ReadLine();
        }
    }

    public class Hand
    {
        public static readonly int HANDVALUE_GUU = 0; // グ ーを表す値
        public static readonly int HANDVALUE_CHO = 1; // チョキを表す値
        public static readonly int HANDVALUE_PAA = 2; // パーを表す値
        public static Hand[] hand = {
            new Hand(HANDVALUE_GUU),
            new Hand(HANDVALUE_CHO),
            new Hand(HANDVALUE_PAA),
        };
        private static readonly string[] name = {
            "グー", "チョキ", "パー"
        };
        private int handvalue;
        public static Hand GetHand(int handvalue)
        {
            return hand[handvalue];
        }
        private Hand(int handvalue)
        {
            this.handvalue = handvalue;
        }

        public bool IsStrongerThan(Hand h)
        {
            return Fight(h) == 1;
        }

        public bool IsWeakerThan(Hand h)
        {
            return Fight(h) == -1;
        }

        private int Fight(Hand h)
        {
            if (this == h)
            {
                return 0;
            }
            else if ((this.handvalue + 1) % 3 == h.handvalue)
            {
                return 1;
            }
            else
            {
                return -1;
            }
        }

        public override string ToString()
        {
            return name[handvalue];
        }
    }

    // Strategy
    // ・戦略を定めるためのインターフェース(API)を定める
    public interface Strategy
    {
        Hand NextHand();
        void Study(bool win); 
    }

    // ConcreteStrategy
    // ・Strategyのインターフェース(API)を実装する
    public class WinningStrategy : Strategy
    {
        private Random random;
        private bool won = false;
        private Hand prevHand;
        public WinningStrategy(int seed)
        {
            random = new Random(seed);
        }

        public Hand NextHand()
        {
            if (!won)
            {
                prevHand = Hand.GetHand(random.Next(3));
            }
            return prevHand;
        }

        public void Study(bool win)
        {
            won = win;
        }
    }

    // ConcreteStrategy
    public class ProbStrategy : Strategy
    {
        private Random random;
        private int prevHandValue = 0;
        private int currentHandValue = 0;
        private int[][] history =
        {
            new int[] { 1, 1, 1, },
            new int[] { 1, 1, 1, },
            new int[] { 1, 1, 1, },
        };
        public ProbStrategy(int seed)
        {
            random = new Random(seed);
        }


        public Hand NextHand()
        {
            int bet = random.Next(GetSum(currentHandValue));
            int handvalue = 0;
            if (bet < history[currentHandValue][0])
            {
                handvalue = 0;
            }
            else if (bet < history[currentHandValue][0] + history[currentHandValue][1])
            {
                handvalue = 1;
            }
            else
            {
                handvalue = 2;
            }
            prevHandValue = currentHandValue;
            currentHandValue = handvalue;
            return Hand.GetHand(handvalue);
        }

        private int GetSum(int hv)
        {
            int sum = 0;
            for (int i = 0; i < 3; i++)
            {
                sum += history[hv][i];
            }
            return sum;
        }

        public void Study(bool win)
        {
            if (win)
            {
                history[prevHandValue][currentHandValue]++;
            }
            else
            {
                history[prevHandValue][(currentHandValue + 1) % 3]++;
                history[prevHandValue][(currentHandValue + 2) % 3]++;
            }
        }
    }

    // Context
    // ・Strategyを利用する
    // ・ConcreteStrategyのインスタンスを持っていて、必要に応じて利用する
    // ・呼び出すのはStrategyのインターフェース
    public class Player
    {
        private string name;
        private Strategy strategy;
        private int wincount;
        private int losecount;
        private int gamecount;
        public Player(string name, Strategy strategy)
        {
            this.name = name;
            this.strategy = strategy;
        }

        public Hand NextHand()
        {
            return strategy.NextHand();
        }

        public void Win()
        {
            strategy.Study(true);
            wincount++;
            gamecount++;
        }

        public void Lose()
        {
            strategy.Study(false);
            losecount++;
            gamecount++;
        }

        public void Even()
        {
            gamecount++;
        }

        public override string ToString()
        {
            return $"[{name}:{gamecount} games, {wincount} win, {losecount} lose]";
        }
    }
}

効能

  • Strategy(アルゴリズム)の切り替えが容易なため、アルゴリズムの性能比較がしやすい
  • アルゴリズムをそれ以外の部分から分離し、委譲によりゆるやかに結びつけることができる
  • 同一の結果を得るために別々の方法を試すことができる(計算の検算をしやすい)
  • プログラムの実行環境の状況に依ってもStrategyを切り替えることができる
    • メモリが少ない環境で省メモリ(だが時間はかかる)なアルゴリズムを使用する

関連しているパターン

感想、疑問、メモ

C#で学ぶデザインパターン入門

①Iterator
②Adapter
③Template Method
④Factory Method
⑤Singleton
⑥Prototype
⑦Builder
⑧AbstractFactory
⑨Bridge
⑩Strategy Pattern
⑪Composite Pattern
⑫Decorator Pattern

toshi0607
最近はGoとServerlessが好きです!
https://toshi0607.com/
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