// <copyright file="Distance.cs" company="Math.NET">
// Math.NET Numerics, part of the Math.NET Project
// http://numerics.mathdotnet.com
// http://github.com/mathnet/mathnet-numerics
//
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//
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using System;
using System.Collections.Generic;
using MathNet.Numerics.LinearAlgebra;
using MathNet.Numerics.Properties;
using MathNet.Numerics.Providers.LinearAlgebra;
using MathNet.Numerics.Statistics;

namespace MathNet.Numerics
{
    /// <summary>
    /// Metrics to measure the distance between two structures.
    /// </summary>
    public static class Distance
    {
        /// <summary>
        /// Sum of Absolute Difference (SAD), i.e. the L1-norm (Manhattan) of the difference.
        /// </summary>
        public static double SAD<T>(Vector<T> a, Vector<T> b) where T : struct, IEquatable<T>, IFormattable
        {
            return (a - b).L1Norm();
        }

        /// <summary>
        /// Sum of Absolute Difference (SAD), i.e. the L1-norm (Manhattan) of the difference.
        /// </summary>
        public static double SAD(double[] a, double[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            double sum = 0d;
            for (var i = 0; i < a.Length; i++)
            {
                sum += Math.Abs(a[i] - b[i]);
            }
            return sum;
        }

        /// <summary>
        /// Sum of Absolute Difference (SAD), i.e. the L1-norm (Manhattan) of the difference.
        /// </summary>
        public static float SAD(float[] a, float[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            float sum = 0f;
            for (var i = 0; i < a.Length; i++)
            {
                sum += Math.Abs(a[i] - b[i]);
            }
            return sum;
        }

        /// <summary>
        /// Mean-Absolute Error (MAE), i.e. the normalized L1-norm (Manhattan) of the difference.
        /// </summary>
        public static double MAE<T>(Vector<T> a, Vector<T> b) where T : struct, IEquatable<T>, IFormattable
        {
            return (a - b).L1Norm()/a.Count;
        }

        /// <summary>
        /// Mean-Absolute Error (MAE), i.e. the normalized L1-norm (Manhattan) of the difference.
        /// </summary>
        public static double MAE(double[] a, double[] b)
        {
            return SAD(a, b)/a.Length;
        }

        /// <summary>
        /// Mean-Absolute Error (MAE), i.e. the normalized L1-norm (Manhattan) of the difference.
        /// </summary>
        public static float MAE(float[] a, float[] b)
        {
            return SAD(a, b)/a.Length;
        }

        /// <summary>
        /// Sum of Squared Difference (SSD), i.e. the squared L2-norm (Euclidean) of the difference.
        /// </summary>
        public static double SSD<T>(Vector<T> a, Vector<T> b) where T : struct, IEquatable<T>, IFormattable
        {
            var norm = (a - b).L2Norm();
            return norm*norm;
        }

        /// <summary>
        /// Sum of Squared Difference (SSD), i.e. the squared L2-norm (Euclidean) of the difference.
        /// </summary>
        public static double SSD(double[] a, double[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            var diff = new double[a.Length];
            LinearAlgebraControl.Provider.SubtractArrays(a, b, diff);
            return LinearAlgebraControl.Provider.DotProduct(diff, diff);
        }

        /// <summary>
        /// Sum of Squared Difference (SSD), i.e. the squared L2-norm (Euclidean) of the difference.
        /// </summary>
        public static float SSD(float[] a, float[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            var diff = new float[a.Length];
            LinearAlgebraControl.Provider.SubtractArrays(a, b, diff);
            return LinearAlgebraControl.Provider.DotProduct(diff, diff);
        }

        /// <summary>
        /// Mean-Squared Error (MSE), i.e. the normalized squared L2-norm (Euclidean) of the difference.
        /// </summary>
        public static double MSE<T>(Vector<T> a, Vector<T> b) where T : struct, IEquatable<T>, IFormattable
        {
            var norm = (a - b).L2Norm();
            return norm*norm/a.Count;
        }

        /// <summary>
        /// Mean-Squared Error (MSE), i.e. the normalized squared L2-norm (Euclidean) of the difference.
        /// </summary>
        public static double MSE(double[] a, double[] b)
        {
            return SSD(a, b)/a.Length;
        }

        /// <summary>
        /// Mean-Squared Error (MSE), i.e. the normalized squared L2-norm (Euclidean) of the difference.
        /// </summary>
        public static float MSE(float[] a, float[] b)
        {
            return SSD(a, b)/a.Length;
        }

