682 /**
683 * Constructs a randomly generated BigInteger, uniformly distributed over
684 * the range 0 to (2<sup>{@code numBits}</sup> - 1), inclusive.
685 * The uniformity of the distribution assumes that a fair source of random
686 * bits is provided in {@code rnd}. Note that this constructor always
687 * constructs a non-negative BigInteger.
688 *
689 * @param numBits maximum bitLength of the new BigInteger.
690 * @param rnd source of randomness to be used in computing the new
691 * BigInteger.
692 * @throws IllegalArgumentException {@code numBits} is negative.
693 * @see #bitLength()
694 */
695 public BigInteger(int numBits, Random rnd) {
696 this(1, randomBits(numBits, rnd));
697 }
698
699 private static byte[] randomBits(int numBits, Random rnd) {
700 if (numBits < 0)
701 throw new IllegalArgumentException("numBits must be non-negative");
702 int numBytes = (int)(((long)numBits+7)/8); // avoid overflow
703 byte[] randomBits = new byte[numBytes];
704
705 // Generate random bytes and mask out any excess bits
706 if (numBytes > 0) {
707 rnd.nextBytes(randomBits);
708 int excessBits = 8*numBytes - numBits;
709 randomBits[0] &= (1 << (8-excessBits)) - 1;
710 }
711 return randomBits;
712 }
713
714 /**
715 * Constructs a randomly generated positive BigInteger that is probably
716 * prime, with the specified bitLength.
717 *
718 * @apiNote It is recommended that the {@link #probablePrime probablePrime}
719 * method be used in preference to this constructor unless there
720 * is a compelling need to specify a certainty.
721 *
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682 /**
683 * Constructs a randomly generated BigInteger, uniformly distributed over
684 * the range 0 to (2<sup>{@code numBits}</sup> - 1), inclusive.
685 * The uniformity of the distribution assumes that a fair source of random
686 * bits is provided in {@code rnd}. Note that this constructor always
687 * constructs a non-negative BigInteger.
688 *
689 * @param numBits maximum bitLength of the new BigInteger.
690 * @param rnd source of randomness to be used in computing the new
691 * BigInteger.
692 * @throws IllegalArgumentException {@code numBits} is negative.
693 * @see #bitLength()
694 */
695 public BigInteger(int numBits, Random rnd) {
696 this(1, randomBits(numBits, rnd));
697 }
698
699 private static byte[] randomBits(int numBits, Random rnd) {
700 if (numBits < 0)
701 throw new IllegalArgumentException("numBits must be non-negative");
702
703 // It is highly likely that the first, or one of the first few, bytes
704 // generated by Random.nextBytes() will be non-zero thereby making the
705 // distribution highly non-uniform by skewing it to the upper end of
706 // the interval [0, 2^numBits). To improve the uniformity of the
707 // distribution, adjust numBits to a random value in the interval
708 // [0, numBits].
709 if (numBits < Integer.MAX_VALUE) {
710 final int numBitsOriginal = numBits;
711 numBits = rnd.nextInt(numBits + 1);
712 // Increment the adjusted maximum bit length to help prevent the
713 // very largest numbers from being underrepresented.
714 if (numBits < numBitsOriginal) {
715 numBits++;
716 }
717 }
718
719 int numBytes = (int)(((long)numBits+7)/8); // avoid overflow
720 byte[] randomBits = new byte[numBytes];
721
722 // Generate random bytes and mask out any excess bits
723 if (numBytes > 0) {
724 rnd.nextBytes(randomBits);
725 int excessBits = 8*numBytes - numBits;
726 randomBits[0] &= (1 << (8-excessBits)) - 1;
727 }
728 return randomBits;
729 }
730
731 /**
732 * Constructs a randomly generated positive BigInteger that is probably
733 * prime, with the specified bitLength.
734 *
735 * @apiNote It is recommended that the {@link #probablePrime probablePrime}
736 * method be used in preference to this constructor unless there
737 * is a compelling need to specify a certainty.
738 *
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