ersatztv/ErsatzTV.Core.Tests/FFmpeg/WatermarkSelectorDecoResolutionTests.cs

using ErsatzTV.Core.Domain;
using ErsatzTV.Core.Domain.Filler;
using ErsatzTV.Core.Domain.Scheduling;
using ErsatzTV.Core.FFmpeg;
using ErsatzTV.Core.Images;
using ErsatzTV.Core.Interfaces.FFmpeg;
using ErsatzTV.Core.Interfaces.Images;
using Microsoft.Extensions.Logging.Abstractions;
using NSubstitute;
using NUnit.Framework;
using Shouldly;
using Testably.Abstractions.Testing;

namespace ErsatzTV.Core.Tests.FFmpeg;

/// <summary>
///     Pins ersatztv#510: a watermark attached through a DECO resolves by exactly the same policy as the three
///     precedence levels (playout item, channel, global).
/// </summary>
/// <remarks>
///     Before #510 the deco path had its own copy of the image-source switch that resolved paths *unchecked*, so
///     one channel could disagree with itself about whether a bug rendered purely by how the watermark was
///     attached. The divergence covered all three <see cref="ChannelWatermarkImageSource" /> values, not just
///     <c>ChannelLogo</c>:
///     <list type="bullet">
///         <item>a missing local file was handed downstream as a dead path (and a dead LOCAL path can reach
///         ffmpeg as a bare <c>-i</c> argument via <c>CanUseFFmpegNativeWatermark</c>, so it is worse than a
///         skipped overlay);</item>
///         <item>an un-migrated external-URL logo was handed down as a renderable URL, which
///         <c>graphics.channel-logo-caching</c> (#525) forbids the render path from fetching;</item>
///         <item>a channel with no logo artwork got the generated-initials localhost URL, which a live-E2E on a
///         real transcoded frame confirmed DID render — the deco path only. #510 resolved that split in favour
///         of "no on-screen bug" everywhere.</item>
///     </list>
///     The <c>Deco_And_Channel_Level_Should_Resolve_Identically</c> cases are the structural guard: they assert
///     the two callers agree, so re-introducing a per-caller policy fails here rather than silently in prod.
/// </remarks>
[TestFixture]
public class WatermarkSelectorDecoResolutionTests
{
    private const string ExternalLogoUrl = "https://cdn.example.com/logos/channel.png";
    private const string LogoStoredPath = "abc123.png";
    private const string LogoCachePath = "/cache/logos/ab/abc123.png";
    private const string CustomStoredPath = "def456.png";
    private const string CustomCachePath = "/cache/watermarks/de/def456.png";
    private const string ResourceImage = "song-progress.png";

    private static string ResourcePath => Path.Combine(FileSystemLayout.ResourcesCacheFolder, ResourceImage);

    /// <summary>Builds a selector whose mock filesystem contains exactly <paramref name="existingFiles" />.</summary>
    private static WatermarkSelector Selector(Deco playoutDeco, params string[] existingFiles)
    {
        // one Initialize() call, chained -- calling it per file would leave "does a second Initialize()
        // preserve the first file?" untested, and a silently under-seeded filesystem makes a
        // "resolves to nothing" assertion pass for the wrong reason
        var mockFileSystem = new MockFileSystem();
        if (existingFiles.Length > 0)
        {
            var initialized = mockFileSystem.Initialize().WithFile(existingFiles[0]);
            foreach (string file in existingFiles.Skip(1))
            {
                initialized = initialized.WithFile(file);
            }
        }

        var fakeImageCache = Substitute.For<IImageCache>();
        fakeImageCache.GetPathForImage(Arg.Any<string>(), Arg.Is(ArtworkKind.Logo), Arg.Any<Option<int>>())
            .Returns(_ => LogoCachePath);
        fakeImageCache.GetPathForImage(Arg.Any<string>(), Arg.Is(ArtworkKind.Watermark), Arg.Any<Option<int>>())
            .Returns(_ => CustomCachePath);

