/// <summary>
/// Given a Unicode scalar value, gets the number of UTF-16 code units required to represent this value.
/// </summary>
public static int GetUtf16SequenceLength(uint value)
{
UnicodeDebug.AssertIsValidScalar(value);
value -= 0x10000; // if value < 0x10000, high byte = 0xFF; else high byte = 0x00
value += (2 << 24); // if value < 0x10000, high byte = 0x01; else high byte = 0x02
value >>= 24; // shift high byte down
return((int)value); // and return it
}
/// <summary>
/// Given a Unicode scalar value, gets the number of UTF-8 code units required to represent this value.
/// </summary>
public static int GetUtf8SequenceLength(uint value)
{
UnicodeDebug.AssertIsValidScalar(value);
// The logic below can handle all valid scalar values branchlessly.
// It gives generally good performance across all inputs, and on x86
// it's only six instructions: lea, sar, xor, add, shr, lea.
// 'a' will be -1 if input is < 0x800; else 'a' will be 0
// => 'a' will be -1 if input is 1 or 2 UTF-8 code units; else 'a' will be 0
int a = ((int)value - 0x0800) >> 31;
// The number of UTF-8 code units for a given scalar is as follows:
// - U+0000..U+007F => 1 code unit
// - U+0080..U+07FF => 2 code units
// - U+0800..U+FFFF => 3 code units
// - U+10000+ => 4 code units
//
// If we XOR the incoming scalar with 0xF800, the chart mutates:
// - U+0000..U+F7FF => 3 code units
// - U+F800..U+F87F => 1 code unit
// - U+F880..U+FFFF => 2 code units
// - U+10000+ => 4 code units
//
// Since the 1- and 3-code unit cases are now clustered, they can
// both be checked together very cheaply.
value ^= 0xF800u;
value -= 0xF880u; // if scalar is 1 or 3 code units, high byte = 0xFF; else high byte = 0x00
value += (4 << 24); // if scalar is 1 or 3 code units, high byte = 0x03; else high byte = 0x04
value >>= 24; // shift high byte down
// Final return value:
// - U+0000..U+007F => 3 + (-1) * 2 = 1
// - U+0080..U+07FF => 4 + (-1) * 2 = 2
// - U+0800..U+FFFF => 3 + ( 0) * 2 = 3
// - U+10000+ => 4 + ( 0) * 2 = 4
return((int)value + (a * 2));
}
