漢数字.cs

for (uint i = 0; i < 12; i++)
{
    Console.WriteLine(format(i));
}

for (uint i = 15; i < 120; i+=7)
{
    Console.WriteLine(format(i));
}
Console.WriteLine(format(1234));
Console.WriteLine(format(987_654_321));
Console.WriteLine(format(5_1230_1230));
Console.WriteLine(format(1020304050));

for (uint i = 10; i < 10_0000_0000; i*=10)
{
    Console.WriteLine(format(i));
}

Console.WriteLine(format(1222_2222_2222_2222_2222));
Console.WriteLine(format(12_3456_7890_1234_5678));
Console.WriteLine(format(1000_0000_0001_1010_0000));
Console.WriteLine(format(ulong.MaxValue));

static string format(ulong x)
{
    if (x == 0) return "〇";

    ReadOnlySpan<char> DigitChars = "一二三四五六七八九";
    ReadOnlySpan<char> PowerChars = "十百千";
    ReadOnlySpan<char> MyriadChars = "万億兆京";

    Span<byte> digitBuffer = stackalloc byte[24];
    int digits = 0;
    for (; x != 0 && digits < 21; digits++)
    {
        (x, var r) = ulong.DivRem(x, 10);
        digitBuffer[digits] = (byte)r;
    }

    Span<char> charBuffer = stackalloc char[40];
    var len = 0;

    for (var myriads = digits / 4; myriads >= 0; myriads--)
    {
        var myriadSlice = digitBuffer.Slice((myriads * 4), 4);

        var prev = len;
        for (int n = 3; n > 0; n--)
        {
            var d = myriadSlice[n];
            if (d != 0)
            {
                if (d != 1) charBuffer[len++] = DigitChars[d - 1];
                charBuffer[len++] = PowerChars[n - 1];
            }
        }

        var d0 = myriadSlice[0];
        if (d0 != 0) charBuffer[len++] = DigitChars[d0 - 1];

        if (myriads != 0 && prev != len) charBuffer[len++] = MyriadChars[myriads - 1];
    }

    return new(charBuffer[..len]);
}