        /// <summary>
        /// Euclidean Distance, i.e. the L2-norm of the difference.
        /// </summary>
        public static double Euclidean<T>(Vector<T> a, Vector<T> b) where T : struct, IEquatable<T>, IFormattable
        {
            return (a - b).L2Norm();
        }

        /// <summary>
        /// Euclidean Distance, i.e. the L2-norm of the difference.
        /// </summary>
        public static double Euclidean(double[] a, double[] b)
        {
            return Math.Sqrt(SSD(a, b));
        }

        /// <summary>
        /// Euclidean Distance, i.e. the L2-norm of the difference.
        /// </summary>
        public static float Euclidean(float[] a, float[] b)
        {
            return (float) Math.Sqrt(SSD(a, b));
        }

        /// <summary>
        /// Manhattan Distance, i.e. the L1-norm of the difference.
        /// </summary>
        public static double Manhattan<T>(Vector<T> a, Vector<T> b) where T : struct, IEquatable<T>, IFormattable
        {
            return (a - b).L1Norm();
        }

        /// <summary>
        /// Manhattan Distance, i.e. the L1-norm of the difference.
        /// </summary>
        public static double Manhattan(double[] a, double[] b)
        {
            return SAD(a, b);
        }

        /// <summary>
        /// Manhattan Distance, i.e. the L1-norm of the difference.
        /// </summary>
        public static float Manhattan(float[] a, float[] b)
        {
            return SAD(a, b);
        }

        /// <summary>
        /// Chebyshev Distance, i.e. the Infinity-norm of the difference.
        /// </summary>
        public static double Chebyshev<T>(Vector<T> a, Vector<T> b) where T : struct, IEquatable<T>, IFormattable
        {
            return (a - b).InfinityNorm();
        }

        /// <summary>
        /// Chebyshev Distance, i.e. the Infinity-norm of the difference.
        /// </summary>
        public static double Chebyshev(double[] a, double[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            double max = Math.Abs(a[0] - b[0]);
            for (int i = 1; i < a.Length; i++)
            {
                var next = Math.Abs(a[i] - b[i]);
                if (next > max)
                {
                    max = next;
                }
            }
            return max;
        }

        /// <summary>
        /// Chebyshev Distance, i.e. the Infinity-norm of the difference.
        /// </summary>
        public static float Chebyshev(float[] a, float[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            float max = Math.Abs(a[0] - b[0]);
            for (int i = 1; i < a.Length; i++)
            {
                var next = Math.Abs(a[i] - b[i]);
                if (next > max)
                {
                    max = next;
                }
            }
            return max;
        }

        /// <summary>
        /// Minkowski Distance, i.e. the generalized p-norm of the difference.
        /// </summary>
        public static double Minkowski<T>(double p, Vector<T> a, Vector<T> b) where T : struct, IEquatable<T>, IFormattable
        {
            return (a - b).Norm(p);
        }

        /// <summary>
        /// Minkowski Distance, i.e. the generalized p-norm of the difference.
        /// </summary>
        public static double Minkowski(double p, double[] a, double[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            if (p < 0d)
            {
                throw new ArgumentOutOfRangeException(nameof(p));
            }

            if (p == 1d)
            {
                return Manhattan(a, b);
            }

            if (p == 2d)
            {
                return Euclidean(a, b);
            }

            if (double.IsPositiveInfinity(p))
            {
                return Chebyshev(a, b);
            }

            double sum = 0d;
            for (var i = 0; i < a.Length; i++)
            {
                sum += Math.Pow(Math.Abs(a[i] - b[i]), p);
            }
            return Math.Pow(sum, 1.0 / p);
        }

        /// <summary>
        /// Minkowski Distance, i.e. the generalized p-norm of the difference.
        /// </summary>
        public static float Minkowski(double p, float[] a, float[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            if (p < 0d)
            {
                throw new ArgumentOutOfRangeException(nameof(p));
            }

            if (p == 1d)
            {
                return Manhattan(a, b);
            }

            if (p == 2d)
            {
                return Euclidean(a, b);
            }

            if (double.IsPositiveInfinity(p))
            {
                return Chebyshev(a, b);
            }

            double sum = 0d;
            for (var i = 0; i < a.Length; i++)
            {
                sum += Math.Pow(Math.Abs(a[i] - b[i]), p);
            }
            return (float) Math.Pow(sum, 1.0/p);
        }

        /// <summary>
        /// Canberra Distance, a weighted version of the L1-norm of the difference.
        /// </summary>
        public static double Canberra(double[] a, double[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            double sum = 0d;
            for (var i = 0; i < a.Length; i++)
            {
                sum += Math.Abs(a[i] - b[i]) / (Math.Abs(a[i]) + Math.Abs(b[i]));
            }
            return sum;
        }

