private static Int32 ReadRuneFromString(String input, Int32 index)
{
if (input is null)
{
throw new ArgumentNullException(nameof(input));
}
if ((UInt32)index >= (UInt32)input !.Length)
{
throw new ArgumentOutOfRangeException(nameof(index));
}
// Optimistically assume input is within BMP.
UInt32 returnValue = input[index];
if (Utf16.IsSurrogate(returnValue))
{
if (!Utf16.IsHighSurrogate(returnValue))
{
return(-1);
}
// Treat 'returnValue' as the high surrogate.
//
// If this becomes a hot code path, we can skip the below bounds check by reading
// off the end of the string using unsafe code. Since strings are null-terminated,
// we're guaranteed not to read a valid low surrogate, so we'll fail correctly if
// the string terminates unexpectedly.
index++;
if ((UInt32)index >= (UInt32)input.Length)
{
return(-1); // not an argument exception - just a "bad data" failure
}
UInt32 potentialLowSurrogate = input[index];
if (!Utf16.IsLowSurrogate(potentialLowSurrogate))
{
return(-1);
}
returnValue = Unsafe.Utf16Decode(returnValue, potentialLowSurrogate);
}
return((Int32)returnValue);
}
// returns a negative number on failure
internal static int ReadFirstRuneFromUtf16Buffer(ReadOnlySpan <char> input)
{
if (input.IsEmpty)
{
return(-1);
}
// Optimistically assume input is within BMP.
uint returnValue = input[0];
if (Utf16.IsSurrogate(returnValue))
{
if (!Utf16.IsHighSurrogate(returnValue))
{
return(-1);
}
// Treat 'returnValue' as the high surrogate.
if (1 >= (uint)input.Length)
{
return(-1); // not an argument exception - just a "bad data" failure
}
uint potentialLowSurrogate = input[1];
if (!Utf16.IsLowSurrogate(potentialLowSurrogate))
{
return(-1);
}
returnValue = Unsafe.Utf16Decode(returnValue, potentialLowSurrogate);
}
return((int)returnValue);
}