outputUnicodeEscape

fn outputUnicodeEscape(codepoint: u21, w: *Writer) Error!void {
    if (codepoint <= 0xFFFF) {
        // If the character is in the Basic Multilingual Plane (U+0000 through U+FFFF),
        // then it may be represented as a six-character sequence: a reverse solidus, followed
        // by the lowercase letter u, followed by four hexadecimal digits that encode the character's code point.
        try w.writeAll("\\u");
        try w.printInt(codepoint, 16, .lower, .{ .width = 4, .fill = '0' });
    } else {
        assert(codepoint <= 0x10FFFF);
        // To escape an extended character that is not in the Basic Multilingual Plane,
        // the character is represented as a 12-character sequence, encoding the UTF-16 surrogate pair.
        const high = @as(u16, @intCast((codepoint - 0x10000) >> 10)) + 0xD800;
        const low = @as(u16, @intCast(codepoint & 0x3FF)) + 0xDC00;
        try w.writeAll("\\u");
        try w.printInt(high, 16, .lower, .{ .width = 4, .fill = '0' });
        try w.writeAll("\\u");
        try w.printInt(low, 16, .lower, .{ .width = 4, .fill = '0' });
    }
}