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Apache Commons Text is a library focused on algorithms working on strings.
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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.commons.text.lookup;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;

import org.apache.commons.lang3.time.FastDateFormat;
import org.apache.commons.text.StringSubstitutor;

/**
 * Formats the current date with the format given in the key in a format compatible with
 * {@link java.text.SimpleDateFormat}.
 * 
 * Using a {@link StringLookup} from the {@link StringLookupFactory}:
 * 
 *
 *  * StringLookupFactory.INSTANCE.dateStringLookup().lookup("yyyy-MM-dd");
 * 
 * 
 * Using a {@link StringSubstitutor}:
 * 
 *
 *  * StringSubstitutor.createInterpolator().replace("... ${date:yyyy-MM-dd} ..."));
 * 
 * 
 * The above examples convert {@code "yyyy-MM-dd"} to today's date, for example, {@code "2019-08-04"}.
 * 
 */
final class DateStringLookup extends AbstractStringLookup {

    /**
     * Defines the singleton for this class.
     */
    static final DateStringLookup INSTANCE = new DateStringLookup();

    /**
     * No need to build instances for now.
     */
    private DateStringLookup() {
        // empty
    }

    /**
     * Formats the given {@code date} long with the given {@code format}.
     *
     * @param dateMillis the date to format
     * @param format the format string for {@link SimpleDateFormat}.
     * @return The formatted date
     */
    private String formatDate(final long dateMillis, final String format) {
        FastDateFormat dateFormat = null;
        if (format != null) {
            try {
                dateFormat = FastDateFormat.getInstance(format);
            } catch (final Exception ex) {
                throw IllegalArgumentExceptions.format(ex, "Invalid date format: [%s]", format);
            }
        }
        if (dateFormat == null) {
            dateFormat = FastDateFormat.getInstance();
        }
        return dateFormat.format(new Date(dateMillis));
    }

    /**
     * Formats the current date with the format given in the key in a format compatible with
     * {@link java.text.SimpleDateFormat}.
     *
     * @param key the format to use. If null, the default {@link DateFormat} will be used.
     * @return The value of the environment variable.
     */
    @Override
    public String lookup(final String key) {
        return formatDate(System.currentTimeMillis(), key);
    }
}