using System;
using System.Collections.Generic;

namespace MfGames.Gallium;

/// <summary>
/// Extension methods for selecting components from a list.
/// </summary>
public static class SelectComponentOrDefaultExtensions
{
    /// <summary>
    /// Retrieves a component from an entity and return it. If the entity does not have
    /// the component, then null will be returned.
    /// </summary>
    /// <param name="entities">The entities to process.</param>
    /// <param name="t1">The component type being searched.</param>
    /// <returns>A sequence of T1 or nulls.</returns>
    public static IEnumerable<object?> SelectComponent(
        IEnumerable<Entity> entities,
        Type t1)
    {
        foreach (Entity entity in entities)
        {
            if (entity.Has(t1))
            {
                yield return entity.Get<object>(t1);
            }

            yield return null;
        }
    }

    /// <summary>
    /// Retrieves a component from an entity and return it. If the entity does not have
    /// the component, then the default value will be returned.
    /// </summary>
    /// <param name="entities">The entities to process.</param>
    /// <typeparam name="T1">The component type being searched.</typeparam>
    /// <returns>A sequence of T1.</returns>
    public static IEnumerable<T1?> SelectComponentOrDefault<T1>(this IEnumerable<Entity> entities)
    {
        foreach (Entity entity in entities)
        {
            if (entity.TryGet(out T1 v1))
            {
                yield return v1;
            }

            yield return default;
        }
    }
}