Converts from Unicode codepoint to UTF 8 and vice versa. Courtesy of GPT-4

CodePointUTF8.cs

using System.Text;

static byte[] UnicodeCodepointToUtf8(int codepoint)
{
    if (codepoint >= 0 && codepoint <= 127)
    {
        return new byte[] { (byte)codepoint };
    }
    else if (codepoint >= 128 && codepoint <= 2047)
    {
        byte firstByte = (byte)(192 + (codepoint / 64));
        byte secondByte = (byte)(128 + (codepoint % 64));
        return new byte[] { firstByte, secondByte };
    }
    else if (codepoint >= 2048 && codepoint <= 65535)
    {
        byte firstByte = (byte)(224 + (codepoint / 4096));
        int remainder = codepoint % 4096;
        byte secondByte = (byte)(128 + (remainder / 64));
        byte thirdByte = (byte)(128 + (remainder % 64));
        return new byte[] { firstByte, secondByte, thirdByte };
    }
    else if (codepoint >= 65536 && codepoint <= 1114111)
    {
        byte firstByte = (byte)(240 + (codepoint / 262144));
        int remainder = codepoint % 262144;
        byte secondByte = (byte)(128 + (remainder / 4096));
        remainder = remainder % 4096;
        byte thirdByte = (byte)(128 + (remainder / 64));
        byte fourthByte = (byte)(128 + (remainder % 64));
        return new byte[] { firstByte, secondByte, thirdByte, fourthByte };
    }
    else
    {
        throw new ArgumentOutOfRangeException("Invalid Unicode codepoint");
    }
}

static uint Utf8BytesToCodepoint(MemoryStream ms)
{
    int firstByte = ms.ReadByte();
    if (firstByte < 0) return 0; // End of stream

    uint codepoint = 0;
    int additionalBytes = 0;

    if (firstByte < 128)
    {
        codepoint = (uint)firstByte;
    }
    else if (firstByte >= 192 && firstByte < 224)
    {
        codepoint = (uint)(firstByte - 192);
        additionalBytes = 1;
    }
    else if (firstByte >= 224 && firstByte < 240)
    {
        codepoint = (uint)(firstByte - 224);
        additionalBytes = 2;
    }
    else if (firstByte >= 240 && firstByte < 248)
    {
        codepoint = (uint)(firstByte - 240);
        additionalBytes = 3;
    }
    else
    {
        throw new ArgumentException("Invalid UTF-8 byte sequence.");
    }

    for (int i = 0; i < additionalBytes; i++)
    {
        int nextByte = ms.ReadByte();
        if (nextByte < 128 || nextByte >= 192)
        {
            throw new ArgumentException("Invalid UTF-8 byte sequence.");
        }

        codepoint = codepoint * 64 + (uint)(nextByte - 128);
    }

    return codepoint;
}