// Exercise 1: a ValueTask cache that deduplicates concurrent loads (medium)
//
// UserCache.GetAsync(id) returns a ValueTask<string>:
//   - On a cache hit it must complete SYNCHRONOUSLY (vt.IsCompletedSuccessfully == true) without allocating a Task.
//   - On a miss it calls the loader. If several callers ask for the same missing id concurrently, the loader
//     must run only ONCE: keep in-flight loads in a ConcurrentDictionary<int, Task<string>> and share them.
//   - When the load completes, store the value in _values so later calls are synchronous hits, and remove
//     the in-flight entry.
//
// Run with: dotnet run (inside a console project).

using System;
using System.Collections.Concurrent;
using System.Threading.Tasks;

class UserCache
{
    readonly Func<int, Task<string>> _loader;
    readonly ConcurrentDictionary<int, string> _values = new();
    readonly ConcurrentDictionary<int, Task<string>> _inflight = new();
    public UserCache(Func<int, Task<string>> loader) { _loader = loader; }

    public ValueTask<string> GetAsync(int id)
    {
        // TODO
        return new ValueTask<string>(_loader(id));
    }
}

class Program
{

    static async Task Main()
    {
        int pass = 0, total = 0;
        void Check(string name, bool ok) { total++; if (ok) pass++; Console.WriteLine($"{(ok ? "PASS" : "FAIL")}: {name}"); }

        int calls = 0;
        var cache = new UserCache(async id => { System.Threading.Interlocked.Increment(ref calls); await Task.Delay(100); return "user-" + id; });

        var t1 = cache.GetAsync(7).AsTask();
        var t2 = cache.GetAsync(7).AsTask();
        var t3 = cache.GetAsync(7).AsTask();
        var results = await Task.WhenAll(t1, t2, t3);
        Check("all callers get the value", results[0] == "user-7" && results[1] == "user-7" && results[2] == "user-7");
        Check($"loader ran once for concurrent misses (ran {calls})", calls == 1);

        var hit = cache.GetAsync(7);
        Check("cache hit completes synchronously", hit.IsCompletedSuccessfully && hit.Result == "user-7");
        Check("no extra load on hit", calls == 1);

        await cache.GetAsync(8);
        Check("different key loads separately", calls == 2);

        Console.WriteLine($"--- {pass}/{total} checks passed ---");
    }
}