#region CPL License
/*
Nuclex Framework
Copyright (C) 2002-2014 Nuclex Development Labs

This library is free software; you can redistribute it and/or
modify it under the terms of the IBM Common Public License as
published by the IBM Corporation; either version 1.0 of the
License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
IBM Common Public License for more details.

You should have received a copy of the IBM Common Public
License along with this library
*/
#endregion

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace Nuclex.Support {

  /// <summary>Helper routines for working with floating point numbers</summary>
  /// <remarks>
  ///   <para>
  ///     The floating point comparison code is based on this excellent article:
  ///     http://www.cygnus-software.com/papers/comparingfloats/comparingfloats.htm
  ///   </para>
  ///   <para>
  ///     "ULP" means Unit in the Last Place and in the context of this library refers to
  ///     the distance between two adjacent floating point numbers. IEEE floating point
  ///     numbers can only represent a finite subset of natural numbers, with greater
  ///     accuracy for smaller numbers and lower accuracy for very large numbers.
  ///   </para>
  ///   <para>
  ///     If a comparison is allowed "2 ulps" of deviation, that means the values are
  ///     allowed to deviate by up to 2 adjacent floating point values, which might be
  ///     as low as 0.0000001 for small numbers or as high as 10.0 for large numbers.
  ///   </para>
  /// </remarks>
  public static class FloatHelper {

    #region struct FloatIntUnion

    /// <summary>Union of a floating point variable and an integer</summary>
    [StructLayout(LayoutKind.Explicit)]
    private struct FloatIntUnion {

      /// <summary>The union's value as a floating point variable</summary>
      [FieldOffset(0)]
      public float Float;

      /// <summary>The union's value as an integer</summary>
      [FieldOffset(0)]
      public int Int;

      /// <summary>The union's value as an unsigned integer</summary>
      [FieldOffset(0)]
      public uint UInt;

    }

    #endregion // struct FloatIntUnion

    #region struct DoubleLongUnion

    /// <summary>Union of a double precision floating point variable and a long</summary>
    [StructLayout(LayoutKind.Explicit)]
    private struct DoubleLongUnion {

      /// <summary>The union's value as a double precision floating point variable</summary>
      [FieldOffset(0)]
      public double Double;

      /// <summary>The union's value as a long</summary>
      [FieldOffset(0)]
      public long Long;

      /// <summary>The union's value as an unsigned long</summary>
      [FieldOffset(0)]
      public ulong ULong;

    }

    #endregion // struct DoubleLongUnion

    /// <summary>A floating point value that holds a positive zero</summary>
    public const float PositiveZeroFloat = +0.0f;

    /// <summary>A floating point value that holds a negative zero</summary>
    /// <remarks>
    ///   Negative zeros have a special representation in IEEE 752 floating point math
    /// </remarks>
    public const float NegativeZeroFloat = -0.0f;

    /// <summary>A double precision floating point value that holds a positive zero</summary>
    public const double PositiveZeroDouble = +0.0;

    /// <summary>A doublep precision floating point value that holds a negative zero</summary>
    /// <remarks>
    ///   Negative zeros have a special representation in IEEE 752 floating point math
    /// </remarks>
    public const double NegativeZeroDouble = -0.0;

    /// <summary>Checks whether the floating point value is exactly zero</summary>
    /// <param name="value">Value that will be checked for being zero</param>
    /// <returns>True if the value is zero, false otherwise</returns>
    public static bool IsZero(float value) {
      return (value == PositiveZeroFloat) || (value == NegativeZeroFloat);
    }

    /// <summary>
    ///   Checks whether the double precision floating point value is exactly zero
    /// </summary>
    /// <param name="value">Value that will be checked for being zero</param>
    /// <returns>True if the value is zero, false otherwise</returns>
    public static bool IsZero(double value) {
      return (value == PositiveZeroDouble) || (value == NegativeZeroDouble);
    }

    /// <summary>Compares two floating point values for equality</summary>
    /// <param name="left">First floating point value to be compared</param>
    /// <param name="right">Second floating point value t be compared</param>
    /// <param name="maxUlps">
    ///   Maximum number of representable floating point values that are allowed to
    ///   be between the left and the right floating point values
    /// </param>
    /// <returns>True if both numbers are equal or close to being equal</returns>
    /// <remarks>
    ///   <para>
    ///     Floating point values can only represent a finite subset of natural numbers.
    ///     For example, the values 2.00000000 and 2.00000024 can be stored in a float,
    ///     but nothing inbetween them.
    ///   </para>
    ///   <para>
    ///     This comparison will count how many possible floating point values are between
    ///     the left and the right number. If the number of possible values between both
    ///     numbers is less than or equal to maxUlps, then the numbers are considered as
    ///     being equal.
    ///   </para>
    ///   <para>
    ///     Implementation partially follows the code outlined here (link now defunct):
    ///     http://www.anttirt.net/2007/08/19/proper-floating-point-comparisons/
    ///     And here:
    ///     http://www.altdevblogaday.com/2012/02/22/comparing-floating-point-numbers-2012-edition/
    ///   </para>
    /// </remarks>
    public static bool AreAlmostEqual(float left, float right, int maxUlps) {
      var leftUnion = new FloatIntUnion();
      var rightUnion = new FloatIntUnion();

      leftUnion.Float = left;
      rightUnion.Float = right;

      if(leftUnion.Int < 0) {
        leftUnion.Int = unchecked((int)0x80000000 - leftUnion.Int);
      }
      if(rightUnion.Int < 0) {
        rightUnion.Int = unchecked((int)0x80000000 - rightUnion.Int);
      }

