// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System.Collections.Generic;
 
namespace System.Linq.Expressions.Compiler
{
    /// <summary>
    /// A simple dictionary of stacks, keyed off a particular type
    /// This is useful for storing free lists of variables
    /// </summary>
    internal sealed class KeyedStack<TKey, TValue> where TValue : class where TKey : notnull
    {
        private readonly Dictionary<TKey, Stack<TValue>> _data = new Dictionary<TKey, Stack<TValue>>();
 
        internal void Push(TKey key, TValue value)
        {
            if (!_data.TryGetValue(key, out Stack<TValue>? stack))
            {
                _data.Add(key, stack = new Stack<TValue>());
            }
 
            stack.Push(value);
        }
 
        internal TValue? TryPop(TKey key) =>
            _data.TryGetValue(key, out Stack<TValue>? stack) && stack.TryPop(out TValue? value) ? value : null;
    }
}