using MoonTools.ECS;

namespace Nerfed.Runtime.Scheduling;

public sealed class SystemScheduler
{
    private readonly List<SystemScheduleEntry> entries = new List<SystemScheduleEntry>();
    private readonly SystemSchedulerOptions options;

    public SystemScheduler(SystemSchedulerOptions options)
    {
        this.options = options ?? new SystemSchedulerOptions();
    }

    public IReadOnlyList<SystemScheduleEntry> Entries => entries;

    public void Register(MoonTools.ECS.System system, SystemSchedulePhase phase, params string[] dependsOn)
    {
        ArgumentNullException.ThrowIfNull(system);
        string[] dependencies = dependsOn?.Where(static value => !string.IsNullOrWhiteSpace(value)).ToArray() ?? Array.Empty<string>();
        entries.Add(new SystemScheduleEntry(system, phase, dependencies, entries.Count));
    }

    public void Execute(TimeSpan delta)
    {
        if (entries.Count == 0)
        {
            return;
        }

        Dictionary<string, SystemScheduleEntry> byName = BuildNameIndex(entries);
        ValidateDependencies(byName, entries);

        foreach (SystemSchedulePhase phase in Enum.GetValues<SystemSchedulePhase>())
        {
            ExecutePhase(phase, delta, byName);
        }
    }

    private static Dictionary<string, SystemScheduleEntry> BuildNameIndex(List<SystemScheduleEntry> allEntries)
    {
        var byName = new Dictionary<string, SystemScheduleEntry>(StringComparer.Ordinal);
        foreach (SystemScheduleEntry entry in allEntries)
        {
            if (byName.ContainsKey(entry.Name))
            {
                throw new InvalidOperationException($"Duplicate scheduled system name '{entry.Name}'.");
            }

            byName.Add(entry.Name, entry);
        }

        return byName;
    }

    private void ValidateDependencies(Dictionary<string, SystemScheduleEntry> byName, List<SystemScheduleEntry> allEntries)
    {
        foreach (SystemScheduleEntry entry in allEntries)
        {
            foreach (string dependency in entry.DependsOn)
            {
                if (!byName.TryGetValue(dependency, out SystemScheduleEntry dependencyEntry))
                {
                    if (options.StrictDependencyValidation)
                    {
                        throw new InvalidOperationException($"Scheduled system '{entry.Name}' depends on missing system '{dependency}'.");
                    }

                    continue;
                }

                if (dependencyEntry.Phase > entry.Phase)
                {
                    throw new InvalidOperationException(
                        $"Scheduled system '{entry.Name}' in phase {entry.Phase} cannot depend on later phase system '{dependency}' in phase {dependencyEntry.Phase}.");
                }
            }
        }
    }

    private void ExecutePhase(SystemSchedulePhase phase, TimeSpan delta, Dictionary<string, SystemScheduleEntry> byName)
    {
        List<SystemScheduleEntry> phaseEntries = entries
            .Where(entry => entry.Phase == phase)
            .OrderBy(entry => entry.RegistrationIndex)
            .ToList();

        if (phaseEntries.Count == 0)
        {
            return;
        }

        var completed = new HashSet<string>(StringComparer.Ordinal);

        while (completed.Count < phaseEntries.Count)
        {
            List<SystemScheduleEntry> ready = phaseEntries
                .Where(entry => !completed.Contains(entry.Name) && DependenciesSatisfied(entry, completed, byName))
                .OrderBy(entry => entry.RegistrationIndex)
                .ToList();

            if (ready.Count == 0)
            {
                throw new InvalidOperationException($"Cyclic or unsatisfied dependencies detected in phase {phase}.");
            }

            List<SystemScheduleEntry> batch = BuildNonConflictingBatch(ready);
            ExecuteBatch(batch, delta, phase);

            foreach (SystemScheduleEntry entry in batch)
            {
                completed.Add(entry.Name);
            }
        }
    }

    private static bool DependenciesSatisfied(SystemScheduleEntry entry, HashSet<string> completed, Dictionary<string, SystemScheduleEntry> byName)
    {
        foreach (string dependency in entry.DependsOn)
        {
            if (!byName.TryGetValue(dependency, out SystemScheduleEntry dependencyEntry))
            {
                continue;
            }

            if (dependencyEntry.Phase == entry.Phase && !completed.Contains(dependency))
            {
                return false;
            }
        }

        return true;
    }

    private static List<SystemScheduleEntry> BuildNonConflictingBatch(List<SystemScheduleEntry> ready)
    {
        var batch = new List<SystemScheduleEntry>(ready.Count);

        for (int i = 0; i < ready.Count; i++)
        {
            SystemScheduleEntry candidate = ready[i];
            bool conflicts = false;

            for (int j = 0; j < batch.Count; j++)
            {
                if (candidate.Access.ConflictsWith(batch[j].Access))
                {
                    conflicts = true;
                    break;
                }
            }

            if (!conflicts)
            {
                batch.Add(candidate);
            }
        }

        if (batch.Count == 0)
        {
            batch.Add(ready[0]);
        }

        return batch;
    }

    private void ExecuteBatch(List<SystemScheduleEntry> batch, TimeSpan delta, SystemSchedulePhase phase)
    {
        using ProfilerScope phaseScope = new($"Schedule.{phase}.Batch[{batch.Count}]");

        if (options.EnableParallelSystemExecution && batch.Count > 1)
        {
            Parallel.ForEach(batch, entry => ExecuteSystem(entry, delta));
            return;
        }

        for (int i = 0; i < batch.Count; i++)
        {
            ExecuteSystem(batch[i], delta);
        }
    }

    private static void ExecuteSystem(SystemScheduleEntry entry, TimeSpan delta)
    {
        using ProfilerScope scope = new(entry.Name);
        entry.System.Update(delta);
    }
}