ImageSharp/SixLabors.Fonts/Tables/Cff/RefStack{T}.cs

// Copyright (c) Six Labors.
// Licensed under the Six Labors Split License.

using System;
using System.Runtime.CompilerServices;

namespace SixLabors.Fonts.Tables.Cff {
  /// <summary>
  /// A ref struct stack implementation that uses a pooled span to store the data.
  /// </summary>
  /// <typeparam name="T">The type of elements in the stack.</typeparam>
  internal ref struct RefStack<T>
      where T : unmanaged
  {
      private const int MaxLength = 0X7FFFFFC7;
      private Buffer<T> buffer;
      private Span<T> stack;
      private bool isDisposed;

      /// <summary>
      /// Initializes a new instance of the <see cref="RefStack{T}"/> struct with the specified initial capacity.
      /// </summary>
      /// <param name="capacity">The initial capacity of the stack. Values less than 1 default to 4.</param>
      public RefStack(int capacity)
      {
          if (capacity < 1)
          {
              capacity = 4;
          }

          this.buffer = new Buffer<T>(capacity);
          this.stack = this.buffer.GetSpan();
          this.isDisposed = false;
          this.Length = 0;
      }

      /// <summary>
      /// Gets the number of elements currently in the stack.
      /// </summary>
      public int Length { get; private set; }

      /// <summary>
      /// Gets or sets the element at the specified index in the stack.
      /// </summary>
      /// <param name="index">The zero-based index of the element.</param>
      /// <returns>The element at the specified index.</returns>
      public T this[int index]
      {
          readonly get
          {
              if ((uint)index >= (uint)this.Length)
              {
                  ThrowForOutOfRange();
              }

              return this.stack[index];
          }

          set
          {
              if ((uint)index >= (uint)this.Length)
              {
                  this.Push(value);
                  return;
              }

              this.stack[index] = value;
          }
      }

      /// <summary>
      /// Adds an item to the stack.
      /// </summary>
      /// <param name="value">The item to add.</param>
      public void Push(T value)
      {
          if ((uint)this.Length < (uint)this.stack.Length)
          {
              this.stack[this.Length++] = value;
          }
          else
          {
              int capacity = this.stack.Length * 2;
              if ((uint)capacity > MaxLength)
              {
                  capacity = MaxLength;
              }

              var newBuffer = new Buffer<T>(capacity);
              Span<T> newStack = newBuffer.GetSpan();

              this.stack.CopyTo(newStack);
              this.buffer.Dispose();

              this.buffer = newBuffer;
              this.stack = newStack;

              this.stack[this.Length++] = value;
          }
      }

      /// <summary>
      /// Removes the first element of the stack.
      /// </summary>
      /// <returns>The <typeparamref name="T"/> element.</returns>
      public T Shift()
      {
          int newSize = this.Length - 1;
          if (newSize < 0)
          {
              ThrowForEmptyStack();
          }

          T item = this.stack[0];
          this.stack = this.stack.Slice(1);
          this.Length = newSize;
          return item;
      }

      /// <summary>
      /// Removes the last element of the stack.
      /// </summary>
      /// <returns>The <typeparamref name="T"/> element.</returns>
      public T Pop()
      {
          int newSize = this.Length - 1;
          if (newSize < 0)
          {
              ThrowForEmptyStack();
          }

          this.Length = newSize;
          return this.stack[newSize];
      }

      /// <summary>
      /// Clears the current stack.
      /// </summary>
      public void Clear()
      {
          this.Length = 0;
          this.stack = this.buffer.GetSpan();
      }

      /// <summary>
      /// Releases the pooled buffer used by this stack.
      /// </summary>
      public void Dispose()
      {
          if (this.isDisposed)
          {
              return;
          }

          this.buffer.Dispose();
          this.isDisposed = true;
      }

      /// <summary>
      /// Throws an <see cref="InvalidOperationException"/> for an out-of-range index access.
      /// </summary>
      [MethodImpl(MethodImplOptions.NoInlining)]
      private static void ThrowForOutOfRange()
          => throw new InvalidOperationException("Index must be greater or equal to zero or less than the stack length.");

      /// <summary>
      /// Throws an <see cref="InvalidOperationException"/> when attempting to pop or shift from an empty stack.
      /// </summary>
      [MethodImpl(MethodImplOptions.NoInlining)]
      private static void ThrowForEmptyStack() => throw new InvalidOperationException("Empty stack!");
  }
}