PKHeX/PKHeX.Core/Util/BinaryCodedDecimal.cs

using System;
using System.Runtime.CompilerServices;

namespace PKHeX.Core
{
    /// <summary>
    /// 4-bit decimal encoding used by some Generation 1 save file values.
    /// </summary>
    public static class BinaryCodedDecimal
    {
        /// <summary>
        /// Returns a 32-bit signed integer converted from bytes in a Binary Coded Decimal format byte array.
        /// </summary>
        /// <param name="input">Input byte array to read from.</param>
        public static int ToInt32BE(ReadOnlySpan<byte> input)
        {
            int result = 0;
            foreach (var b in input)
                PushDigits(ref result, b);
            return result;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static void PushDigits(ref int result, byte b) => result = (result * 100) + (10 * (b >> 4)) + (b & 0xf);

        /// <summary>
        /// Writes the <see cref="value"/> to the <see cref="data"/> buffer.
        /// </summary>
        public static void WriteBytesBE(Span<byte> data, int value)
        {
            for (int i = data.Length - 1; i >= 0; i--, value /= 100)
                data[i] = (byte)((((value / 10) % 10) << 4) | (value % 10));
        }

        /// <inheritdoc cref="ToInt32BE(ReadOnlySpan{byte})"/>
        public static int ToInt32LE(ReadOnlySpan<byte> input)
        {
            int result = 0;
            for (int i = input.Length - 1; i >= 0; i--)
                PushDigits(ref result, input[i]);
            return result;
        }

        /// <summary>
        /// Writes the <see cref="value"/> to the <see cref="data"/> buffer.
        /// </summary>
        public static void WriteBytesLE(Span<byte> data, int value)
        {
            for (int i = 0; i < data.Length; i++, value /= 100)
                data[i] = (byte)((((value / 10) % 10) << 4) | (value % 10));
        }
    }
}