// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System;
using System.Diagnostics.Contracts;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace Microsoft.Toolkit.Extensions
{
    /// <summary>
    /// Helpers for working with value types.
    /// </summary>
    public static class ValueTypeExtensions
    {
        /// <summary>
        /// Gets the table of hex characters (doesn't allocate, maps to .text section, see <see href="https://github.com/dotnet/roslyn/pull/24621"/>)
        /// </summary>
        private static ReadOnlySpan<byte> HexCharactersTable => new[]
        {
            (byte)'0', (byte)'1', (byte)'2', (byte)'3',
            (byte)'4', (byte)'5', (byte)'6', (byte)'7',
            (byte)'8', (byte)'9', (byte)'A', (byte)'B',
            (byte)'C', (byte)'D', (byte)'E', (byte)'F'
        };

        /// <summary>
        /// Returns a hexadecimal <see cref="string"/> representation of a given <typeparamref name="T"/> value, left-padded and ordered as big-endian.
        /// </summary>
        /// <typeparam name="T">The input type to format to <see cref="string"/>.</typeparam>
        /// <param name="value">The input value to format to <see cref="string"/>.</param>
        /// <returns>
        /// The hexadecimal representation of <paramref name="value"/> (with the '0x' prefix), left-padded to byte boundaries and ordered as big-endian.
        /// </returns>
        /// <remarks>
        /// As a byte (8 bits) is represented by two hexadecimal digits (each representing a group of 4 bytes), each <see cref="string"/>
        /// representation will always contain an even number of digits. For instance:
        /// <code>
        /// Console.WriteLine(1.ToHexString()); // "0x01"
        /// Console.WriteLine(((byte)255).ToHexString()); // "0xFF"
        /// Console.WriteLine((-1).ToHexString()); // "0xFFFFFFFF"
        /// </code>
        /// </remarks>
        [Pure]
        [MethodImpl(MethodImplOptions.NoInlining)]
        public static unsafe string ToHexString<T>(this T value)
            where T : unmanaged
        {
            int
                sizeOfT = Unsafe.SizeOf<T>(),
                bufferSize = (2 * sizeOfT) + 2;
            char* p = stackalloc char[bufferSize];

            p[0] = '0';
            p[1] = 'x';

            ref byte r0 = ref Unsafe.As<T, byte>(ref value);
            ref byte rh = ref MemoryMarshal.GetReference(HexCharactersTable);

            for (int i = 0, j = bufferSize - 2; i < sizeOfT; i++, j -= 2)
            {
                byte b = Unsafe.Add(ref r0, i);
                int
                    low = b & 0x0F,
                    high = (b & 0xF0) >> 4;

                p[j + 1] = (char)Unsafe.Add(ref rh, low);
                p[j] = (char)Unsafe.Add(ref rh, high);
            }

            return new string(p, 0, bufferSize);
        }
    }
}