/// <summary>
/// Given that the next character is an ampersand ('&'), attempt to interpret the
/// following characters as an XML entity. On success, populate the out parameters
/// with the low and high UTF-16 surrogates for the character represented by the
/// entity.
/// </summary>
/// <param name="ch">e.g. '<' for &lt;.</param>
/// <param name="surrogate">e.g. '\uDC00' for &#x10000; (ch == '\uD800').</param>
/// <returns>True if a valid XML entity was consumed.</returns>
/// <remarks>
/// NOTE: Always advances, even on failure.
/// </remarks>
public bool TryScanXmlEntity(out char ch, out char surrogate)
{
Debug.Assert(this.PeekChar() == '&');
ch = '&';
this.AdvanceChar();
surrogate = InvalidCharacter;
switch (this.PeekChar())
{
case 'l':
if (AdvanceIfMatches("lt;"))
{
ch = '<';
return(true);
}
break;
case 'g':
if (AdvanceIfMatches("gt;"))
{
ch = '>';
return(true);
}
break;
case 'a':
if (AdvanceIfMatches("amp;"))
{
ch = '&';
return(true);
}
else if (AdvanceIfMatches("apos;"))
{
ch = '\'';
return(true);
}
break;
case 'q':
if (AdvanceIfMatches("quot;"))
{
ch = '"';
return(true);
}
break;
case '#':
{
this.AdvanceChar(); //#
uint uintChar = 0;
if (AdvanceIfMatches("x"))
{
char digit;
while (SyntaxFacts.IsHexDigit(digit = this.PeekChar()))
{
this.AdvanceChar();
// disallow overflow
if (uintChar <= 0x7FFFFFF)
{
uintChar = (uintChar << 4) + (uint)SyntaxFacts.HexValue(digit);
}
else
{
return(false);
}
}
}
else
{
char digit;
while (SyntaxFacts.IsDecDigit(digit = this.PeekChar()))
{
this.AdvanceChar();
// disallow overflow
if (uintChar <= 0x7FFFFFF)
{
uintChar = (uintChar << 3) + (uintChar << 1) + (uint)SyntaxFacts.DecValue(digit);
}
else
{
return(false);
}
}
}
if (AdvanceIfMatches(";"))
{
ch = GetCharsFromUtf32(uintChar, out surrogate);
return(true);
}
break;
}
}
return(false);
}