DateUtil.java

import java.time.Duration;
import java.time.LocalDateTime;
import java.time.temporal.ChronoUnit;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.regex.MatchResult;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;

/**
 * Utility class for parsing dates.
 */
class DateUtil {

  /**
   * Regex matching an amount of some sort of time unit e.g. 4d see <a
   * href="https://regex101.com/r/ErO2oC/1/">Regex101</a> for mor info
   *
   * This regex has to groups the amount and the unit
   */
  public static final Pattern REGEX = Pattern.compile("(\\d)([yMwdhms])");

  /**
   * Converts a String formatted like 1M2d4m to an LocalDateTime containing the date in the future with the specified string added.
   *
   * @param input the string to parse
   * @return an {@link LocalDateTime} containing the amount of converted milliseconds or {@code null} if
   * the input was not formatted correctly
   * @throws NumberFormatException when the amount group of any input is not a valid number
   */
  @Nullable
  public static LocalDateTime parseDate(@NotNull String input) {
    List<MatchResult> allResults = findAll(input);
    if (allResults.isEmpty()) {
      return null;
    }
    LocalDateTime time = LocalDateTime.now();
    for(MatchResult matchResult : allResults) {
      int amount = Integer.parseInt(matchResult.group(1));
      switch (matchResult.group(2)) {
        case "y":
          time = time.plus(amount, ChronoUnit.YEARS);
          break;
        case "M":
          time = time.plus(amount, ChronoUnit.MONTHS);
          break;
        case "w":
          time = time.plus(amount, ChronoUnit.WEEKS);
          break;
        case "d":
          time = time.plus(amount, ChronoUnit.DAYS);
          break;
        case "h":
          time = time.plus(amount, ChronoUnit.HOURS);
          break;
        case "m":
          time = time.plus(amount, ChronoUnit.MINUTES);
          break;
        case "s":
          time = time.plus(amount, ChronoUnit.SECONDS);
          break;
      }
    }
    return time;
  }

  @NotNull
  private static List<MatchResult> findAll(@NotNull String input) {
    List<MatchResult> results = new ArrayList<>();
    Matcher matcher = DateUtil.REGEX.matcher(input);
    for (int i = 0; matcher.find(i); ) {
      MatchResult result = matcher.toMatchResult();
      i = result.end();
      results.add(result);
    }
    return Collections.unmodifiableList(results);
  }
}