
/*
 * Copyright (c) 2016 Vivid Solutions.
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License 2.0
 * and Eclipse Distribution License v. 1.0 which accompanies this distribution.
 * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
 * and the Eclipse Distribution License is available at
 *
 * http://www.eclipse.org/org/documents/edl-v10.php.
 */
package org.locationtech.jts.index.quadtree;

/**
 * DoubleBits manipulates Double numbers
 * by using bit manipulation and bit-field extraction.
 * For some operations (such as determining the exponent)
 * this is more accurate than using mathematical operations
 * (which suffer from round-off error).
 * <p>
 * The algorithms and constants in this class
 * apply only to IEEE-754 double-precision floating point format.
 *
 * @version 1.7
 */
public class DoubleBits {

  public static final int EXPONENT_BIAS = 1023;

  public static double powerOf2(int exp)
  {
    if (exp > 1023 || exp < -1022)
      throw new IllegalArgumentException("Exponent out of bounds");
    long expBias = exp + EXPONENT_BIAS;
    long bits = expBias << 52;
    return Double.longBitsToDouble(bits);
  }

  public static int exponent(double d)
  {
    DoubleBits db = new DoubleBits(d);
    return db.getExponent();
  }

  public static double truncateToPowerOfTwo(double d)
  {
    DoubleBits db = new DoubleBits(d);
    db.zeroLowerBits(52);
    return db.getDouble();
  }

  public static String toBinaryString(double d)
  {
    DoubleBits db = new DoubleBits(d);
    return db.toString();
  }

  public static double maximumCommonMantissa(double d1, double d2)
  {
    if (d1 == 0.0 || d2 == 0.0) return 0.0;

    DoubleBits db1 = new DoubleBits(d1);
    DoubleBits db2 = new DoubleBits(d2);

    if (db1.getExponent() != db2.getExponent()) return 0.0;

    int maxCommon = db1.numCommonMantissaBits(db2);
    db1.zeroLowerBits(64 - (12 + maxCommon));
    return db1.getDouble();
  }

  private double x;
  private long xBits;

  public DoubleBits(double x)
  {
    this.x = x;
    xBits = Double.doubleToLongBits(x);
  }

  public double getDouble()
  {
    return Double.longBitsToDouble(xBits);
  }

  /**
   * Determines the exponent for the number
   */
  public int biasedExponent()
  {
    int signExp = (int) (xBits >> 52);
    int exp = signExp & 0x07ff;
    return exp;
  }

  /**
   * Determines the exponent for the number
   */
  public int getExponent()
  {
    return biasedExponent() - EXPONENT_BIAS;
  }

  public void zeroLowerBits(int nBits)
  {
    long invMask = (1L << nBits) - 1L;
    long mask = ~ invMask;
    xBits &= mask;
  }

  public int getBit(int i)
  {
    long mask = (1L << i);
    return (xBits & mask) != 0 ? 1 : 0;
  }

  /**
   * This computes the number of common most-significant bits in the mantissa.
   * It does not count the hidden bit, which is always 1.
   * It does not determine whether the numbers have the same exponent - if they do
   * not, the value computed by this function is meaningless.
   * @param db
   * @return the number of common most-significant mantissa bits
   */
  public int numCommonMantissaBits(DoubleBits db)
  {
    for (int i = 0; i < 52; i++)
    {
      if (getBit(i) != db.getBit(i))
        return i;
    }
    return 52;
  }

  /**
   * A representation of the Double bits formatted for easy readability
   */
  public String toString()
  {
    String numStr = Long.toBinaryString(xBits);
    // 64 zeroes!
    String zero64 = "0000000000000000000000000000000000000000000000000000000000000000";
    String padStr =  zero64 + numStr;
    String bitStr = padStr.substring(padStr.length() - 64);
    String str = bitStr.substring(0, 1) + "  "
        + bitStr.substring(1, 12) + "(" + getExponent() + ") "
        + bitStr.substring(12)
        + " [ " + x + " ]";
    return str;
  }
}
