#nullable enable
using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.ExceptionServices;
using System.Threading;
using OpenNest.Engine.Jobs;
using OpenNest.Engine.NestingEngines.Circles;
using OpenNest.Engine.NestingEngines.Irregular;
using OpenNest.Engine.NestingEngines.Rectangles;

namespace OpenNest.Engine.NestingEngines.Default;

/// <summary>
/// The engine used when the caller names none. It runs each candidate engine on the whole job,
/// checks every result with <see cref="NestLayoutCheck"/> and returns the best one: a valid
/// layout before an invalid one, then the fewest unplaced parts, then the lowest
/// <see cref="NestJobCost"/>; remaining ties keep candidate order.
///
/// Candidates are Irregular, then Rectangles, then Circles. Neither of the first two wins every
/// job: Rectangles packs each part as its box, which is often best for plain plates and is a
/// cheap, valid fallback for the rest. Circles bids only when every part is round, and otherwise
/// returns everything unplaced at once.
/// A candidate that throws or returns no result is skipped; when every candidate throws, the
/// exception of the first in tie-break order is rethrown. Cancellation stops the search.
/// Candidates' plate commits are not forwarded; the chosen result's commits are reported once it
/// is selected.
///
/// Candidates' whole-job plans (<see cref="NestJobStage.PlanImproved"/>) are scored the same way
/// and reported only when one beats every plan reported so far, so a later candidate never shows a
/// worse plan after a better one. A candidate result that beats them is reported too, and the
/// chosen result is always the last plan reported.
///
/// Rectangles, the quick and always-valid fallback, runs first, then Circles, then Irregular, so
/// a caller that stops the search early, such as <see cref="DeadlineNestingEngine"/>, already
/// holds a valid plan, and Irregular can search until cancelled
/// (<see cref="NestJobOptions.SearchUntilCancelled"/>) last. The choice is made after all have
/// run, in candidate (tie-break) order, so the run order never changes it.
///
/// Deterministic: candidates run one after another and the choice uses no clock.
/// </summary>
public sealed class DefaultNestingEngine : INestingEngine
{
    private readonly IReadOnlyList<Func<INestingEngine>> candidates;
    private readonly IReadOnlyList<int> runOrder;

    public DefaultNestingEngine()
        : this(
            new Func<INestingEngine>[]
            {
                () => new IrregularNestingEngine(),
                () => new RectanglesNestingEngine(),
                () => new CirclesNestingEngine(),
            },
            new[] { 1, 2, 0 })
    {
    }

    /// <summary>Test seam: candidate engines in tie-break order, run in that order.</summary>
    internal DefaultNestingEngine(IReadOnlyList<Func<INestingEngine>> candidates)
        : this(candidates, Enumerable.Range(0, candidates?.Count ?? 0).ToArray())
    {
    }

    /// <summary>Test seam: candidate engines in tie-break order, and the order to run them in
    /// (a permutation of their indexes); the run order affects progress, never the choice.
    /// </summary>
    internal DefaultNestingEngine(
        IReadOnlyList<Func<INestingEngine>> candidates,
        IReadOnlyList<int> runOrder
    )
    {
        ArgumentNullException.ThrowIfNull(candidates);
        ArgumentNullException.ThrowIfNull(runOrder);
        if (candidates.Count == 0)
            throw new ArgumentException(
                "At least one candidate engine is required.",
                nameof(candidates)
            );
        if (
            runOrder.Count != candidates.Count
            || runOrder.Distinct().Count() != candidates.Count
            || runOrder.Any(i => i < 0 || i >= candidates.Count)
        )
            throw new ArgumentException(
                "The run order must list every candidate index once.",
                nameof(runOrder)
            );
        this.candidates = candidates;
        this.runOrder = runOrder;
    }

    /// <summary>Candidate indexes in the order they run.</summary>
    internal IReadOnlyList<int> RunOrder => runOrder;

    public NestJobResult Solve(
        NestJob job,
        IProgress<NestJobProgress>? progress = null,
        CancellationToken token = default
    )
    {
        ArgumentNullException.ThrowIfNull(job);
        token.ThrowIfCancellationRequested();
        var forward = progress == null ? null : new PlanFilter(job, progress, token);
        var scores = new CandidateScore?[candidates.Count];
        ExceptionDispatchInfo? firstFailure = null;
        var firstFailureOrder = int.MaxValue;

        foreach (var order in runOrder)
        {
            token.ThrowIfCancellationRequested();
            NestJobResult? result;
            if (forward != null)
                forward.Order = order;
            try
            {
                result = candidates[order]().Solve(job, forward, token);
            }
            catch (Exception ex)
            {
                // A canceled solve is rethrown by the token checks around each candidate.
                if (order < firstFailureOrder)
                {
                    firstFailure = ExceptionDispatchInfo.Capture(ex);
                    firstFailureOrder = order;
                }
                continue;
            }
            if (result == null)
                continue;

            var scored = CandidateScore.Of(job, result, order);
            scores[order] = scored;
            forward?.Offer(scored);
        }

        // Choose in tie-break order, whatever order the candidates ran in.
        CandidateScore? best = null;
        foreach (var scored in scores)
            if (scored != null && (best == null || scored.IsBetterThan(best)))
                best = scored;

        token.ThrowIfCancellationRequested();
        if (best == null)
        {
            firstFailure?.Throw();
            throw new InvalidOperationException("No candidate engine returned a result.");
        }

        forward?.Finish(best);
        CandidateProgress.ReportCommits(best.Result, progress);
        return best.Result;
    }

    /// <summary>
    /// Forwards candidate progress except plate commits, and replaces candidates' raw plan reports
    /// with Default's own best plan so far by <see cref="CandidateScore"/>.
    /// </summary>
    private sealed class PlanFilter(NestJob job, IProgress<NestJobProgress> inner, CancellationToken token)
        : IProgress<NestJobProgress>
    {
        private CandidateScore? shown;

        /// <summary>Candidate order of the plans now arriving, for score ties.</summary>
        public int Order { get; set; }

        public void Report(NestJobProgress value)
        {
            if (value == null || value.Stage == NestJobStage.PlateCommitted)
                return;
            if (value.Stage != NestJobStage.PlanImproved)
            {
                inner.Report(value);
                return;
            }
            if (value.Plan == null)
                return;
            CandidateScore plan;
            try
            {
                plan = CandidateScore.Of(job, value.Plan, Order);
            }
            catch (Exception) when (!token.IsCancellationRequested)
            {
                // A plan that cannot be scored is not shown; throwing would fail the candidate.
                return;
            }
            Offer(plan);
        }

        /// <summary>Reports the plan when it beats every plan reported so far.</summary>
        public void Offer(CandidateScore plan)
        {
            if (shown != null && !plan.IsBetterThan(shown))
                return;
            shown = plan;
            CandidateProgress.ReportPlan(plan.Result, inner);
        }

        /// <summary>Makes the chosen result the last plan reported. A candidate's plan can outscore
        /// every returned result (its engine judged a later plan better), so this may follow it.</summary>
        public void Finish(CandidateScore chosen)
        {
            if (shown != null && CandidateProgress.SameLayout(shown.Result, chosen.Result))
                return;
            shown = chosen;
            CandidateProgress.ReportPlan(chosen.Result, inner);
        }
    }
}