PluralKit/PluralKit.Core/Utils/DateUtils.cs

using System.Text.RegularExpressions;

using NodaTime;
using NodaTime.Text;

namespace PluralKit.Core;

public class DateUtils
{
    public static Duration? ParsePeriod(string str)
    {
        var d = Duration.Zero;

        foreach (Match match in Regex.Matches(str, "(\\d{1,6})(\\w)"))
        {
            var amount = int.Parse(match.Groups[1].Value);
            var type = match.Groups[2].Value.ToLowerInvariant();

            if (type == "w") d += Duration.FromDays(7) * amount;
            else if (type == "d") d += Duration.FromDays(1) * amount;
            else if (type == "h") d += Duration.FromHours(1) * amount;
            else if (type == "m") d += Duration.FromMinutes(1) * amount;
            else if (type == "s") d += Duration.FromSeconds(1) * amount;
            else return null;
        }

        if (d == Duration.Zero) return null;
        return d;
    }

    public static LocalDate? ParseDate(string str, bool allowNullYear = false)
    {
        // NodaTime can't parse constructs like "1st" and "2nd" so we quietly replace those away
        // Gotta make sure to do the regex otherwise we'll catch things like the "st" in "August" too
        str = Regex.Replace(str, "(\\d+)(st|nd|rd|th)", "$1");

        var patterns = new[]
        {
            "MMM d yyyy",   // Jan 1 2019
            "MMM d, yyyy",  // Jan 1, 2019
            "MMMM d yyyy",  // January 1 2019
            "MMMM d, yyyy", // January 1, 2019
            "yyyy-MM-dd",   // 2019-01-01
            "yyyy MM dd",   // 2019 01 01
            "yyyy/MM/dd"    // 2019/01/01
        }.ToList();

        if (allowNullYear)
            patterns.AddRange(new[]
            {
            "MMM d",        // Jan 1
            "MMMM d",       // January 1
            "MM-dd",        // 01-01
            "MM dd",        // 01 01
            "MM/dd"         // 01/01
            });

        // Giving a template value so year will be parsed as 0004 if not present
        // This means we can later disambiguate whether a null year was given
        // We use the basis year 0004 (rather than, say, 0001) because 0004 is a leap year in the Gregorian calendar
        // which means the date "Feb 29, 0004" is a valid date. 0001 is still accepted as a null year for legacy reasons.
        // TODO: should we be using invariant culture here?
        foreach (var pattern in patterns.Select(p =>
                     LocalDatePattern.CreateWithInvariantCulture(p).WithTemplateValue(new LocalDate(0004, 1, 1))))
        {
            var result = pattern.Parse(str);
            if (result.Success) return result.Value;
        }

        return null;
    }

    public static ZonedDateTime? ParseDateTime(string str, bool nudgeToPast = false, DateTimeZone zone = null)
    {
        if (zone == null) zone = DateTimeZone.Utc;

        // Find the current timestamp in the given zone, find the (naive) midnight timestamp, then put that into the same zone (and make it naive again)
        // Should yield a <current *local @ zone* date> 12:00:00 AM.
        var now = SystemClock.Instance.GetCurrentInstant().InZone(zone).LocalDateTime;
        var midnight = now.Date.AtMidnight();

        // First we try to parse the string as a relative time using the period parser
        var relResult = ParsePeriod(str);
        if (relResult != null)
            // if we can, we just subtract that amount from the 
            return now.InZoneLeniently(zone).Minus(relResult.Value);

        var timePatterns = new[]
        {
            "H:mm",         // 4:30
            "HHmm",         // 2330
            "HH:mm",        // 23:30
            "H:mm:ss",      // 4:30:29
            "HH:mm:ss",     // 23:30:29
            "h tt",         // 2 PM
            "htt",          // 2PM
            "h:mm tt",      // 4:30 PM
            "h:mmtt",       // 4:30PM
            "h:mm:ss tt",   // 4:30:29 PM
            "h:mm:sstt",    // 4:30:29PM
            "hh:mm tt",     // 11:30 PM
            "hh:mmtt",      // 11:30PM
            "hh:mm:ss tt",  // 11:30:29 PM
            "hh:mm:sstt"    // 11:30:29PM
            };

        var datePatterns = new[]
        {
            "MMM d yyyy",   // Jan 1 2019
            "MMM d, yyyy",  // Jan 1, 2019
            "MMMM d yyyy",  // January 1 2019
            "MMMM d, yyyy", // January 1, 2019
            "yyyy-MM-dd",   // 2019-01-01
            "yyyy MM dd",   // 2019 01 01
            "yyyy/MM/dd",   // 2019/01/01
            "MMM d",        // Jan 1
            "MMMM d",       // January 1
            "MM-dd",        // 01-01
            "MM dd",        // 01 01
            "MM/dd"         // 01-01
            };

        // First, we try all the timestamps that only have a time
        foreach (var timePattern in timePatterns)
        {
            var pat = LocalDateTimePattern.CreateWithInvariantCulture(timePattern).WithTemplateValue(midnight);
            var result = pat.Parse(str);
            if (result.Success)
            {
                // If we have a successful match and we need a time in the past, we try to shove a future-time a date before
                // Example: "4:30 pm" at 3:30 pm likely refers to 4:30 pm the previous day
                var val = result.Value;

                // If we need to nudge, we just subtract a day. This only occurs when we're parsing specifically *just time*, so
                // we know we won't nudge it by more than a day since we use today's midnight timestamp as a date template.

                // Since this is a naive datetime, this ensures we're actually moving by one calendar day even if
                // DST changes occur, since they'll be resolved later wrt. the right side of the boundary
                if (val > now && nudgeToPast) val = val.PlusDays(-1);
                return val.InZoneLeniently(zone);
            }
        }

        // Then we try specific date+time combinations, both date first and time first, with and without commas
        foreach (var timePattern in timePatterns)
            foreach (var datePattern in datePatterns)
                foreach (var patternStr in new[]
                {
                     $"{timePattern}, {datePattern}", $"{datePattern}, {timePattern}",
                     $"{timePattern} {datePattern}", $"{datePattern} {timePattern}"
                })
                {
                    var pattern = LocalDateTimePattern.CreateWithInvariantCulture(patternStr).WithTemplateValue(midnight);
                    var res = pattern.Parse(str);
                    if (res.Success) return res.Value.InZoneLeniently(zone);
                }

        // Finally, just date patterns, still using midnight as the template
        foreach (var datePattern in datePatterns)
        {
            var pat = LocalDateTimePattern.CreateWithInvariantCulture(datePattern).WithTemplateValue(midnight);
            var res = pat.Parse(str);
            if (res.Success) return res.Value.InZoneLeniently(zone);
        }

        // Still haven't parsed something, we just give up lmao
        return null;
    }
}