Beam/Beam.Fluent/DownloadBuilder.cs

using aeqw89.DataKeys;
using Beam;
using Microsoft.Extensions.Logging;
using System;
using System.Collections.Generic;
using Beam.Data;
using Beam.Downloaders;
using Beam.Models;

namespace Beam.Fluent {
    /// <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 partial class DownloadBuilder<RawType, OutType> {
        /* ──────────────────────────── Entry points ─────────────────────────── */
        
        public static ILinkStage FromResource(DataKey<ResourceDictionary> dict, string kind, BeamDataContext beamDataDictionary)
            => Create(dict, beamDataDictionary, kind);

        public static IAlternativeLinkStage FromScratch()
            => new LinkStage(null!, null!, null!, new());

        private static ILinkStage Create(DataKey<ResourceDictionary> resourceDict, BeamDataContext data, string kind) {
            var (source, initial) = Resolve(resourceDict, kind, data);
            var ctxBuilder = new DownloadContextBuilder<RawType>().WithLinks([]); // placeholder, filled later.
            return new LinkStage(source, initial, data, ctxBuilder);
        }

        private static (WebResource Source, State Initial) Resolve(DataKey<ResourceDictionary> resourceDict, string kind, BeamDataContext data) {
            if (!data.ResourceDictionaries.TryGetValue(resourceDict, out var dict))
                throw new KeyNotFoundException($"Novel '{resourceDict}' not found in BeamDataDictionary.");
            if (!dict.Resources.TryGetValue(kind, out var sourceKey))
                throw new KeyNotFoundException($"Novel kind '{kind}' not found in '{resourceDict}'");
            if (!data.Resources.TryGetValue(sourceKey, out var source))
                throw new KeyNotFoundException($"Novel source '{sourceKey}' was not found");
            if (!data.InitialStates.TryGetValue(sourceKey.To<ImmutableState>(), out var istate))
                throw new KeyNotFoundException($"Immutable state for kind '{kind}' not found");
            return (source, istate);
        }
    }
}