Date library for parsing human readable arithmetic calculations

Question

I am writing a QA test automation program where people are going to input human readable duration strings with arithmetic operations with values like now - 5d, yesterday, now + 8 days, and I need parse them into java 8 LocalDateTime instances.

So simply stated, I need a utility like this:

LocalDateTime nowMinus5Days = DurationUtil.parseHumanReadableDuration("now - 5d")

How can I code this in Java? Or is there some library that already exists that I should use instead of trying to reinvent the wheel?

I think your Question is too broad. But yes, the java.time api has methods to add days, hours, minutes and a now(). I think you'll have to add some more details for anyone to be able to provide with an actual answer though. — Scratte, Mar 30 '20 at 21:50
It did :) I didn't understand that you wanted to parse text. No, as far as I'm aware, the standard java SE does not provide with a parser for this. — Scratte, Mar 30 '20 at 21:54
yeah that's the feeling i'm getting. but there's gotta be some sort of library that does it. — Nicholas DiPiazza, Mar 30 '20 at 21:55
Hi I would like this re-opened. I should have asked "how to code this in java" versus "what library can i use". but i also will ask, should i use an existing library to prevent reinventing the wheel? — Nicholas DiPiazza, Apr 06 '20 at 19:47

Ole V.V. · Accepted Answer · 2020-04-04T12:02:52.507

Here’s a starting point. I am guessing that you will want to elaborate it further.

private static Pattern relativeTimePattern
        = Pattern.compile("(\\w+)\\s*(?:([+-])\\s*(\\w+))?");
private static Map<String, Supplier<LocalDateTime>> bases
        = Map.of("now", () -> LocalDateTime.now(),
                "yesterday", () -> LocalDate.now().minusDays(1).atStartOfDay()); 

public static LocalDateTime parseRelativeTime(String timeString) {
    Matcher m = relativeTimePattern.matcher(timeString);
    if (m.matches()) {
        String baseString = m.group(1);
        LocalDateTime result = bases.get(baseString).get();
        String signString = m.group(2);
        if (signString != null) {
            boolean subtract = signString.equals("-");
            String diffString = m.group(3);
            TemporalAmount diff;
            try {
                diff = Period.parse("P" + diffString);
            } catch (DateTimeParseException dtpe) {
                // try a Duration instead
                diff = Duration.parse("PT" + diffString);
            }
            if (subtract) {
                result = result.minus(diff);
            } else {
                result = result.plus(diff);
            }
        }
        return result;
    } else {
        throw new IllegalArgumentException();
    }
}

Let’s try it out:

    System.out.println(parseRelativeTime("now - 5d"));
    System.out.println(parseRelativeTime("yesterday"));
    System.out.println(parseRelativeTime("now + 8d"));

Output when I ran just now:

2020-03-30T09:49:18.300731
2020-04-03T00:00
2020-04-12T09:49:18.307784

As my method stands, it accepts either of now and yesterday in lower case optionally followed by a sign (+ or -) and either a period of years-months-weeks-days or a duration of hours-minutes-seconds. Each of the latter must use one letter abbreviations for the time units (y, m, w, d, h, m, s; when the ambiguous m comes alone, it is taken as months). No spaces are allowed inside the period or duration.

Thinkable further developments include:

Add more words: today, tomorrow; consider allowing upper case and mixed case.
Allow units in full and with space, for example 8 days. A further regex will convert this to 8d before the final parsing into a Period.
Disallow the ambiguous 2m forcing the user to specify for example 2m0d (2 months 0 days) or 2m0s (2 minutes 0 seconds). Or force uppercase M for months and lowercase m for minutes.
The complicated part: provide useful error messages for strings that cannot be parsed.

I had something similar I was doing, but I like what you have here better so I am swapping it out. It appears to be working fine. Thanks. — Nicholas DiPiazza, Apr 06 '20 at 19:44

score 1 · Answer 2 · answered Mar 30 '20 at 22:00

I think it is pretty much impossible (or at least hard) to cover conversion of every possible String input to a valid date. It could even need some intelligent Natural Language Processing.

I would suggest using a set of fields exposed for input and convert them to a desired object. If there is an end user, you can have a user select whether they want to add or subtract date and then present a Text Input where they can enter the number of days. You can handle inputs like "Yesterday" and "Tomorrow" as special inputs.

Java's Date Package is pretty useful and has methods for adding and subtracting a given number of days.

Date library for parsing human readable arithmetic calculations

2 Answers2