Beam/Beam.Temporary.Cli/DownloadBuilder.cs

using aeqw89.DataKeys;
using Beam.Dynamic;
using Microsoft.Extensions.Logging;
using System;
using System.Collections.Generic;

namespace Beam.Temporary.Cli {
    /// <summary>
    ///     Type‑safe, staged builder that prevents callers from forgetting the mandatory steps
    ///     (source → link selection → transformer) and surfaces operational knobs as first‑class
    ///     methods instead of magic parameters.
    /// </summary>
    public static class DownloadBuilder<T> {
        /* ──────────────────────────── Entry points ─────────────────────────── */

        public static ILinkStage FromMeta(DataKey<TextResource> novelKey, BeamDataDictionary data) =>
            Create(novelKey, data, SourceKind.Meta);

        public static ILinkStage FromText(DataKey<TextResource> novelKey, BeamDataDictionary data) =>
            Create(novelKey, data, SourceKind.Text);

        /* ────────────────────────────── Stages ─────────────────────────────── */

        public interface ILinkStage {
            ITransformStage WithLink();
            ITransformStage WithLinkGenerator();
        }

        public interface ITransformStage {
            IContextStage WithTransformer(Func<DataBindings, HtmlTransformer<T>> factory);
        }

        public interface IContextStage {
            IContextStage Configure(Action<DownloadContextBuilder<T>> configure);
            IContextStage WithParallelism(int degree);
            IContextStage WithTimeout(TimeSpan timeout);
            IContextStage WithRetryReporter(IProgress<RetryReport> reporter);
            DownloadEnumerable<T> Build();
        }

        /* ────────────────────────── Implementation ────────────────────────── */

        private enum SourceKind { Meta, Text }

        private static ILinkStage Create(DataKey<TextResource> novelKey, BeamDataDictionary data, SourceKind kind) {
            var (source, initial) = Resolve(novelKey, data, kind);
            var ctxBuilder = new DownloadContextBuilder<T>().WithLinks(Array.Empty<SourceLink>()); // placeholder, filled later.
            return new LinkStage(source, initial, data, ctxBuilder);
        }

        private static (WebResource Source, State Initial) Resolve(DataKey<TextResource> novelKey, BeamDataDictionary data, SourceKind kind) {
            if (!data.Novels.TryGetValue(novelKey, out var tr))
                throw new KeyNotFoundException($"Novel '{novelKey}' not found in BeamDataDictionary.");

            var textRecord = tr.ToRecord(data);
            WebResource? source;
            State? initial;

            if (kind == SourceKind.Meta) {
                source = textRecord.AssociatedMetaSource ?? throw new InvalidOperationException($"Meta source missing for '{novelKey}'.");
                initial = textRecord.Resource.MetaTemplateInitialData ?? throw new InvalidOperationException("Meta template data missing.");
            } else {
                source = textRecord.AssociatedSource ?? throw new InvalidOperationException($"Text source missing for '{novelKey}'.");
                initial = textRecord.Resource.TemplateInitialData ?? throw new InvalidOperationException("Template initial data missing.");
            }

            return (source, initial);
        }

        /* ──────────────────────────── Stage types ─────────────────────────── */

        private sealed record LinkStage(
            WebResource Source,
            State Initial,
            BeamDataDictionary Data,
            DownloadContextBuilder<T> CtxBuilder) : ILinkStage {
            public ITransformStage WithLink() {
                var link = Data.Templates[Source.Key].Builder.Build(Initial);
                CtxBuilder.WithLinks(new[] { link });
                return new TransformStage(Source, Data, CtxBuilder);
            }

            public ITransformStage WithLinkGenerator() {
                var template = Data.Templates[Source.Key];
                var generator = SourceLinkEnumerable.FromGenerator(new OrderedSourceLinkGenerator(
                    template.Builder,
                    new NumberedStateChanger(template.Factory.Behavior),
                    Initial));
                CtxBuilder.WithLinks(generator);
                return new TransformStage(Source, Data, CtxBuilder);
            }
        }

        private sealed record TransformStage(
            WebResource Source,
            BeamDataDictionary Data,
            DownloadContextBuilder<T> CtxBuilder) : ITransformStage {
            public IContextStage WithTransformer(Func<DataBindings, HtmlTransformer<T>> factory) {
                var transformer = factory(Data.Bindings[Source.Bindings]);
                CtxBuilder.WithTransformer(transformer);
                return new ContextStage(CtxBuilder);
            }
        }

        private sealed class ContextStage : IContextStage {
            private readonly DownloadContextBuilder<T> _ctxBuilder;
            private int _parallelism = 4;

            public ContextStage(DownloadContextBuilder<T> ctxBuilder) => _ctxBuilder = ctxBuilder;

            public IContextStage Configure(Action<DownloadContextBuilder<T>> configure) {
                configure(_ctxBuilder);
                return this;
            }

            public IContextStage WithParallelism(int degree) {
                _parallelism = Math.Max(1, degree);
                return this;
            }

            public IContextStage WithTimeout(TimeSpan timeout) {
                _ctxBuilder.WithTimeOut(timeout);
                return this;
            }

            public IContextStage WithRetryReporter(IProgress<RetryReport> reporter) {
                _ctxBuilder.WithRetryReporter(reporter);
                return this;
            }

            public DownloadEnumerable<T> Build() {
                var context = _ctxBuilder.Build();
                SequentialFragmentDownloader<T> sequentialDownloader = new(
                        context,
                        ctx => new UnitFragmentDownloader<T>(
                            context.Web,
                            context.AsyncTranformer,
                            context.AsyncFailurePredicates,
                            _parallelism,
                            context.DownloadLogger),
                        context.DownloadLogger);
                var enumerable = new DownloadEnumerable<T>(
                    sequentialDownloader
                    .UnwrapFragmented());
                sequentialDownloader.DisposeAsync().AsTask().Wait();
                return enumerable;
            }
        }
    }
}