using System;
using System.Collections.Generic;
using System.Linq;

using FluentValidation;

using MfGames.Gallium;

namespace MfGames.Nitride.Entities;

/// <summary>
/// Links a series of entities together in some manner. This assumes that
/// the entities coming into the operation are already ordered.
/// </summary>
[WithProperties]
public partial class LinkEntitySequence : OperationBase, IResolvingOperation
{
    private readonly IValidator<LinkEntitySequence> validator;

    public LinkEntitySequence(IValidator<LinkEntitySequence> validator)
    {
        this.validator = validator;

        this.CreateSequenceIndex = (
            list,
            index) => new EntitySequence(list, index);

        this.AddSequenceIndex = (
            entity,
            sequence) => entity.Add(sequence);
    }

    /// <summary>
    /// Gets or sets a callback to add a sequence into a given entity.
    /// </summary>
    public Func<Entity, EntitySequence, Entity> AddSequenceIndex { get; set; }

    /// <summary>
    /// Gets or sets the function used to create the sequence index.
    /// </summary>
    public Func<IReadOnlyList<Entity>, int, EntitySequence> CreateSequenceIndex
    {
        get;
        set;
    }

    /// <inheritdoc />
    public override IEnumerable<Entity> Run(IEnumerable<Entity> input)
    {
        // Make sure everything is good.
        this.validator.ValidateAndThrow(this);

        // We pull all the entities into a list so we can walk through them
        // more than once along with index access. We also don't reorder the
        // list since we assumed it has been ordered already.
        var list = input.ToList();

        // Go through the list and assign each entity in order with a unique
        // copy of the sequence.
        for (int i = 0; i < list.Count; i++)
        {
            Entity entity = list[i];
            EntitySequence index = this.CreateSequenceIndex(
                list.AsReadOnly(),
                i);

            yield return this.AddSequenceIndex(entity, index);
        }
    }
}