using System.Text;

namespace NpgsqlRest;

/// <summary>

/// A simple thread-safe StringBuilder pool to reduce allocations in high-throughput scenarios.

/// Uses a lock-free stack-based approach for fast rent/return operations.

/// </summary>

internal static class StringBuilderPool

{

private const int MaxPoolSize = 64;

private const int DefaultCapacity = 256;

private const int MaxCapacity = 8192;

private static readonly StringBuilder?[] _pool = new StringBuilder?[MaxPoolSize];

private static int _index = -1;

/// <summary>

/// Rents a StringBuilder from the pool. Returns a new instance if the pool is empty.

/// </summary>

public static StringBuilder Rent()

{

StringBuilder? sb = null;

int currentIndex = Interlocked.Decrement(ref _index);

if (currentIndex >= 0 && currentIndex < MaxPoolSize)

{

sb = Interlocked.Exchange(ref _pool[currentIndex], null);

}

else

{

// Pool was empty or index went negative, restore it

Interlocked.Increment(ref _index);

}

return sb ?? new StringBuilder(DefaultCapacity);

}

/// <summary>

/// Rents a StringBuilder from the pool with a minimum capacity.

/// </summary>

public static StringBuilder Rent(int minimumCapacity)

{

var sb = Rent();

if (sb.Capacity < minimumCapacity)

{

sb.EnsureCapacity(minimumCapacity);

}

return sb;

}

/// <summary>

/// Returns a StringBuilder to the pool. The StringBuilder is cleared before being pooled.

/// Large StringBuilders (capacity > MaxCapacity) are not pooled to avoid memory bloat.

/// </summary>

public static void Return(StringBuilder sb)

{

if (sb.Capacity > MaxCapacity)

{

// Don't pool oversized builders - let GC collect them

return;

}

sb.Clear();

int currentIndex = Interlocked.Increment(ref _index);

if (currentIndex >= 0 && currentIndex < MaxPoolSize)

{

_pool[currentIndex] = sb;

}

else

{

// Pool is full, decrement index back and let the StringBuilder be collected

Interlocked.Decrement(ref _index);

}

}

/// <summary>

/// Gets the string value and returns the StringBuilder to the pool.

/// </summary>

public static string ToStringAndReturn(StringBuilder sb)

{

var result = sb.ToString();

Return(sb);

return result;

}

}