O(n) way of parsing golang-formatted duration strings to TimeSpans in C#

parser.cs

/// <summary>
/// Highly performant method for parsing strings using the GoLang TimeSpan format (e.g. 23h59m59s)
/// If no time unit is specified, seconds will be assumed.
/// </summary>
/// <param name="str">the string to parse</param>
/// <returns>a TimeSpan representing the parsed value</returns>
public static TimeSpan ParseTimeSpan(string str)
{
    if (str == null || string.IsNullOrWhiteSpace(str))
        throw new ParseException("input may not be empty");

    int? hours = null;
    int? minutes = null;
    int? seconds = null;

    int tmp = 0;
    bool valid = false;
    for (int i = 0; i < str.Length; i++)
    {
        var current = str[i];
        switch (current)
        {
            case 'h':
            case 'H':
                if (hours != null)
                    throw new ParseException("a value for hours has already been set: " + hours);
                hours = tmp;
                tmp = 0;
                if (!valid) 
                    throw new ParseException("empty values for hours are not allowed.");
                valid = false;
                break;
            case 'm':
            case 'M':
                if (hours != null)
                    throw new ParseException("a value for minutes has already been set: " + minutes);
                minutes = tmp;
                tmp = 0;
                if (!valid) throw new ParseException("empty values for minutes are not allowed.");
                valid = false;
                break;
            case 's':
            case 'S':
                if (hours != null)
                    throw new ParseException("a value for seconds has already been set: " + seconds);
                seconds = tmp;
                tmp = 0;
                if (!valid) throw new ParseException("empty values for seconds are not allowed.");
                valid = false;
                break;
            default:
                if (current < 48 || current > 57)
                    throw new ParseException("invalid character: " + current);
                tmp = tmp * 10 + (current - 48);
                valid = true;
                break;
        }
        if (hours != null && minutes != null && seconds != null)
            break;
    }
    if (tmp > 0)
        if (seconds == null)
            seconds = tmp;
        else
            throw new ParseException("input ending implies that seconds should be set, but they already have a value: " + seconds);
    return new TimeSpan(hours ?? 0, minutes ?? 0, seconds ?? 0);
}

you may omit the 's' for seconds, eg. 3h4m2 instead of 3h4m2s.