Project: A Task Tracker

using System.Text.Json;
using System.Text.Json.Serialization;

var today = new DateOnly(2026, 10, 8);   // DateOnly.FromDateTime(DateTime.Today) in real use
var store = TaskStore.Load("tasks.json");
Console.WriteLine($"Task tracker: {store.Count} tasks loaded. Commands: add, list, done, remove, stats, quit");

while (true)
{
    Console.Write("> ");
    string? line = Console.ReadLine();
    if (line is null) break;                 // end of input

    string[] words = line.Split(' ', StringSplitOptions.RemoveEmptyEntries);
    try
    {
        switch (words)
        {
            case []:
                Console.WriteLine();
                break;
            case ["quit" or "exit"]:
                store.Save();
                Console.WriteLine($"Saved {store.Count} tasks. Bye!");
                return;
            case ["add", .. var rest]:
                TaskItem added = store.Add(ParseNewTask(rest));
                Console.WriteLine($"Added #{added.Id}: {added.Title}");
                break;
            case ["list"]:
                Print(store.All.Where(t => !t.Done));
                break;
            case ["list", "all"]:
                Print(store.All);
                break;
            case ["list", "due"]:
                Print(store.All.Where(t => !t.Done && t.Due <= today.AddDays(7)));
                break;
            case ["done", var id]:
                Console.WriteLine($"Completed: {store.Complete(ParseId(id)).Title}");
                break;
            case ["remove", var id]:
                Console.WriteLine($"Removed: {store.Remove(ParseId(id)).Title}");
                break;
            case ["stats"]:
                PrintStats();
                break;
            default:
                Console.WriteLine($"Unknown command '{line.Trim()}'");
                break;
        }
    }
    catch (Exception ex) when (ex is ArgumentException or FormatException or KeyNotFoundException)
    {
        Console.WriteLine($"Error: {ex.Message}");
    }
}
store.Save();
Console.WriteLine($"Saved {store.Count} tasks.");

NewTask ParseNewTask(string[] words)
{
    var priority = Priority.Normal;
    DateOnly? due = null;
    var title = new List<string>();

    foreach (string w in words)
    {
        switch (w)
        {
            case ['!', .. var p]:
                priority = Enum.TryParse(p, ignoreCase: true, out Priority parsed)
                    ? parsed
                    : throw new FormatException($"Unknown priority '{p}'. Use low, normal or high.");
                break;
            case ['@', .. var d]:
                due = DateOnly.TryParseExact(d, "yyyy-MM-dd", out DateOnly date)
                    ? date
                    : throw new FormatException($"Bad date '{d}'. Use yyyy-MM-dd.");
                break;
            default:
                title.Add(w);
                break;
        }
    }
    if (title.Count == 0) throw new ArgumentException("A task needs a title.");
    return new NewTask(string.Join(' ', title), priority, due);
}

static int ParseId(string text) =>
    int.TryParse(text.TrimStart('#'), out int id) ? id : throw new FormatException($"'{text}' is not a task number.");

void Print(IEnumerable<TaskItem> tasks)
{
    var sorted = tasks
        .OrderBy(t => t.Done)
        .ThenByDescending(t => t.Priority)
        .ThenBy(t => t.Due ?? DateOnly.MaxValue)
        .ToList();

    if (sorted.Count == 0)
    {
        Console.WriteLine("(no tasks)");
        return;
    }
    foreach (TaskItem t in sorted)
    {
        string box = t.Done ? "[x]" : "[ ]";
        string due = t.Due switch
        {
            null => "",
            var d when !t.Done && d < today => $"OVERDUE {d:MMM d}",
            var d when d == today => "due today",
            var d => $"due {d:MMM d}",
        };
        Console.WriteLine($"{box} #{t.Id,-3}{t.Title,-24}{t.Priority,-7}{due}");
    }
}

void PrintStats()
{
    var open = store.All.Where(t => !t.Done).ToList();
    Console.WriteLine($"{store.Count} tasks: {open.Count} open, {store.Count - open.Count} done");
    foreach (var (priority, count) in open.CountBy(t => t.Priority).OrderByDescending(p => p.Key))
        Console.WriteLine($"  {priority,-7}{new string('■', count)} {count}");
    int overdue = open.Count(t => t.Due < today);
    if (overdue > 0) Console.WriteLine($"  {overdue} overdue!");
}

enum Priority { Low, Normal, High }

record NewTask(string Title, Priority Priority, DateOnly? Due);

record TaskItem(int Id, string Title, Priority Priority, DateOnly? Due, bool Done = false);

class TaskStore
{
    private readonly List<TaskItem> _tasks;
    private readonly string _path;

    private TaskStore(string path, List<TaskItem> tasks)
    {
        _path = path;
        _tasks = tasks;
    }

    public int Count => _tasks.Count;
    public IReadOnlyList<TaskItem> All => _tasks;

    public static TaskStore Load(string path)
    {
        if (!File.Exists(path)) return new TaskStore(path, []);
        string json = File.ReadAllText(path);
        return new TaskStore(path, JsonSerializer.Deserialize(json, TaskJson.Default.ListTaskItem) ?? []);
    }

    public void Save() => File.WriteAllText(_path, JsonSerializer.Serialize(_tasks, TaskJson.Default.ListTaskItem));

    public TaskItem Add(NewTask t)
    {
        int id = _tasks.Count == 0 ? 1 : _tasks.Max(x => x.Id) + 1;
        var task = new TaskItem(id, t.Title, t.Priority, t.Due);
        _tasks.Add(task);
        return task;
    }

    public TaskItem Complete(int id)
    {
        int i = IndexOf(id);
        _tasks[i] = _tasks[i] with { Done = true };
        return _tasks[i];
    }

    public TaskItem Remove(int id)
    {
        int i = IndexOf(id);
        TaskItem removed = _tasks[i];
        _tasks.RemoveAt(i);
        return removed;
    }

    private int IndexOf(int id)
    {
        int i = _tasks.FindIndex(t => t.Id == id);
        return i >= 0 ? i : throw new KeyNotFoundException($"There is no task #{id}.");
    }
}

[JsonSourceGenerationOptions(JsonSerializerDefaults.Web, WriteIndented = true, UseStringEnumConverter = true)]
[JsonSerializable(typeof(List<TaskItem>))]
partial class TaskJson : JsonSerializerContext;

[
  {
    "id": 1,
    "title": "Renew passport",
    "priority": "High",
    "due": "2026-10-06",
    "done": false
  },
  ...
]