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The set of RFC 3339 compliant date/time scalar types for GraphQL Java implementation

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/*
 *  Copyright 2017 Alexey Zhokhov
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */
package com.zhokhov.graphql.datetime;

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Objects;

import static com.zhokhov.graphql.datetime.DateTimeHelper.DATE_FORMATTERS;

/**
 * @author Alexey Zhokhov
 */
class LocalDateTimeConverter {

    private final boolean zoneConversionEnabled;

    LocalDateTimeConverter(boolean zoneConversionEnabled) {
        this.zoneConversionEnabled = zoneConversionEnabled;
    }

    String formatDate(LocalDate date, DateTimeFormatter formatter) {
        Objects.requireNonNull(date, "date");
        Objects.requireNonNull(formatter, "formatter");

        return formatter.format(date);
    }

    String formatDate(LocalDateTime dateTime, DateTimeFormatter formatter) {
        Objects.requireNonNull(dateTime, "dateTime");
        Objects.requireNonNull(formatter, "formatter");

        return formatter.format(toUTC(dateTime));
    }

    LocalDateTime parseDate(String date) {
        Objects.requireNonNull(date, "date");
        for (DateTimeFormatter formatter : DATE_FORMATTERS) {
            // try to parse as dateTime
            try {
                LocalDateTime dateTime = LocalDateTime.parse(date, formatter);
                return fromUTC(dateTime);
            } catch (java.time.format.DateTimeParseException ignored) {
            }

            // try to parse as date
            try {
                // equals ISO_LOCAL_DATE or custom date format
                LocalDate localDate = LocalDate.parse(date, formatter);
                return localDate.atStartOfDay();
            } catch (java.time.format.DateTimeParseException ignored) {
            }
        }

        return null;
    }

    private LocalDateTime convert(LocalDateTime dateTime, ZoneId from, ZoneId to) {
        if (zoneConversionEnabled) {
            return dateTime.atZone(from).withZoneSameInstant(to).toLocalDateTime();
        }
        return dateTime;
    }

    private LocalDateTime fromUTC(LocalDateTime dateTime) {
        return convert(dateTime, ZoneOffset.UTC, ZoneId.systemDefault());
    }

    private ZonedDateTime toUTC(LocalDateTime dateTime) {
        return ZonedDateTime.of(convert(dateTime, ZoneId.systemDefault(), ZoneOffset.UTC), ZoneOffset.UTC);
    }

}