ill-formed な文字

IllFormedUnicode.cs

using System;
using System.Runtime.InteropServices;
using System.Text;

class Program
{
    static void Main()
    {
        var s = "😀";
        //codepoint: 1F600
        //UTF-16 high: D83D, low: DE00
        //UTF-8: { F0 9F 98 80 }
        //UTF-16 → WTF-8 hight: { ED A0 BD }, low: { ED B8 80 }

        var cp = MemoryMarshal.Cast<byte, int>(Encoding.UTF32.GetBytes(s))[0];

        Console.WriteLine($"codepoint: {cp:X}");

        var hs = (int)s[0];
        var ls = (int)s[1];

        Console.WriteLine($"UTF-16 high: {hs:X}, low: {ls:X}");

        Span<byte> buffer = stackalloc byte[4];
        var utf8 = ToIllFormedUtf8(cp, buffer);

        Console.Write("UTF-8: {");
        foreach (var b in utf8) Console.Write($" {b:X2}");
        Console.WriteLine(" }");

        var wtf8 = ToIllFormedUtf8(hs, buffer);
        Console.Write("UTF-16 → WTF-8 hight: {");
        foreach (var b in wtf8) Console.Write($" {b:X2}");
        wtf8 = ToIllFormedUtf8(ls, buffer);
        Console.Write(" }, low: {");
        foreach (var b in wtf8) Console.Write($" {b:X2}");
        Console.WriteLine(" }");
    }

    /// <summary>
    /// 今の Encoding.UTF8.GetBytes は不正な符号点文字が来たとき U+FFFD への置き換えをする。
    /// そういうのやらずに愚直に符号化。
    /// </summary>
    private static Span<byte> ToIllFormedUtf8(int c, Span<byte> buffer)
    {
        const int Two = 0b1100_0000;
        const int Three = 0b1110_0000;
        const int Four = 0b1111_0000;
        const int Tailing = 0b1000_0000;
        const int Bits6 = 0b11_1111;
        const int Bits5 = 0b1_1111;
        const int Bits4 = 0b1111;
        const int Bits3 = 0b111;

        if (c < 0x100)
        {
            buffer[0] = (byte)c;
            return buffer.Slice(0, 1);
        }
        else if (c < 0x8000)
        {
            buffer[0] = (byte)(((c >> 6) & Bits5) | Two);
            buffer[2] = (byte)((c & Bits6) | Tailing);
            return buffer.Slice(0, 2);
        }
        else if(c < 0x10000)
        {
            buffer[0] = (byte)(((c >> 12) & Bits4) | Three);
            buffer[1] = (byte)(((c >> 6) & Bits6) | Tailing);
            buffer[2] = (byte)((c & Bits6) | Tailing);
            return buffer.Slice(0, 3);
        }
        else
        {
            buffer[0] = (byte)(((c >> 18) & Bits3) | Four);
            buffer[1] = (byte)(((c >> 12) & Bits6) | Tailing);
            buffer[2] = (byte)(((c >> 6) & Bits6) | Tailing);
            buffer[3] = (byte)((c & Bits6) | Tailing);
            return buffer.Slice(0, 4);
        }
    }
}