70 lines
4.0 KiB
C#
70 lines
4.0 KiB
C#
// Licensed to the .NET Foundation under one or more agreements.
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// The .NET Foundation licenses this file to you under the MIT license.
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// See the LICENSE file in the project root for more information.
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using System;
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using System.Runtime.CompilerServices;
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using static System.Math;
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namespace CommunityToolkit.HighPerformance.Memory.Internals;
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/// <summary>
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/// A helper to validate arithmetic operations for <see cref="Memory2D{T}"/> and <see cref="Span2D{T}"/>.
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/// </summary>
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internal static class OverflowHelper
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{
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/// <summary>
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/// Ensures that the input parameters will not exceed the maximum native int value when indexing.
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/// </summary>
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/// <param name="height">The height of the 2D memory area to map.</param>
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/// <param name="width">The width of the 2D memory area to map.</param>
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/// <param name="pitch">The pitch of the 2D memory area to map (the distance between each row).</param>
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/// <exception cref="OverflowException">Throw when the inputs don't fit in the expected range.</exception>
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/// <remarks>The input parameters are assumed to always be positive.</remarks>
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[MethodImpl(MethodImplOptions.AggressiveInlining)]
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public static void EnsureIsInNativeIntRange(int height, int width, int pitch)
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{
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// As per the layout used in the Memory2D<T> and Span2D<T> types, we have the
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// following memory representation with respect to height, width and pitch:
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//
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// _________width_________ ________...
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// / \/
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// | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- |_
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// | -- | -- | XX | XX | XX | XX | XX | XX | -- | -- | |
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// | -- | -- | XX | XX | XX | XX | XX | XX | -- | -- | |
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// | -- | -- | XX | XX | XX | XX | XX | XX | -- | -- | |_height
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// | -- | -- | XX | XX | XX | XX | XX | XX | -- | -- |_|
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// | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- |
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// | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- |
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// ...__pitch__/
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//
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// The indexing logic works on nint values in unchecked mode, with no overflow checks,
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// which means it relies on the maximum element index to always be within <= nint.MaxValue.
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// To ensure no overflows will ever occur there, we need to ensure that no instance can be
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// created with parameters that could cause an overflow in case any item was accessed, so we
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// need to ensure no overflows occurs when calculating the index of the last item in each view.
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// The logic below calculates that index with overflow checks, throwing if one is detected.
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// Note that we're subtracting 1 to the height as we don't want to include the trailing pitch
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// for the 2D memory area, and also 1 to the width as the index is 0-based, as usual.
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// Additionally, we're also ensuring that the stride is never greater than int.MaxValue, for
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// consistency with how ND arrays work (int.MaxValue as upper bound for each axis), and to
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// allow for faster iteration in the RefEnumerable<T> type, when traversing columns.
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_ = checked(((nint)(width + pitch) * Max(unchecked(height - 1), 0)) + Max(unchecked(width - 1), 0));
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}
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/// <summary>
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/// Ensures that the input parameters will not exceed <see cref="int.MaxValue"/> when indexing.
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/// </summary>
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/// <param name="height">The height of the 2D memory area to map.</param>
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/// <param name="width">The width of the 2D memory area to map.</param>
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/// <param name="pitch">The pitch of the 2D memory area to map (the distance between each row).</param>
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/// <returns>The area resulting from the given parameters.</returns>
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/// <exception cref="OverflowException">Throw when the inputs don't fit in the expected range.</exception>
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/// <remarks>The input parameters are assumed to always be positive.</remarks>
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[MethodImpl(MethodImplOptions.AggressiveInlining)]
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public static int ComputeInt32Area(int height, int width, int pitch)
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{
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return checked(((width + pitch) * Max(unchecked(height - 1), 0)) + width);
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}
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}
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