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

using MfGames.Gallium;
using MfGames.Nitride;
using MfGames.Nitride.IO.Contents;
using MfGames.Nitride.IO.Directories;
using MfGames.Nitride.IO.Paths;
using MfGames.Nitride.Pipelines;

namespace CopyFiles;

/// <summary>
/// The single pipeline used by the CopyFiles project.
/// </summary>
public class CopyFilesPipeline : PipelineBase
{
    private readonly AddPathPrefix addPathPrefix;

    private readonly ClearDirectory clearDirectory;

    private readonly ReadFiles readFiles;

    private readonly RemovePathPrefix removePathPrefix;

    private readonly WriteFiles writeFiles;

    public CopyFilesPipeline(
        ReadFiles readFiles,
        ClearDirectory clearDirectory,
        WriteFiles writeFiles,
        RemovePathPrefix removePathPrefix,
        AddPathPrefix addPathPrefix)
    {
        // While we can configure these during runtime, it seems cleaner to
        // build them up during the constructor to call out the ones that
        // require runtime data.
        this.readFiles = readFiles.WithPattern("/input/**/*.txt");
        this.clearDirectory = clearDirectory.WithPath("/output");
        this.writeFiles = writeFiles;
        this.removePathPrefix = removePathPrefix.WithPathPrefix("/input");
        this.addPathPrefix = addPathPrefix.WithPathPrefix("/output");
    }

    /// <inheritdoc />
    public override IAsyncEnumerable<Entity> RunAsync(
        IEnumerable<Entity> _,
        CancellationToken cancellationToken = default)
    {
        // We don't care about the incoming entities which means we can
        // ignore them and use the entities from the ReadFiles operation
        // or we can union the entities pass in with the ReadFiles in case
        // we want to merge them.
        //
        // This will read all the files of the given pattern and return them
        // as an IEnumerable<Entity> with the Zio components (UPath and
        // binary contents) set. As a note, this doesn't actually read the
        // file, just create a pointer to where the file could be read from.
        //
        // The path component will always be the relative path to the root,
        // so `/input/a.txt` in this case since we only have one file.
        IEnumerable<Entity> entities = this.readFiles.Run();

        // Change the path. The path (Zio.UPath component) stays with the
        // entity which is why there is no root directory in the writeFiles
        // operation. Instead, we need to remove the `/input` and replace it
        // with the `/output`.
        //
        // We can do that with a RemovePathPrefix followed by an
        // AddPathPrefix, or use the more complex version of ChangePaths.
        // In this case, we are going for easy to learn, so we'll do the
        // pair.
        //
        // We are going to use the chain extension to make it easier to 
        // read. Coming out of this, we will have one entity that fulfills:
        //
        //   entity.Get<UPath> == "/output/a.txt"
        entities = entities
            .Run(this.removePathPrefix, cancellationToken)
            .Run(this.addPathPrefix, cancellationToken);

        // Then we write out the files to the output. First we make sure we
        // clear out the output. This operation performs an action when it
        // it is first entered, but otherwise passes all the inputs through.
        //
        // We can call the clear directory in three ways. The first is to call
        // the operation with:
        //
        //   this.clearDirectory.Run()
        //
        // The other is to pass in the entities and get a new list of
        // modified ones out. In this case, clear doesn't make any changes,
        // but Entity objects are immutable, so we always work on the list
        // returned.
        //
        //  entities = this.clearDirectory.Run(entity)
        //
        // The third way is to use an extension on entities which lets us
        // chain calls, ala Gulp's pipelines. The below code does this along
        // with writing the files to the output.
        entities = entities
            .Run(this.clearDirectory, cancellationToken)
            .Run(this.writeFiles, cancellationToken);

        // If we are chaining this pipeline into another, we return the
        // entities. Otherwise, we can just return an empty list. The
        // pipeline is async, so it is wrapped in a task, but most
        // operations are not (or are both).
        return entities.ToAsyncEnumerable();
    }
}