// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using Microsoft.Shared.Diagnostics;
 
namespace Microsoft.Extensions.AI;
 
/// <summary>A builder for creating pipelines of <see cref="IImageGenerator"/>.</summary>
[Experimental("MEAI001")]
public sealed class ImageGeneratorBuilder
{
    private readonly Func<IServiceProvider, IImageGenerator> _innerGeneratorFactory;
 
    /// <summary>The registered generator factory instances.</summary>
    private List<Func<IImageGenerator, IServiceProvider, IImageGenerator>>? _generatorFactories;
 
    /// <summary>Initializes a new instance of the <see cref="ImageGeneratorBuilder"/> class.</summary>
    /// <param name="innerGenerator">The inner <see cref="IImageGenerator"/> that represents the underlying backend.</param>
    /// <exception cref="ArgumentNullException"><paramref name="innerGenerator"/> is <see langword="null"/>.</exception>
    public ImageGeneratorBuilder(IImageGenerator innerGenerator)
    {
        _ = Throw.IfNull(innerGenerator);
        _innerGeneratorFactory = _ => innerGenerator;
    }
 
    /// <summary>Initializes a new instance of the <see cref="ImageGeneratorBuilder"/> class.</summary>
    /// <param name="innerGeneratorFactory">A callback that produces the inner <see cref="IImageGenerator"/> that represents the underlying backend.</param>
    /// <exception cref="ArgumentNullException"><paramref name="innerGeneratorFactory"/> is <see langword="null"/>.</exception>
    public ImageGeneratorBuilder(Func<IServiceProvider, IImageGenerator> innerGeneratorFactory)
    {
        _innerGeneratorFactory = Throw.IfNull(innerGeneratorFactory);
    }
 
    /// <summary>Builds an <see cref="IImageGenerator"/> that represents the entire pipeline. Calls to this instance will pass through each of the pipeline stages in turn.</summary>
    /// <param name="services">
    /// The <see cref="IServiceProvider"/> that should provide services to the <see cref="IImageGenerator"/> instances.
    /// If null, an empty <see cref="IServiceProvider"/> will be used.
    /// </param>
    /// <returns>An instance of <see cref="IImageGenerator"/> that represents the entire pipeline.</returns>
    public IImageGenerator Build(IServiceProvider? services = null)
    {
        services ??= EmptyServiceProvider.Instance;
        var imageGenerator = _innerGeneratorFactory(services);
 
        // To match intuitive expectations, apply the factories in reverse order, so that the first factory added is the outermost.
        if (_generatorFactories is not null)
        {
            for (var i = _generatorFactories.Count - 1; i >= 0; i--)
            {
                imageGenerator = _generatorFactories[i](imageGenerator, services) ??
                    throw new InvalidOperationException(
                        $"The {nameof(ImageGeneratorBuilder)} entry at index {i} returned null. " +
                        $"Ensure that the callbacks passed to {nameof(Use)} return non-null {nameof(IImageGenerator)} instances.");
            }
        }
 
        return imageGenerator;
    }
 
    /// <summary>Adds a factory for an intermediate image generator to the image generator pipeline.</summary>
    /// <param name="generatorFactory">The generator factory function.</param>
    /// <returns>The updated <see cref="ImageGeneratorBuilder"/> instance.</returns>
    /// <exception cref="ArgumentNullException"><paramref name="generatorFactory"/> is <see langword="null"/>.</exception>
    public ImageGeneratorBuilder Use(Func<IImageGenerator, IImageGenerator> generatorFactory)
    {
        _ = Throw.IfNull(generatorFactory);
 
        return Use((innerGenerator, _) => generatorFactory(innerGenerator));
    }
 
    /// <summary>Adds a factory for an intermediate image generator to the image generator pipeline.</summary>
    /// <param name="generatorFactory">The generator factory function.</param>
    /// <returns>The updated <see cref="ImageGeneratorBuilder"/> instance.</returns>
    /// <exception cref="ArgumentNullException"><paramref name="generatorFactory"/> is <see langword="null"/>.</exception>
    public ImageGeneratorBuilder Use(Func<IImageGenerator, IServiceProvider, IImageGenerator> generatorFactory)
    {
        _ = Throw.IfNull(generatorFactory);
 
        (_generatorFactories ??= []).Add(generatorFactory);
        return this;
    }
}