/*
    * 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
    *
    *      https://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.lang3;
  
  import java.util.Objects;
  import java.util.Random;
  
  /**
   * Generates random integers of specific bit length.
   *
   * <p>
   * It is more efficient than calling Random.nextInt(1 << nbBits). It uses a cache of cacheSize random bytes that it replenishes when it gets empty. This is
   * especially beneficial for SecureRandom Drbg implementations, which incur a constant cost at each randomness generation.
   * </p>
   *
   * <p>
   * Used internally by RandomStringUtils.
   * </p>
   *
   * <p>
   * #NotThreadSafe#
   * </p>
   */
  final class CachedRandomBits {
  
      /**
       * The maximum size of the cache.
       *
       * <p>
       * This is to prevent the possibility of overflow in the {@code if (bitIndex >> 3 >= cache.length)} in the {@link #nextBits(int)} method.
       * </p>
       */
      private static final int MAX_CACHE_SIZE = Integer.MAX_VALUE >> 3;
  
      /** Maximum number of bits that can be generated (size of an int) */
      private static final int MAX_BITS = 32;
  
      /** Mask to extract the bit offset within a byte (0-7) */
      private static final int BIT_INDEX_MASK = 0x7;
  
      /** Number of bits in a byte */
      private static final int BITS_PER_BYTE = 8;
      private final Random random;
      private final byte[] cache;
      /**
       * Index of the next bit in the cache to be used.
       *
       * <ul>
       * <li>bitIndex=0 means the cache is fully random and none of the bits have been used yet.</li>
       * <li>bitIndex=1 means that only the LSB of cache[0] has been used and all other bits can be used.</li>
       * <li>bitIndex=8 means that only the 8 bits of cache[0] has been used.</li>
       * </ul>
       */
      private int bitIndex;
      /**
       * Creates a new instance.
       *
       * @param cacheSize number of bytes cached (only affects performance)
       * @param random random source
       */
      CachedRandomBits(final int cacheSize, final Random random) {
          if (cacheSize <= 0) {
              throw new IllegalArgumentException("cacheSize must be positive");
          }
          this.cache = cacheSize <= MAX_CACHE_SIZE ? new byte[cacheSize] : new byte[MAX_CACHE_SIZE];
          this.random = Objects.requireNonNull(random, "random");
          this.random.nextBytes(this.cache);
          this.bitIndex = 0;
      }
  
      /**
       * Generates a random integer with the specified number of bits.
       *
       * <p>This method efficiently generates random bits by using a byte cache and bit manipulation:
       * <ul>
       *   <li>Uses a byte array cache to avoid frequent calls to the underlying random number generator</li>
       *   <li>Extracts bits from each byte using bit shifting and masking</li>
       *   <li>Handles partial bytes to avoid wasting random bits</li>
       *   <li>Accumulates bits until the requested number is reached</li>
       * </ul>
       * </p>
       *
       * @param bits number of bits to generate, MUST be between 1 and 32 (inclusive)
       * @return random integer containing exactly the requested number of random bits
       * @throws IllegalArgumentException if bits is not between 1 and 32
      */
     public int nextBits(final int bits) {
         if (bits > MAX_BITS || bits <= 0) {
             throw new IllegalArgumentException("number of bits must be between 1 and " + MAX_BITS);
         }
         int result = 0;
         int generatedBits = 0; // number of generated bits up to now
         while (generatedBits < bits) {
             // Check if we need to refill the cache
             // Convert bitIndex to byte index by dividing by 8 (right shift by 3)
             if (bitIndex >> 3 >= cache.length) {
                 // We exhausted the number of bits in the cache
                 // This should only happen if the bitIndex is exactly matching the cache length
                 assert bitIndex == cache.length * BITS_PER_BYTE;
                 random.nextBytes(cache);
                 bitIndex = 0;
             }
             // Calculate how many bits we can extract from the current byte
             // 1. Get current position within byte (0-7) using bitIndex & 0x7
             // 2. Calculate remaining bits in byte: 8 - (position within byte)
             // 3. Take minimum of remaining bits in byte and bits still needed
             final int generatedBitsInIteration = Math.min(
                 BITS_PER_BYTE - (bitIndex & BIT_INDEX_MASK),
                 bits - generatedBits);
             // Shift existing result left to make room for new bits
             result = result << generatedBitsInIteration;
             // Extract and append new bits:
             // 1. Get byte from cache (bitIndex >> 3 converts bit index to byte index)
             // 2. Shift right by bit position within byte (bitIndex & 0x7)
             // 3. Mask to keep only the bits we want ((1 << generatedBitsInIteration) - 1)
             result |= cache[bitIndex >> 3] >> (bitIndex & BIT_INDEX_MASK) & ((1 << generatedBitsInIteration) - 1);
             // Update counters
             generatedBits += generatedBitsInIteration;
             bitIndex += generatedBitsInIteration;
         }
         return result;
     }
 }