// Exercise 2: an object pool for expensive buffers (medium)
//
// Implement BufferPool (a simplified Microsoft.Extensions.ObjectPool):
//   - Rent(): returns a pooled StringBuilder if one is available (LIFO), otherwise creates a new one
//     (count creations in Created).
//   - Return(sb): clears it (sb.Clear()) and keeps it for reuse, but only if the pool holds fewer than
//     MaxRetained items and sb.Capacity <= 4096 (oversized builders are dropped to avoid hoarding memory).
//   - Thread safety: use a ConcurrentBag or a lock.
//
// Run with: dotnet run (inside a console project).

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

class BufferPool
{
    public const int MaxRetained = 8;
    public int Created;
    public StringBuilder Rent() { Created++; return new StringBuilder(); } // TODO: reuse
    public void Return(StringBuilder sb) { } // TODO
    public int Retained => 0; // TODO
}

class Program
{

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

        var pool = new BufferPool();
        var a = pool.Rent();
        a.Append("hello");
        pool.Return(a);
        var b = pool.Rent();
        Check("reuses returned builder", ReferenceEquals(a, b) && pool.Created == 1);
        Check("returned builder is cleared", b.Length == 0);

        var big = pool.Rent();
        big.Append(new string('x', 10_000));
        pool.Return(big);
        Check("oversized builders are dropped", pool.Retained == 0);

        for (int i = 0; i < 20; i++) pool.Return(new StringBuilder());
        Check("retains at most MaxRetained", pool.Retained == BufferPool.MaxRetained);

        var p2 = new BufferPool();
        Parallel.For(0, 10_000, i => { var sb = p2.Rent(); sb.Append(i); p2.Return(sb); });
        Check($"thread-safe reuse (created {p2.Created})", p2.Created <= 64 && p2.Retained <= BufferPool.MaxRetained);

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