      return Math.Abs(rightUnion.Int - leftUnion.Int) <= maxUlps;
    }
#if false
    public static bool OldAreAlmostEqual(float left, float right, int maxUlps) {
      FloatInt32Union leftUnion = new FloatInt32Union();
      FloatInt32Union rightUnion = new FloatInt32Union();

      leftUnion.Float = left;
      rightUnion.Float = right;

      uint leftSignMask = (leftUnion.UInt >> 31);
      uint rightSignMask = (rightUnion.UInt >> 31);

      uint leftTemp = ((0x80000000 - leftUnion.UInt) & leftSignMask);
      leftUnion.UInt = leftTemp | (leftUnion.UInt & ~leftSignMask);

      uint rightTemp = ((0x80000000 - rightUnion.UInt) & rightSignMask);
      rightUnion.UInt = rightTemp | (rightUnion.UInt & ~rightSignMask);

      return (Math.Abs(leftUnion.Int - rightUnion.Int) <= maxUlps);
    }
#endif

    /// <summary>Compares two double precision floating point values for equality</summary>
    /// <param name="left">First double precision floating point value to be compared</param>
    /// <param name="right">Second double precision floating point value t be compared</param>
    /// <param name="maxUlps">
    ///   Maximum number of representable double precision floating point values that are
    ///   allowed to be between the left and the right double precision floating point values
    /// </param>
    /// <returns>True if both numbers are equal or close to being equal</returns>
    /// <remarks>
    ///   <para>
    ///     Double precision floating point values can only represent a limited series of
    ///     natural numbers. For example, the values 2.0000000000000000 and 2.0000000000000004
    ///     can be stored in a double, but nothing inbetween them.
    ///   </para>
    ///   <para>
    ///     This comparison will count how many possible double precision floating point
    ///     values are between the left and the right number. If the number of possible
    ///     values between both numbers is less than or equal to maxUlps, then the numbers
    ///     are considered as being equal.
    ///   </para>
    ///   <para>
    ///     Implementation partially follows the code outlined here:
    ///     http://www.anttirt.net/2007/08/19/proper-floating-point-comparisons/
    ///     And here:
    ///     http://www.altdevblogaday.com/2012/02/22/comparing-floating-point-numbers-2012-edition/
    ///   </para>
    /// </remarks>
    public static bool AreAlmostEqual(double left, double right, long maxUlps) {
      var leftUnion = new DoubleLongUnion();
      var rightUnion = new DoubleLongUnion();

      leftUnion.Double = left;
      rightUnion.Double = right;

      if(leftUnion.Long < 0) {
        leftUnion.Long = unchecked((long)0x8000000000000000 - leftUnion.Long);
      }
      if(rightUnion.Long < 0) {
        rightUnion.Long = unchecked((long)0x8000000000000000 - rightUnion.Long);
      }

      return Math.Abs(rightUnion.Long - leftUnion.Long) <= maxUlps;
    }
#if false
    public static bool OldAreAlmostEqual(double left, double right, long maxUlps) {
      DoubleInt64Union leftUnion = new DoubleInt64Union();
      DoubleInt64Union rightUnion = new DoubleInt64Union();

      leftUnion.Double = left;
      rightUnion.Double = right;

      ulong leftSignMask = (leftUnion.ULong >> 63);
      ulong rightSignMask = (rightUnion.ULong >> 63);

      ulong leftTemp = ((0x8000000000000000 - leftUnion.ULong) & leftSignMask);
      leftUnion.ULong = leftTemp | (leftUnion.ULong & ~leftSignMask);

      ulong rightTemp = ((0x8000000000000000 - rightUnion.ULong) & rightSignMask);
      rightUnion.ULong = rightTemp | (rightUnion.ULong & ~rightSignMask);

      return (Math.Abs(leftUnion.Long - rightUnion.Long) <= maxUlps);
    }
#endif

    /// <summary>
    ///   Reinterprets the memory contents of a floating point value as an integer value
    /// </summary>
    /// <param name="value">
    ///   Floating point value whose memory contents to reinterpret
    /// </param>
    /// <returns>
    ///   The memory contents of the floating point value interpreted as an integer
    /// </returns>
    public static int ReinterpretAsInt(this float value) {
      FloatIntUnion union = new FloatIntUnion();
      union.Float = value;
      return union.Int;
    }

    /// <summary>
    ///   Reinterprets the memory contents of a double precision floating point
    ///   value as an integer value
    /// </summary>
    /// <param name="value">
    ///   Double precision floating point value whose memory contents to reinterpret
    /// </param>
    /// <returns>
    ///   The memory contents of the double precision floating point value
    ///   interpreted as an integer
    /// </returns>
    public static long ReinterpretAsLong(this double value) {
      DoubleLongUnion union = new DoubleLongUnion();
      union.Double = value;
      return union.Long;
    }

    /// <summary>
    ///   Reinterprets the memory contents of an integer as a floating point value
    /// </summary>
    /// <param name="value">Integer value whose memory contents to reinterpret</param>
    /// <returns>
    ///   The memory contents of the integer value interpreted as a floating point value
    /// </returns>
    public static float ReinterpretAsFloat(this int value) {
      FloatIntUnion union = new FloatIntUnion();
      union.Int = value;
      return union.Float;
    }

    /// <summary>
    ///   Reinterprets the memory contents of an integer value as a double precision
    ///   floating point value
    /// </summary>
    /// <param name="value">Integer whose memory contents to reinterpret</param>
    /// <returns>
    ///   The memory contents of the integer interpreted as a double precision
    ///   floating point value
    /// </returns>
    public static double ReinterpretAsDouble(this long value) {
      DoubleLongUnion union = new DoubleLongUnion();
      union.Long = value;
      return union.Double;
    }

  }

} // namespace Nuclex.Support