/**
 * 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.waveprotocol.box.server.util;

import com.google.common.annotations.VisibleForTesting;

import java.security.SecureRandom;
import java.util.Random;

/**
 * Produces sequence of pseudo-random id strings.
 * Thread-safe.
 *
 *
 */
public class RandomBase64Generator {

  /** The 64 valid web-safe characters. */
  @VisibleForTesting static final char[] WEB64_ALPHABET =
      "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
      .toCharArray();

  private final Random random;

  /**
   * @param random Pseudo-random generator,
   *        use Random for speed, SecureRandom for cryptographic strength.
   */
  public RandomBase64Generator(Random random) {
    this.random = random;
  }

  /**
   * Default constructor using Random.
   */
  public RandomBase64Generator() {
    this(new SecureRandom());
  }

  /**
   * @param length The requested number of random base 64 characters.
   * @return string with length many random base 64 characters.
   */
  public String next(int length) {
    StringBuilder result = new StringBuilder(length);
    int bits = 0;
    int bitCount = 0;
    while (result.length() < length) {
      if (bitCount < 6) {
        bits = random.nextInt();
        bitCount = 32;
      }
      result.append(WEB64_ALPHABET[bits & 0x3F]);
      bits >>= 6;
      bitCount -= 6;
    }
    return result.toString();
  }
}