aspnetcore/src/Microsoft.AspNetCore.Routing/Matching/CandidateSet.cs

// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Runtime.CompilerServices;

namespace Microsoft.AspNetCore.Routing.Matching
{
    /// <summary>
    /// Represents a set of <see cref="Endpoint"/> candidates that have been matched
    /// by the routing system. Used by implementations of <see cref="EndpointSelector"/>
    /// and <see cref="IEndpointSelectorPolicy"/>.
    /// </summary>
    public sealed class CandidateSet
    {
        // We inline storage for 4 candidates here to avoid allocations in common
        // cases. There's no real reason why 4 is important, it just seemed like 
        // a plausible number.
        private CandidateState _state0;
        private CandidateState _state1;
        private CandidateState _state2;
        private CandidateState _state3;

        private CandidateState[] _additionalCandidates;

        /// <summary>
        /// <para>
        /// Initializes a new instances of the candidate set structure with the provided list of endpoints 
        /// and associated scores.
        /// </para>
        /// <para>
        /// The constructor is provided to enable unit tests of implementations of <see cref="EndpointSelector"/>
        /// and <see cref="IEndpointSelectorPolicy"/>.
        /// </para>
        /// </summary>
        /// <param name="endpoints">The list of endpoints, sorted in descending priority order.</param>
        /// <param name="scores">The list of endpoint scores. <see cref="CandidateState.Score"/>.</param>
        public CandidateSet(MatcherEndpoint[] endpoints, int[] scores)
        {
            Count = endpoints.Length;

            switch (endpoints.Length)
            {
                case 0:
                    return;

                case 1:
                    _state0 = new CandidateState(endpoints[0], score: scores[0]);
                    break;

                case 2:
                    _state0 = new CandidateState(endpoints[0], score: scores[0]);
                    _state1 = new CandidateState(endpoints[1], score: scores[1]);
                    break;

                case 3:
                    _state0 = new CandidateState(endpoints[0], score: scores[0]);
                    _state1 = new CandidateState(endpoints[1], score: scores[1]);
                    _state2 = new CandidateState(endpoints[2], score: scores[2]);
                    break;

                case 4:
                    _state0 = new CandidateState(endpoints[0], score: scores[0]);
                    _state1 = new CandidateState(endpoints[1], score: scores[1]);
                    _state2 = new CandidateState(endpoints[2], score: scores[2]);
                    _state3 = new CandidateState(endpoints[3], score: scores[3]);
                    break;

                default:
                    _state0 = new CandidateState(endpoints[0], score: scores[0]);
                    _state1 = new CandidateState(endpoints[1], score: scores[1]);
                    _state2 = new CandidateState(endpoints[2], score: scores[2]);
                    _state3 = new CandidateState(endpoints[3], score: scores[3]);

                    _additionalCandidates = new CandidateState[endpoints.Length - 4];
                    for (var i = 4; i < endpoints.Length; i++)
                    {
                        _additionalCandidates[i - 4] = new CandidateState(endpoints[i], score: scores[i]);
                    }
                    break;
            }
        }

        internal CandidateSet(Candidate[] candidates)
        {
            Count = candidates.Length;

            switch (candidates.Length)
            {
                case 0:
                    return;

                case 1:
                    _state0 = new CandidateState(candidates[0].Endpoint, candidates[0].Score);
                    break;

                case 2:
                    _state0 = new CandidateState(candidates[0].Endpoint, candidates[0].Score);
                    _state1 = new CandidateState(candidates[1].Endpoint, candidates[1].Score);
                    break;

                case 3:
                    _state0 = new CandidateState(candidates[0].Endpoint, candidates[0].Score);
                    _state1 = new CandidateState(candidates[1].Endpoint, candidates[1].Score);
                    _state2 = new CandidateState(candidates[2].Endpoint, candidates[2].Score);
                    break;

                case 4:
                    _state0 = new CandidateState(candidates[0].Endpoint, candidates[0].Score);
                    _state1 = new CandidateState(candidates[1].Endpoint, candidates[1].Score);
                    _state2 = new CandidateState(candidates[2].Endpoint, candidates[2].Score);
                    _state3 = new CandidateState(candidates[3].Endpoint, candidates[3].Score);
                    break;

                default:
                    _state0 = new CandidateState(candidates[0].Endpoint, candidates[0].Score);
                    _state1 = new CandidateState(candidates[1].Endpoint, candidates[1].Score);
                    _state2 = new CandidateState(candidates[2].Endpoint, candidates[2].Score);
                    _state3 = new CandidateState(candidates[3].Endpoint, candidates[3].Score);

                    _additionalCandidates = new CandidateState[candidates.Length - 4];
                    for (var i = 4; i < candidates.Length; i++)
                    {
                        _additionalCandidates[i - 4] = new CandidateState(candidates[i].Endpoint, candidates[i].Score);
                    }
                    break;
            }
        }

        /// <summary>
        /// Gets the count of candidates in the set.
        /// </summary>
        public int Count { get; }

        /// <summary>
        /// Gets the <see cref="CandidateState"/> associated with the candidate <see cref="Endpoint"/>
        /// at <paramref name="index"/>.
        /// </summary>
        /// <param name="index">The candidate index.</param>
        /// <returns>
        /// A reference to the <see cref="CandidateState"/>. The result is returned by reference
        /// and intended to be mutated.
        /// </returns>
        public ref CandidateState this[int index]
        {
            // Note that this is a ref-return because of both mutability and performance.
            // We don't want to copy these fat structs if it can be avoided.

            // PERF: Force inlining
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get
            {
                // Friendliness for inlining
                if ((uint)index >= Count)
                {
                    ThrowIndexArgumentOutOfRangeException();
                }

                switch (index)
                {
                    case 0:
                        return ref _state0;

                    case 1:
                        return ref _state1;

                    case 2:
                        return ref _state2;

                    case 3:
                        return ref _state3;

                    default:
                        return ref _additionalCandidates[index - 4];
                }
            }
        }

        private static void ThrowIndexArgumentOutOfRangeException()
        {
            throw new ArgumentOutOfRangeException("index");
        }
    }
}