// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
//
// DoubleMinMaxAggregationOperator.cs
//
// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
 
using System.Collections.Generic;
using System.Diagnostics;
using System.Threading;
 
namespace System.Linq.Parallel
{
    /// <summary>
    /// An inlined min/max aggregation and its enumerator, for doubles.
    ///
    /// Notes:
    ///     Note that normally double.NaN &lt; anything is false, as is anything &lt; NaN.  This would
    ///     lead to some strangeness in Min and Max, e.g. Min({ NaN, 5.0 } == NaN, yet
    ///     Min({ 5.0, NaN }) == 5.0!  We impose a total ordering so that NaN is smaller than
    ///     everything, including -infinity, which is consistent with Comparer_T.
    /// </summary>
    internal sealed class DoubleMinMaxAggregationOperator : InlinedAggregationOperator<double, double, double>
    {
        private readonly int _sign; // The sign (-1 for min, 1 for max).
 
        //---------------------------------------------------------------------------------------
        // Constructs a new instance of a min/max associative operator.
        //
 
        internal DoubleMinMaxAggregationOperator(IEnumerable<double> child, int sign) : base(child)
        {
            Debug.Assert(sign == -1 || sign == 1, "invalid sign");
            _sign = sign;
        }
 
        //---------------------------------------------------------------------------------------
        // Executes the entire query tree, and aggregates the intermediate results into the
        // final result based on the binary operators and final reduction.
        //
        // Return Value:
        //     The single result of aggregation.
        //
 
        protected override double InternalAggregate(ref Exception? singularExceptionToThrow)
        {
            // Because the final reduction is typically much cheaper than the intermediate
            // reductions over the individual partitions, and because each parallel partition
            // will do a lot of work to produce a single output element, we prefer to turn off
            // pipelining, and process the final reductions serially.
            using (IEnumerator<double> enumerator = GetEnumerator(ParallelMergeOptions.FullyBuffered, true))
            {
                // Throw an error for empty results.
                if (!enumerator.MoveNext())
                {
                    singularExceptionToThrow = new InvalidOperationException(SR.NoElements);
                    return default(double);
                }
 
                double best = enumerator.Current;
 
                // Based on the sign, do either a min or max reduction.
                if (_sign == -1)
                {
                    while (enumerator.MoveNext())
                    {
                        double current = enumerator.Current;
                        if (current < best || double.IsNaN(current))
                        {
                            best = current;
                        }
                    }
                }
                else
                {
                    while (enumerator.MoveNext())
                    {
                        double current = enumerator.Current;
                        if (current > best || double.IsNaN(best))
                        {
                            best = current;
                        }
                    }
                }
 
                return best;
            }
        }
 
        //---------------------------------------------------------------------------------------
        // Creates an enumerator that is used internally for the final aggregation step.
        //
 
        protected override QueryOperatorEnumerator<double, int> CreateEnumerator<TKey>(
            int index, int count, QueryOperatorEnumerator<double, TKey> source, object? sharedData,
            CancellationToken cancellationToken)
        {
            return new DoubleMinMaxAggregationOperatorEnumerator<TKey>(source, index, _sign, cancellationToken);
        }
 
        //---------------------------------------------------------------------------------------
        // This enumerator type encapsulates the intermediary aggregation over the underlying
        // (possibly partitioned) data source.
        //
 
        private sealed class DoubleMinMaxAggregationOperatorEnumerator<TKey> : InlinedAggregationOperatorEnumerator<double>
        {
            private readonly QueryOperatorEnumerator<double, TKey> _source; // The source data.
            private readonly int _sign; // The sign for comparisons (-1 means min, 1 means max).
 
            //---------------------------------------------------------------------------------------
            // Instantiates a new aggregation operator.
            //
 
            internal DoubleMinMaxAggregationOperatorEnumerator(QueryOperatorEnumerator<double, TKey> source, int partitionIndex, int sign,
                CancellationToken cancellationToken) :
                base(partitionIndex, cancellationToken)
            {
                Debug.Assert(source != null);
                _source = source;
                _sign = sign;
            }
 
            //---------------------------------------------------------------------------------------
            // Tallies up the min/max of the underlying data source, walking the entire thing the first
            // time MoveNext is called on this object.
            //
 
            protected override bool MoveNextCore(ref double currentElement)
            {
                // Based on the sign, do either a min or max reduction.
                QueryOperatorEnumerator<double, TKey> source = _source;
                TKey keyUnused = default(TKey)!;
 
                if (source.MoveNext(ref currentElement, ref keyUnused))
                {
                    int i = 0;
                    // We just scroll through the enumerator and find the min or max.
                    if (_sign == -1)
                    {
                        double elem = default(double);
                        while (source.MoveNext(ref elem, ref keyUnused))
                        {
                            if ((i++ & CancellationState.POLL_INTERVAL) == 0)
                                _cancellationToken.ThrowIfCancellationRequested();
 
                            if (elem < currentElement || double.IsNaN(elem))
                            {
                                currentElement = elem;
                            }
                        }
                    }
                    else
                    {
                        double elem = default(double);
                        while (source.MoveNext(ref elem, ref keyUnused))
                        {
                            if ((i++ & CancellationState.POLL_INTERVAL) == 0)
                                _cancellationToken.ThrowIfCancellationRequested();
 
                            if (elem > currentElement || double.IsNaN(currentElement))
                            {
                                currentElement = elem;
                            }
                        }
                    }
 
                    // The sum has been calculated. Now just return.
                    return true;
                }
 
                return false;
            }
 
            //---------------------------------------------------------------------------------------
            // Dispose of resources associated with the underlying enumerator.
            //
 
            protected override void Dispose(bool disposing)
            {
                Debug.Assert(_source != null);
                _source.Dispose();
            }
        }
    }
}