third_party/rust_crates/vendor/utf8-ranges/src/char_utf8.rs

// Pulled from std::char until encode_utf8 stabilizes. ---AG

// UTF-8 ranges and tags for encoding characters
const TAG_CONT: u8    = 0b1000_0000;
const TAG_TWO_B: u8   = 0b1100_0000;
const TAG_THREE_B: u8 = 0b1110_0000;
const TAG_FOUR_B: u8  = 0b1111_0000;
const MAX_ONE_B: u32   =     0x80;
const MAX_TWO_B: u32   =    0x800;
const MAX_THREE_B: u32 =  0x10000;

#[inline]
pub fn encode_utf8(character: char, dst: &mut [u8]) -> Option<usize> {
    let code = character as u32;
    if code < MAX_ONE_B && !dst.is_empty() {
        dst[0] = code as u8;
        Some(1)
    } else if code < MAX_TWO_B && dst.len() >= 2 {
        dst[0] = (code >> 6 & 0x1F) as u8 | TAG_TWO_B;
        dst[1] = (code & 0x3F) as u8 | TAG_CONT;
        Some(2)
    } else if code < MAX_THREE_B && dst.len() >= 3  {
        dst[0] = (code >> 12 & 0x0F) as u8 | TAG_THREE_B;
        dst[1] = (code >>  6 & 0x3F) as u8 | TAG_CONT;
        dst[2] = (code & 0x3F) as u8 | TAG_CONT;
        Some(3)
    } else if dst.len() >= 4 {
        dst[0] = (code >> 18 & 0x07) as u8 | TAG_FOUR_B;
        dst[1] = (code >> 12 & 0x3F) as u8 | TAG_CONT;
        dst[2] = (code >>  6 & 0x3F) as u8 | TAG_CONT;
        dst[3] = (code & 0x3F) as u8 | TAG_CONT;
        Some(4)
    } else {
        None
    }
}