internal static void ToLower(ReadOnlySpan <char> source, Span <char> destination)
{
Debug.Assert(GlobalizationMode.Invariant);
Debug.Assert(source.Length <= destination.Length);
for (int i = 0; i < source.Length; i++)
{
char c = source[i];
if (char.IsHighSurrogate(c) && i < source.Length - 1)
{
char cl = source[i + 1];
if (char.IsLowSurrogate(cl))
{
// well formed surrogates
SurrogateCasing.ToLower(c, cl, out char h, out char l);
destination[i] = h;
destination[i + 1] = l;
i++; // skip the low surrogate
continue;
}
}
destination[i] = ToLower(c);
}
}
internal static string ToLower(string s)
{
if (s.Length == 0)
{
return(string.Empty);
}
ReadOnlySpan <char> source = s;
int i = 0;
while (i < s.Length)
{
if (char.IsHighSurrogate(source[i]) && i < s.Length - 1 && char.IsLowSurrogate(source[i + 1]))
{
SurrogateCasing.ToLower(source[i], source[i + 1], out char h, out char l);
if (source[i] != h || source[i + 1] != l)
{
break;
}
i += 2;
continue;
}
if (ToLower(source[i]) != source[i])
{
break;
}
i++;
}
if (i >= s.Length)
{
return(s);
}
return(string.Create(s.Length, (s, i), static (destination, state) =>
{
ReadOnlySpan <char> src = state.s;
src.Slice(0, state.i).CopyTo(destination);
InvariantModeCasing.ToLower(src.Slice(state.i), destination.Slice(state.i));
}));
}