        /// <summary>
        /// Canberra Distance, a weighted version of the L1-norm of the difference.
        /// </summary>
        public static float Canberra(float[] a, float[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            float sum = 0f;
            for (var i = 0; i < a.Length; i++)
            {
                sum += Math.Abs(a[i] - b[i]) / (Math.Abs(a[i]) + Math.Abs(b[i]));
            }
            return sum;
        }

        /// <summary>
        /// Cosine Distance, representing the angular distance while ignoring the scale.
        /// </summary>
        public static double Cosine(double[] a, double[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            var ab = LinearAlgebraControl.Provider.DotProduct(a, b);
            var a2 = LinearAlgebraControl.Provider.DotProduct(a, a);
            var b2 = LinearAlgebraControl.Provider.DotProduct(b, b);
            return 1d - ab/Math.Sqrt(a2*b2);
        }

        /// <summary>
        /// Cosine Distance, representing the angular distance while ignoring the scale.
        /// </summary>
        public static float Cosine(float[] a, float[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            var ab = LinearAlgebraControl.Provider.DotProduct(a, b);
            var a2 = LinearAlgebraControl.Provider.DotProduct(a, a);
            var b2 = LinearAlgebraControl.Provider.DotProduct(b, b);
            return (float)(1d - ab/Math.Sqrt(a2*b2));
        }

        /// <summary>
        /// Hamming Distance, i.e. the number of positions that have different values in the vectors.
        /// </summary>
        public static double Hamming(double[] a, double[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            int count = 0;
            for (int i = 0; i < a.Length; i++)
            {
                if (a[i] != b[i])
                {
                    count++;
                }
            }
            return count;
        }

        /// <summary>
        /// Hamming Distance, i.e. the number of positions that have different values in the vectors.
        /// </summary>
        public static float Hamming(float[] a, float[] b)
        {
            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            int count = 0;
            for (int i = 0; i < a.Length; i++)
            {
                if (a[i] != b[i])
                {
                    count++;
                }
            }
            return count;
        }

        /// <summary>
        /// Pearson's distance, i.e. 1 - the person correlation coefficient.
        /// </summary>
        public static double Pearson(IEnumerable<double> a, IEnumerable<double> b)
        {
            return 1.0 - Correlation.Pearson(a, b);
        }

        /// <summary>
        /// Jaccard distance, i.e. 1 - the Jaccard index.
        /// </summary>
        /// <exception cref="ArgumentNullException">Thrown if a or b are null.</exception>
        /// <exception cref="ArgumentException">Throw if a and b are of different lengths.</exception>
        /// <returns>Jaccard distance.</returns>
        public static double Jaccard(double[] a, double[] b)
        {
            int intersection = 0, union = 0;

            if (a == null)
            {
                throw new ArgumentNullException(nameof(a));
            }

            if (b == null)
            {
                throw new ArgumentNullException(nameof(b));
            }

            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            if (a.Length == 0 && b.Length == 0)
            {
                return 0;
            }

            for (int x = 0, len = a.Length; x < len; x++)
            {
                if (a[x] != 0 && b[x] != 0)
                {
                    if (a[x] == b[x])
                    {
                        intersection++;
                    }

                    union++;
                }
            }

            return 1.0 - ((double)intersection / (double)union);
        }

        /// <summary>
        /// Jaccard distance, i.e. 1 - the Jaccard index.
        /// </summary>
        /// <exception cref="ArgumentNullException">Thrown if a or b are null.</exception>
        /// <exception cref="ArgumentException">Throw if a and b are of different lengths.</exception>
        /// <returns>Jaccard distance.</returns>
        public static double Jaccard(float[] a, float[] b)
        {
            int intersection = 0, union = 0;

            if (a == null)
            {
                throw new ArgumentNullException(nameof(a));
            }

            if (b == null)
            {
                throw new ArgumentNullException(nameof(b));
            }

            if (a.Length != b.Length)
            {
                throw new ArgumentException(Resources.ArgumentVectorsSameLength);
            }

            if (a.Length == 0 && b.Length == 0)
            {
                return 0;
            }

            for (int x = 0, len = a.Length; x < len; x++)
            {
                if (a[x] != 0 && b[x] != 0)
                {
                    if (a[x] == b[x])
                    {
                        intersection++;
                    }

                    union++;
                }
            }

            return 1.0 - ((float)intersection / (float)union);
        }
    }
}
