Base64 Encoding with Span<T>

base64encode.linq

void Main()
{
	/*
		Input: [0], Output: [A] (length=1)
Input: [1], Output: [B] (length=1)
Input: [18446744073709551615], Output: [~~~~~~~~~~P] (length=11)
Input: [0], Output: [A] (length=1)
Input: [34343], Output: [nYI] (length=3)
	*/
	
	var values = new long[] { 0L, 1L, long.MaxValue,  34343};

	foreach (var val in values) {
		var encoded = Base64Encode(val);
		Console.WriteLine("Input: [{0}], Output: [{1}] (length={2})", val, encoded, encoded.Length);
	}
}

const string Base64Chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789+~";

/*
 * Can't accept negative values (will throw an IndexOutOfRange) but due to compatability
 * constraints, can't change this function to accept ulong
 */
string Base64Encode(long value, string prefix = "$")
{
	Span<char> sb = stackalloc char[11 + prefix?.Length ?? 0];
	var spanIndex = 0;
	if (!string.IsNullOrEmpty(prefix) && prefix.Length > 0) {
		prefix.AsSpan().CopyTo(sb);
		spanIndex = prefix.Length;
	}	
	
	var next = value;
	do
	{
		var index = (int)(next & 63);
		sb[spanIndex++] = Base64Chars[index];
		next = next >> 6;
	} while (next != 0);
	return sb.Slice(0, spanIndex).ToString();
}