(C# : Basic) 7. LINQ

public enum ClassType
{
    Knight,
    Archer,
    Mage,
}

public class Player
{
    public ClassType ClassType { get; set; }
    public int Level { get; set; }
    public int Hp { get; set; }
    public int Attack { get; set; }
    public List<int> items { get; set; }
}

class Program
{
    static List<Player> _players = new List<Player>();

    static void Main(string[] args)
    {
        Random rand = new Random();

        for(int i = 0; i < 100; i++)
        {
            ClassType type = ClassType.Knight;

            switch(rand.Next(0, 3))
            {
                case 0:
                    type = ClassType.Knight;
                    break;
                case 1:
                    type = ClassType.Archer;
                    break;
                case 2:
                    type = ClassType.Mage;
                    break;
            }

            Player player = new Player()
            {
                ClassType = type,
                Level = rand.Next(1, 100),
                Hp = rand.Next(100, 1000),
                Attack = rand.Next(5, 50)
            };

            _players.Add(player);
        }

        // 레벨이 50이상인 Knight만 뽑아
        // 높은순으로 정렬해보자
        List<Player> players = GetHighLevelKnights();
    }

    // 아래처럼 구현이 가능하지만 LINQ로도 구현이 가능하다
    static public List<Player> GetHighLevelKnights()
    {
        List<Player> players = new List<Player>();

        foreash(Player player in _players)
        {
            if(player.ClassType != ClassType.Knight)
                continue;
            if(player.Level < 50)
                continue;

            players.Add(player);
        }

        players.Sort((lhs, rhs) => { return lhs.Level - rhs.Level; });

        return players;
    }
}

var players =
    from p in _players
    where p.ClassType == ClassType.Knight && p.Level >= 50
    orderby p.Level ascending
    select p;

// 훨씬 직관적이고 간단하게 구현이 가능하다

var playersItem =
    from p in _players
    from i in p.items
    where i < 30
    select new {p ,i};

var li = platerItems.ToList();

// 같은 레벨별로 묶어보자
var playersByLevel = 
    from p in _players
    group p by p.Level into g
    orderby g.Key
    select new { g.Key, Players = g };

List<int> levels = new List<int>() {1, 5, 10};

var playerLevels = 
    from p in _players
    join l in levels
    on p.Level equals l
    select p;

var players =
    from p in _players
    where p.ClassType == ClassType.Knight && p.Level >= 50
    orderby p.Level ascending
    select p;

// 위와 동일한 문법이다
var players2 = _players
    .Where(p => p.ClassType == ClassType.Knight && p.Level >= 50)
    .OrderBy(p => p.Level)
    .Select(p => p);