public final class Duration {
    public static final Duration ZERO = new Duration(0, 0);
    static final int MINUTES_PER_HOUR = 60;
    static final int SECONDS_PER_MINUTE = 60;
    static final int SECONDS_PER_HOUR = SECONDS_PER_MINUTE * MINUTES_PER_HOUR;
    static final long NANOS_PER_SECOND = 1000_000_000L;

    private final long seconds;
    private final int nanos;
    
    public Duration(long seconds, int nanos) {
        this.seconds = seconds;
        this.nanos = nanos;
    }
    
    @Override
    public String toString() {
        if (this == ZERO) {
            return "PT0S";
        }
        long hours = seconds / SECONDS_PER_HOUR;
        int minutes = (int) ((seconds % SECONDS_PER_HOUR) / SECONDS_PER_MINUTE);
        int secs = (int) (seconds % SECONDS_PER_MINUTE);
        StringBuilder buf = new StringBuilder(24);
        buf.append("PT");
        if (hours != 0) {
            buf.append(hours).append('H');
        }
        if (minutes != 0) {
            buf.append(minutes).append('M');
        }
        if (secs == 0 && nanos == 0 && buf.length() > 2) {
            return buf.toString();
        }
        if (secs < 0 && nanos > 0) {
            if (secs == -1) {
                buf.append("-0");
            } else {
                buf.append(secs + 1);
            }
        } else {
            buf.append(secs);
        }
        if (nanos > 0) {
            int pos = buf.length();
            if (secs < 0) {
                buf.append(2 * NANOS_PER_SECOND - nanos);
            } else {
                buf.append(nanos + NANOS_PER_SECOND);
            }
            while (buf.charAt(buf.length() - 1) == '0') {
                buf.setLength(buf.length() - 1);
            }
            buf.setCharAt(pos, '.');
        }
        buf.append('S');
        return buf.toString();
    }
}