// <copyright file="ResidualStopCriterion.cs" company="Math.NET">
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using System;
using System.Diagnostics;

namespace IStation.Numerics.LinearAlgebra.Solvers
{
    /// <summary>
    /// Defines an <see cref="IIterationStopCriterion{T}"/> that monitors residuals as stop criterion.
    /// </summary>
    public sealed class ResidualStopCriterion<T> : IIterationStopCriterion<T> where T : struct, IEquatable<T>, IFormattable
    {
        /// <summary>
        /// The maximum value for the residual below which the calculation is considered converged.
        /// </summary>
        double _maximum;

        /// <summary>
        /// The minimum number of iterations for which the residual has to be below the maximum before
        /// the calculation is considered converged.
        /// </summary>
        int _minimumIterationsBelowMaximum;

        /// <summary>
        /// The status of the calculation
        /// </summary>
        IterationStatus _status = IterationStatus.Continue;

        /// <summary>
        /// The number of iterations since the residuals got below the maximum.
        /// </summary>
        int _iterationCount;

        /// <summary>
        /// The iteration number of the last iteration.
        /// </summary>
        int _lastIteration = -1;

        /// <summary>
        /// Initializes a new instance of the <see cref="ResidualStopCriterion{T}"/> class with the specified
        /// maximum residual and minimum number of iterations.
        /// </summary>
        /// <param name="maximum">
        /// The maximum value for the residual below which the calculation is considered converged.
        /// </param>
        /// <param name="minimumIterationsBelowMaximum">
        /// The minimum number of iterations for which the residual has to be below the maximum before
        /// the calculation is considered converged.
        /// </param>
        public ResidualStopCriterion(double maximum, int minimumIterationsBelowMaximum = 0)
        {
            if (maximum < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(maximum));
            }

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

            _maximum = maximum;
            _minimumIterationsBelowMaximum = minimumIterationsBelowMaximum;
        }

        /// <summary>
        /// Gets or sets the maximum value for the residual below which the calculation is considered
        /// converged.
        /// </summary>
        /// <exception cref="ArgumentOutOfRangeException">Thrown if the <c>Maximum</c> is set to a negative value.</exception>
        public double Maximum
        {
            [DebuggerStepThrough]
            get => _maximum;

            [DebuggerStepThrough]
            set
            {
                if (value < 0)
                {
                    throw new ArgumentOutOfRangeException(nameof(value));
                }

                _maximum = value;
            }
        }

        /// <summary>
        /// Gets or sets the minimum number of iterations for which the residual has to be
        /// below the maximum before the calculation is considered converged.
        /// </summary>
        /// <exception cref="ArgumentOutOfRangeException">Thrown if the <c>BelowMaximumFor</c> is set to a value less than 1.</exception>
        public int MinimumIterationsBelowMaximum
        {
            [DebuggerStepThrough]
            get => _minimumIterationsBelowMaximum;

            [DebuggerStepThrough]
            set
            {
                if (value < 0)
                {
                    throw new ArgumentOutOfRangeException(nameof(value));
                }

                _minimumIterationsBelowMaximum = value;
            }
        }

        /// <summary>
        /// Determines the status of the iterative calculation based on the stop criteria stored
        /// by the current <see cref="IIterationStopCriterion{T}"/>. Result is set into <c>Status</c> field.
        /// </summary>
        /// <param name="iterationNumber">The number of iterations that have passed so far.</param>
        /// <param name="solutionVector">The vector containing the current solution values.</param>
        /// <param name="sourceVector">The right hand side vector.</param>
        /// <param name="residualVector">The vector containing the current residual vectors.</param>
        /// <remarks>
        /// The individual stop criteria may internally track the progress of the calculation based
        /// on the invocation of this method. Therefore this method should only be called if the
        /// calculation has moved forwards at least one step.
        /// </remarks>
        public IterationStatus DetermineStatus(int iterationNumber, Vector<T> solutionVector, Vector<T> sourceVector, Vector<T> residualVector)
        {
            if (iterationNumber < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(iterationNumber));
            }

            if (solutionVector.Count != sourceVector.Count)
            {
                throw new ArgumentException("All vectors must have the same dimensionality.", nameof(sourceVector));
            }

            if (solutionVector.Count != residualVector.Count)
            {
                throw new ArgumentException("All vectors must have the same dimensionality.", nameof(residualVector));
            }


            // Store the infinity norms of both the solution and residual vectors
            // These values will be used to calculate the relative drop in residuals
            // later on.
            var residualNorm = residualVector.InfinityNorm();


            // This is criterion 1 from Templates for the solution of linear systems.
            // The problem with this criterion is that it's not limiting enough. For now
            // we won't use it. Later on we might get back to it.
            // return mMaximumResidual * (System.Math.Abs(mMatrixNorm) * System.Math.Abs(solutionNorm) + System.Math.Abs(mVectorNorm));

            // For now use criterion 2 from Templates for the solution of linear systems. See page 60.

            // Check the residuals by calculating:
            // ||r_i|| <= stop_tol * ||b||
            var stopCriterion = _maximum*sourceVector.InfinityNorm();


            // First check that we have real numbers not NaN's.
            // NaN's can occur when the iterative process diverges so we
            // stop if that is the case.
            if (double.IsNaN(stopCriterion) || double.IsNaN(residualNorm))
            {
                _iterationCount = 0;
                _status = IterationStatus.Diverged;
                return _status;
            }

            // ||r_i|| <= stop_tol * ||b||
            // Stop the calculation if it's clearly smaller than the tolerance
            if (residualNorm < stopCriterion)
            {
                if (_lastIteration <= iterationNumber)
                {
                    _iterationCount = iterationNumber - _lastIteration;
                    _status = _iterationCount >= _minimumIterationsBelowMaximum ? IterationStatus.Converged : IterationStatus.Continue;
                }
            }
            else
            {
                _iterationCount = 0;
                _status = IterationStatus.Continue;
            }

            _lastIteration = iterationNumber;
            return _status;
        }

        /// <summary>
        /// Gets the current calculation status.
        /// </summary>
        public IterationStatus Status
        {
            [DebuggerStepThrough]
            get => _status;
        }

        /// <summary>
        /// Resets the <see cref="IIterationStopCriterion{T}"/> to the pre-calculation state.
        /// </summary>
        public void Reset()
        {
            _status = IterationStatus.Continue;
            _iterationCount = 0;
            _lastIteration = -1;
        }

        /// <summary>
        /// Clones the current <see cref="ResidualStopCriterion{T}"/> and its settings.
        /// </summary>
        /// <returns>A new instance of the <see cref="ResidualStopCriterion{T}"/> class.</returns>
        public IIterationStopCriterion<T> Clone()
        {
            return new ResidualStopCriterion<T>(_maximum, _minimumIterationsBelowMaximum);
        }
    }
}