Method bigToDouble

output/java_guava/1.4.17/DoubleUtils.java:94–136

(BigInteger x)

  static double bigToDouble(BigInteger x) {
    // This is an extremely fast implementation of BigInteger.doubleValue(). JDK patch pending.
    BigInteger absX = x.abs();
    int exponent = absX.bitLength() - 1;
    // exponent == floor(log2(abs(x)))
    if (exponent < Long.SIZE - 1) {
      return x.longValue();
    } else if (exponent > MAX_EXPONENT) {
      return x.signum() * POSITIVE_INFINITY;
    }

    /*
     * We need the top SIGNIFICAND_BITS + 1 bits, including the "implicit" one bit. To make rounding
     * easier, we pick out the top SIGNIFICAND_BITS + 2 bits, so we have one to help us round up or
     * down. twiceSignifFloor will contain the top SIGNIFICAND_BITS + 2 bits, and signifFloor the
     * top SIGNIFICAND_BITS + 1.
     *
     * It helps to consider the real number signif = absX * 2^(SIGNIFICAND_BITS - exponent).
     */
    int shift = exponent - SIGNIFICAND_BITS - 1;
    long twiceSignifFloor = absX.shiftRight(shift).longValue();
    long signifFloor = twiceSignifFloor >> 1;
    signifFloor &= SIGNIFICAND_MASK; // remove the implied bit

    /*
     * We round up if either the fractional part of signif is strictly greater than 0.5 (which is
     * true if the 0.5 bit is set and any lower bit is set), or if the fractional part of signif is
     * >= 0.5 and signifFloor is odd (which is true if both the 0.5 bit and the 1 bit are set).
     */
    boolean increment = (twiceSignifFloor & 1) != 0
&& ((signifFloor & 1) != 0 || absX.getLowestSetBit() < shift);
    long signifRounded = increment? signifFloor + 1 : signifFloor;
    long bits = (long) ((exponent + EXPONENT_BIAS)) << SIGNIFICAND_BITS;
    bits += signifRounded;
    /*
     * If signifRounded == 2^53, we'd need to set all of the significand bits to zero and add 1 to
     * the exponent. This is exactly the behavior we get from just adding signifRounded to bits
     * directly. If the exponent is MAX_DOUBLE_EXPONENT, we round up (correctly) to
     * Double.POSITIVE_INFINITY.
     */
    bits |= x.signum() & SIGN_MASK;
    return longBitsToDouble(bits);
  }

  /**
   * Returns its argument if it is non-negative, zero if it is negative.