        // Faithful to the real ImageCache.GetPathForImage, which does fileName[..2] and therefore THROWS on a
        // blank/null name. Modelling that is what makes the blank-image guard tests mutation-sensitive: before
        // #510 the channel and global arms had no guard and this threw out of stream startup.
        fakeImageCache
            .GetPathForImage(
                Arg.Is<string>(s => string.IsNullOrWhiteSpace(s)),
                Arg.Any<ArtworkKind>(),
                Arg.Any<Option<int>>())
            .Returns<string>(_ => throw new ArgumentOutOfRangeException(nameof(IImageCache.GetPathForImage)));

        var decoSelector = Substitute.For<IDecoSelector>();
        decoSelector.GetDecoEntries(Arg.Any<Playout>(), Arg.Any<DateTimeOffset>())
            .Returns(new DecoEntries(Option<Deco>.None, Optional(playoutDeco)));

        return new WatermarkSelector(
            mockFileSystem,
            fakeImageCache,
            decoSelector,
            NullLogger<WatermarkSelector>.Instance);
    }

    private static ChannelWatermark Watermark(ChannelWatermarkImageSource source, string image = "") =>
        new()
        {
            Id = 7,
            Name = "Deco Bug",
            ImageSource = source,
            Image = image,
            Mode = ChannelWatermarkMode.Permanent
        };

    private static Deco DecoWith(ChannelWatermark watermark) =>
        new()
        {
            Id = 1,
            Name = "Test Deco",
            WatermarkMode = DecoMode.Override,
            UseWatermarkDuringFiller = true,
            DecoWatermarks = [new DecoWatermark { WatermarkId = watermark.Id, Watermark = watermark }],
            Watermarks = []
        };

    private static Channel ChannelWith(string logoPath, ChannelWatermark channelWatermark = null)
    {
        var channel = new Channel(Guid.Empty)
        {
            Id = 1,
            Number = "1",
            Name = "Test",
            StreamingMode = StreamingMode.TransportStream,
            Artwork = [],
            Watermark = channelWatermark,
            WatermarkId = channelWatermark?.Id
        };

        if (logoPath is not null)
        {
            channel.Artwork.Add(new Artwork { ArtworkKind = ArtworkKind.Logo, Path = logoPath });
        }

        return channel;
    }

    private static PlayoutItem PlayoutItem() =>
        new()
        {
            FillerKind = FillerKind.None,
            DisableWatermarks = false,
            Watermarks = [],
            Playout = new Playout()
        };

    private static List<WatermarkOptions> SelectViaDeco(
        ChannelWatermark watermark,
        Channel channel,
        params string[] existingFiles)
    {
        WatermarkSelector selector = Selector(DecoWith(watermark), existingFiles);
        return selector.SelectWatermarks(
            Option<ChannelWatermark>.None,
            channel,
            PlayoutItem(),
            DateTimeOffset.Now);
    }

    // ---- positive control: the arrangement CAN produce a watermark ------------------------------
    //
    // Without this, every "resolves to nothing" assertion below could pass vacuously (a broken deco
    // arrangement that never reaches the resolver at all looks identical to a correct refusal).

    [Test]
    public void Deco_ChannelLogo_Should_Use_Cached_Path_When_Local_Logo_Exists()
    {
        ChannelWatermark watermark = Watermark(ChannelWatermarkImageSource.ChannelLogo);
        Channel channel = ChannelWith(LogoStoredPath);

        List<WatermarkOptions> result = SelectViaDeco(watermark, channel, LogoCachePath);

        result.Count.ShouldBe(1);
        result[0].ImagePath.ShouldBe(LogoCachePath);
    }

    // ---- ChannelLogo: the three cases #510 was filed for ----------------------------------------

    [Test]
    public void Deco_ChannelLogo_Should_Be_Ignored_When_Logo_Is_An_External_Url()
    {
        ChannelWatermark watermark = Watermark(ChannelWatermarkImageSource.ChannelLogo);
        Channel channel = ChannelWith(ExternalLogoUrl);

        List<WatermarkOptions> result = SelectViaDeco(watermark, channel);

        result.ShouldBeEmpty();
    }

    [Test]
    public void Deco_ChannelLogo_Should_Be_Ignored_When_Local_Logo_File_Is_Missing()
    {
        ChannelWatermark watermark = Watermark(ChannelWatermarkImageSource.ChannelLogo);
        Channel channel = ChannelWith(LogoStoredPath);

