emitCharacter

            fn emitCharacter(self: *Windows, code_unit: u16, last_emitted_code_unit: ?u16) ?u16 {
                // Because we are emitting WTF-8, we need to
                // check to see if we've emitted two consecutive surrogate
                // codepoints that form a valid surrogate pair in order
                // to ensure that we're always emitting well-formed WTF-8
                // (https://wtf-8.codeberg.page/#concatenating).
                //
                // If we do have a valid surrogate pair, we need to emit
                // the UTF-8 sequence for the codepoint that they encode
                // instead of the WTF-8 encoding for the two surrogate pairs
                // separately.
                //
                // This is relevant when dealing with a WTF-16 encoded
                // command line like this:
                // "<0xD801>"<0xDC37>
                // which would get parsed and converted to WTF-8 as:
                // <0xED><0xA0><0x81><0xED><0xB0><0xB7>
                // but instead, we need to recognize the surrogate pair
                // and emit the codepoint it encodes, which in this
                // example is U+10437 (𐐷), which is encoded in UTF-8 as:
                // <0xF0><0x90><0x90><0xB7>
                if (last_emitted_code_unit != null and
                    std.unicode.utf16IsLowSurrogate(code_unit) and
                    std.unicode.utf16IsHighSurrogate(last_emitted_code_unit.?))
                {
                    const codepoint = std.unicode.utf16DecodeSurrogatePair(&.{ last_emitted_code_unit.?, code_unit }) catch unreachable;

                    // Unpaired surrogate is 3 bytes long
                    const dest = self.buffer[self.end - 3 ..];
                    const len = std.unicode.utf8Encode(codepoint, dest) catch unreachable;
                    // All codepoints that require a surrogate pair (> U+FFFF) are encoded as 4 bytes
                    assert(len == 4);
                    self.end += 1;
                    return null;
                }

                const wtf8_len = std.unicode.wtf8Encode(code_unit, self.buffer[self.end..]) catch unreachable;
                self.end += wtf8_len;
                return code_unit;
            }