package eu.dnetlib.dhp.oa.dedup;

import static java.util.Collections.reverseOrder;
import static java.util.Map.Entry.comparingByValue;
import static java.util.stream.Collectors.toMap;

import static org.apache.commons.lang.StringUtils.endsWith;
import static org.apache.commons.lang.StringUtils.substringBefore;

import java.time.Year;
import java.util.*;
import java.util.stream.Collectors;

import org.apache.commons.lang.StringUtils;

import eu.dnetlib.dhp.schema.oaf.Field;

public class DatePicker {

	public static final String DATE_PATTERN = "^(\\d{4})-(\\d{2})-(\\d{2})";
	private static final String DATE_DEFAULT_SUFFIX = "01-01";
	private static final int YEAR_LB = 1300;
	private static final int YEAR_UB = Year.now().getValue() + 5;

	public static Field<String> pick(final Collection<String> dateofacceptance) {

		final Map<String, Integer> frequencies = dateofacceptance
			.parallelStream()
			.filter(StringUtils::isNotBlank)
			.map(d -> substringBefore(d, "T"))
			.collect(Collectors.toConcurrentMap(w -> w, w -> 1, Integer::sum));

		if (frequencies.isEmpty()) {
			return new Field<>();
		}

		final Field<String> date = new Field<>();
		date.setValue(frequencies.keySet().iterator().next());

		// let's sort this map by values first, filtering out invalid dates
		final Map<String, Integer> sorted = frequencies
			.entrySet()
			.stream()
			.filter(d -> StringUtils.isNotBlank(d.getKey()))
			.filter(d -> d.getKey().matches(DATE_PATTERN))
			.filter(d -> inRange(d.getKey()))
			.sorted(reverseOrder(comparingByValue()))
			.collect(
				toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e2, LinkedHashMap::new));

		// shortcut
		if (sorted.size() == 0) {
			return date;
		}

		// voting method (1/3 + 1) wins
		if (sorted.size() >= 3) {
			final int acceptThreshold = (sorted.size() / 3) + 1;
			final List<String> accepted = sorted
				.entrySet()
				.stream()
				.filter(e -> e.getValue() >= acceptThreshold)
				.map(e -> e.getKey())
				.collect(Collectors.toList());

			// cannot find strong majority
			if (accepted.isEmpty()) {
				final int max = sorted.values().iterator().next();
				Optional<String> first = sorted
					.entrySet()
					.stream()
					.filter(e -> e.getValue() == max && !endsWith(e.getKey(), DATE_DEFAULT_SUFFIX))
					.map(Map.Entry::getKey)
					.findFirst();
				if (first.isPresent()) {
					date.setValue(first.get());
					return date;
				}

				date.setValue(sorted.keySet().iterator().next());
				return date;
			}

			if (accepted.size() == 1) {
				date.setValue(accepted.get(0));
				return date;
			} else {
				final Optional<String> first = accepted
					.stream()
					.filter(d -> !endsWith(d, DATE_DEFAULT_SUFFIX))
					.findFirst();
				if (first.isPresent()) {
					date.setValue(first.get());
					return date;
				}

				return date;
			}

			// 1st non YYYY-01-01 is returned
		} else {
			if (sorted.size() == 2) {
				for (Map.Entry<String, Integer> e : sorted.entrySet()) {
					if (!endsWith(e.getKey(), DATE_DEFAULT_SUFFIX)) {
						date.setValue(e.getKey());
						return date;
					}
				}
			}

			// none of the dates seems good enough, return the 1st one
			date.setValue(sorted.keySet().iterator().next());
			return date;
		}
	}

	public static boolean inRange(final String date) {
		final int year = Integer.parseInt(substringBefore(date, "-"));
		return year >= YEAR_LB && year <= YEAR_UB;
	}
}