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using System;
using System.Diagnostics;

namespace IStation.Numerics.LinearAlgebra.Solvers
{
    /// <summary>
    /// Defines an <see cref="IIterationStopCriterion{T}"/> that monitors residuals for NaN's.
    /// </summary>
    public sealed class FailureStopCriterion<T> : IIterationStopCriterion<T> where T : struct, IEquatable<T>, IFormattable
    {
        /// <summary>
        /// The status of the calculation
        /// </summary>
        IterationStatus _status = IterationStatus.Continue;

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

        /// <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 != residualVector.Count)
            {
                throw new ArgumentException("The array arguments must have the same length.");
            }

            if (_lastIteration >= iterationNumber)
            {
                // We have already stored the actual last iteration number
                // For now do nothing. We only care about the next step.
                return _status;
            }

            // Store the infinity norms of both the solution and residual vectors
            double residualNorm = residualVector.InfinityNorm();
            double solutionNorm = solutionVector.InfinityNorm();

            _status = double.IsNaN(solutionNorm) || double.IsNaN(residualNorm) ? IterationStatus.Failure : 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;
            _lastIteration = -1;
        }

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