private static string?FormatBigIntegerToHex(bool targetSpan, BigInteger value, char format, int digits, NumberFormatInfo info, Span <char> destination, out int charsWritten, out bool spanSuccess)
{
Debug.Assert(format == 'x' || format == 'X');
// Get the bytes that make up the BigInteger.
byte[]? arrayToReturnToPool = null;
Span <byte> bits = stackalloc byte[64]; // arbitrary threshold
if (!value.TryWriteOrCountBytes(bits, out int bytesWrittenOrNeeded))
{
bits = arrayToReturnToPool = ArrayPool <byte> .Shared.Rent(bytesWrittenOrNeeded);
bool success = value.TryWriteBytes(bits, out bytesWrittenOrNeeded);
Debug.Assert(success);
}
bits = bits.Slice(0, bytesWrittenOrNeeded);
var sb = new ValueStringBuilder(stackalloc char[128]); // each byte is typically two chars
int cur = bits.Length - 1;
if (cur > -1)
{
// [FF..F8] drop the high F as the two's complement negative number remains clear
// [F7..08] retain the high bits as the two's complement number is wrong without it
// [07..00] drop the high 0 as the two's complement positive number remains clear
bool clearHighF = false;
byte head = bits[cur];
if (head > 0xF7)
{
head -= 0xF0;
clearHighF = true;
}
if (head < 0x08 || clearHighF)
{
// {0xF8-0xFF} print as {8-F}
// {0x00-0x07} print as {0-7}
sb.Append(head < 10 ?
(char)(head + '0') :
format == 'X' ? (char)((head & 0xF) - 10 + 'A') : (char)((head & 0xF) - 10 + 'a'));
cur--;
}
}
if (cur > -1)
{
Span <char> chars = sb.AppendSpan((cur + 1) * 2);
int charsPos = 0;
string hexValues = format == 'x' ? "0123456789abcdef" : "0123456789ABCDEF";
while (cur > -1)
{
byte b = bits[cur--];
chars[charsPos++] = hexValues[b >> 4];
chars[charsPos++] = hexValues[b & 0xF];
}
}
if (digits > sb.Length)
{
// Insert leading zeros, e.g. user specified "X5" so we create "0ABCD" instead of "ABCD"
sb.Insert(
0,
value._sign >= 0 ? '0' : (format == 'x') ? 'f' : 'F',
digits - sb.Length);
}
if (arrayToReturnToPool != null)
{
ArrayPool <byte> .Shared.Return(arrayToReturnToPool);
}
if (targetSpan)
{
spanSuccess = sb.TryCopyTo(destination, out charsWritten);
return(null);
}
else
{
charsWritten = 0;
spanSuccess = false;
return(sb.ToString());
}
}