Beam/Beam.Fluent/DownloadBuilder.LinkStage.cs

using Beam.Models;

namespace Beam.Fluent {
    public static partial class DownloadBuilder<RawType, OutType> {
        /* ──────────────────────────── Stage types ─────────────────────────── */


        private sealed record LinkStage(
            WebResource Source,
            State Initial,
            BeamDataContext Data,
            DownloadContextBuilder<RawType> CtxBuilder) : ILinkStage, IAlternativeLinkStage {

            private State? endState;
            private bool linksFrozen = false;

            public ITransformStage WithLink() {
                var link = Data.Templates[Source.Key].Builder.Build(Initial);
                CtxBuilder.WithLinks([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, endState));
                CtxBuilder.WithLinks(generator);
                linksFrozen = true;
                return new TransformStage(Source, Data, CtxBuilder);
            }

            public IAlternativeTransformStage WithLinks(IEnumerable<SourceLink> links) {
                CtxBuilder.WithLinks(links);
                return new TransformStage(Source, Data, CtxBuilder);
            }

            public ILinkStage WithRange(Range range) {
                if (linksFrozen)
                    throw new InvalidOperationException($"WithRange must be called before WithLinkGenerator");
                if (range.End.Value < range.Start.Value)
                    throw new ArgumentOutOfRangeException(nameof(range), $" start must be < end");
                var template = Data.Templates[Source.Key];
                var stateChanger = new NumberedStateChanger(template.Factory.Behavior);
                endState = Initial.Copy();
                stateChanger.Apply(Initial, range.Start.Value - 1);
                stateChanger.Apply(endState, range.End.Value - 1);
                return this;
            }
        }
    }
}