ULID.java

/*
 * The MIT License (MIT)
 *
 * Copyright (c) 2016 Lachlan Dowding
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

package permafrost.tundra.id;

/**
 * A Java implementation of ULID, as per https://github.com/alizain/ulid.
 */
public class ULID {
    /**
     * Table of characters used to base32-encode the ULID time and random components.
     */
    private static final char[] ENCODE_TABLE = {
        '0','1','2','3','4','5','6','7','8','9',
        'a','b','c','d','e','f','g','h','j','k',
        'm','n','p','q','r','s','t','v','w','x',
        'y','z'
    };
    /**
     * The number of characters in the ULID devoted to the time component.
     */
    private static final int TIME_LENGTH = 10;
    /**
     * The number of characters in the ULID devoted to the random component.
     */
    private static final int RANDOM_LENGTH = 16;
    /**
     * The multiplication factor used for 40-bits of randomness.
     */
    private static final double RANDOM_MULTIPLIER = Math.pow(2, 40);

    /**
     * Returns a newly generated ULID.
     *
     * @return A newly generated ULID.
     */
    public static String generate() {
        StringBuilder buffer = new StringBuilder();
        encodeRandom(buffer);
        encodeTime(buffer);
        return buffer.reverse().toString();
    }

    /**
     * Base-32 encodes the given value for the given length into the given buffer.
     *
     * @param buffer    The buffer to encode the value into.
     * @param value     The value to be encoded.
     * @param length    The number of base-32 characters the value is to be represented by.
     */
    private static void encode(StringBuilder buffer, long value, int length) {
        for (int i = 0; i < length; i++) {
            int remainder = (int)(value % ENCODE_TABLE.length);
            value = (value - remainder) / ENCODE_TABLE.length;
            buffer.append(ENCODE_TABLE[remainder]);
        }
    }

    /**
     * Encode the current time into this ULID's string buffer.
     *
     * @param buffer    The buffer into which the time is encoded.
     */
    private static void encodeTime(StringBuilder buffer) {
        encode(buffer, System.currentTimeMillis(), TIME_LENGTH);
    }

    /**
     * Encodes a randomness component into this ULID's string buffer.
     *
     * @param buffer    The buffer into which the random number is encoded.
     */
    private static void encodeRandom(StringBuilder buffer) {
        encode(buffer, (long)(Math.random() * RANDOM_MULTIPLIER), RANDOM_LENGTH/2);
        encode(buffer, (long)(Math.random() * RANDOM_MULTIPLIER), RANDOM_LENGTH/2);
    }
}