        // nothing on disk
        List<WatermarkOptions> result = SelectViaDeco(watermark, channel);

        result.ShouldBeEmpty();
    }

    /// <summary>
    ///     The #510 policy decision: with no logo artwork the generated-initials fallback is NOT used. It
    ///     genuinely rendered here before (confirmed by live-E2E on a real frame), so this is a deliberate,
    ///     recorded behavior change — not a no-op cleanup.
    /// </summary>
    [Test]
    public void Deco_ChannelLogo_Should_Be_Ignored_When_Channel_Has_No_Logo_Artwork()
    {
        ChannelWatermark watermark = Watermark(ChannelWatermarkImageSource.ChannelLogo);
        Channel channel = ChannelWith(null);

        List<WatermarkOptions> result = SelectViaDeco(watermark, channel);

        result.ShouldBeEmpty();

        // Folded in from a separate test that asserted only this. On its own it was vacuous — an empty list
        // trivially contains no URL — so it is a second assertion here rather than a test implying independent
        // coverage. It earns its place by naming the value if this ever starts returning options again (#652).
        result.Select(o => o.ImagePath)
            .ShouldNotContain(ChannelLogoGenerator.GenerateChannelLogoUrl(channel));
    }

    // ---- Custom and Resource: the two arms #510 did not mention but that diverged too -----------

    [Test]
    public void Deco_Custom_Should_Use_Cached_Path_When_File_Exists()
    {
        ChannelWatermark watermark = Watermark(ChannelWatermarkImageSource.Custom, CustomStoredPath);
        Channel channel = ChannelWith(LogoStoredPath);

        List<WatermarkOptions> result = SelectViaDeco(watermark, channel, CustomCachePath);

        result.Count.ShouldBe(1);
        result[0].ImagePath.ShouldBe(CustomCachePath);
    }

    [Test]
    public void Deco_Custom_Should_Be_Ignored_When_File_Is_Missing()
    {
        ChannelWatermark watermark = Watermark(ChannelWatermarkImageSource.Custom, CustomStoredPath);
        Channel channel = ChannelWith(LogoStoredPath);

        List<WatermarkOptions> result = SelectViaDeco(watermark, channel);

        result.ShouldBeEmpty();
    }

    [Test]
    public void Deco_Custom_Should_Be_Ignored_When_Image_Is_Blank()
    {
        ChannelWatermark watermark = Watermark(ChannelWatermarkImageSource.Custom, "   ");
        Channel channel = ChannelWith(LogoStoredPath);

        List<WatermarkOptions> result = SelectViaDeco(watermark, channel, CustomCachePath);

        result.ShouldBeEmpty();
    }

    [Test]
    public void Deco_Resource_Should_Use_Resource_Path_When_File_Exists()
    {
        ChannelWatermark watermark = Watermark(ChannelWatermarkImageSource.Resource, ResourceImage);
        Channel channel = ChannelWith(LogoStoredPath);

        List<WatermarkOptions> result = SelectViaDeco(watermark, channel, ResourcePath);

        result.Count.ShouldBe(1);
        result[0].ImagePath.ShouldBe(ResourcePath);
    }

    [Test]
    public void Deco_Resource_Should_Be_Ignored_When_File_Is_Missing()
    {
        ChannelWatermark watermark = Watermark(ChannelWatermarkImageSource.Resource, ResourceImage);
        Channel channel = ChannelWith(LogoStoredPath);

        List<WatermarkOptions> result = SelectViaDeco(watermark, channel);

        result.ShouldBeEmpty();
    }

    // ---- non-deco consequences of the SAME unification -------------------------------------------
    //
    // These pin precedence-level behavior rather than deco behavior, but they exist because of the #510
    // unification: one is the single piece of per-caller policy deliberately kept, the others are arms that
    // used to throw. Without them a future refactor can delete the survivor, or re-introduce the crash, with
    // a fully green suite.

    /// <summary>
    ///     The one surviving per-caller policy: a playout-item `Custom` watermark with a blank image falls
    ///     THROUGH to the channel/global watermark rather than resolving to "no watermark". Unifying
    ///     resolution must not change which watermark WINS.
    /// </summary>
    [Test]
    public void Blank_Custom_Playout_Item_Watermark_Should_Fall_Through_To_Channel_Watermark()
    {
        ChannelWatermark playoutItemWatermark = Watermark(ChannelWatermarkImageSource.Custom, "   ");
        ChannelWatermark channelWatermark = Watermark(ChannelWatermarkImageSource.Custom, CustomStoredPath);
        channelWatermark.Id = 8;
        Channel channel = ChannelWith(LogoStoredPath, channelWatermark);

        Option<WatermarkOptions> result = Selector(null, CustomCachePath)
            .GetWatermarkOptions(channel, playoutItemWatermark, Option<ChannelWatermark>.None);

        // the CHANNEL watermark wins -- not None, and not the blank playout-item one
        result.IsSome.ShouldBeTrue();
        WatermarkOptions options = result.IfNone(() => throw new InvalidOperationException());
        options.ImagePath.ShouldBe(CustomCachePath);
        options.Watermark.Id.ShouldBe(8);
    }

    /// <summary>
    ///     Before #510 the channel and global arms had no blank-image guard, so they reached
    ///     <c>ImageCache.GetPathForImage</c> whose <c>fileName[..2]</c> threw out of stream startup. Now a
    ///     warning plus no watermark.
    /// </summary>
    [TestCase(null)]
    [TestCase("")]
    [TestCase("   ")]
    public void Channel_Level_Blank_Custom_Watermark_Should_Resolve_To_None_Not_Throw(string image)
    {
        ChannelWatermark channelWatermark = Watermark(ChannelWatermarkImageSource.Custom, image);
        Channel channel = ChannelWith(LogoStoredPath, channelWatermark);

        Option<WatermarkOptions> result = Should.NotThrow(
            () => Selector(null).GetWatermarkOptions(
                channel,
                Option<ChannelWatermark>.None,
                Option<ChannelWatermark>.None));

        result.IsNone.ShouldBeTrue();
    }

    /// <summary>
    ///     Same fall-through, but landing on the GLOBAL watermark — the channel-level variant above cannot
    ///     distinguish "fell through correctly" from "stopped at the channel by accident".
    /// </summary>
    [Test]
    public void Blank_Custom_Playout_Item_Watermark_Should_Fall_Through_To_Global_Watermark()
    {
        ChannelWatermark playoutItemWatermark = Watermark(ChannelWatermarkImageSource.Custom, "   ");
        ChannelWatermark globalWatermark = Watermark(ChannelWatermarkImageSource.Custom, CustomStoredPath);
        globalWatermark.Id = 9;

        // no channel-level watermark, so the only remaining candidate is the global one
        Channel channel = ChannelWith(LogoStoredPath);

        Option<WatermarkOptions> result = Selector(null, CustomCachePath)
            .GetWatermarkOptions(channel, playoutItemWatermark, globalWatermark);

        result.IsSome.ShouldBeTrue();
        WatermarkOptions options = result.IfNone(() => throw new InvalidOperationException());
        options.ImagePath.ShouldBe(CustomCachePath);
        options.Watermark.Id.ShouldBe(9);
    }

    /// <summary>
    ///     The complement of the fall-through cases: a NON-blank custom image whose file is merely missing must
    ///     NOT fall through — it resolves to "no watermark" and the channel watermark never gets a turn.
    ///     Without this, widening the blank-image guard to "any unresolvable custom" would pass unnoticed.
    /// </summary>
    /// <remarks>
    ///     The channel-level fallback is deliberately an INDEPENDENTLY RESOLVABLE `ChannelLogo` watermark whose
    ///     cached file exists. An earlier version of this test gave the fallback the same missing custom path as
    ///     the playout-item watermark, which made it unfalsifiable: a wrongly-widened guard would have fallen
    ///     through to a fallback that also resolved to None, so the assertion held either way.
    /// </remarks>
    [Test]
    public void Missing_But_Named_Custom_Playout_Item_Watermark_Should_Not_Fall_Through()
    {
        ChannelWatermark playoutItemWatermark = Watermark(ChannelWatermarkImageSource.Custom, CustomStoredPath);
        ChannelWatermark channelWatermark = Watermark(ChannelWatermarkImageSource.ChannelLogo);
        channelWatermark.Id = 8;
        Channel channel = ChannelWith(LogoStoredPath, channelWatermark);

        // the channel logo's cached file EXISTS, so a fall-through would return it and fail this test;
        // the custom watermark's file does not, so the playout-item watermark is unresolvable
        Option<WatermarkOptions> result = Selector(null, LogoCachePath)
            .GetWatermarkOptions(channel, playoutItemWatermark, Option<ChannelWatermark>.None);

        result.IsNone.ShouldBeTrue();
    }

    /// <summary>
    ///     Positive control for the test above: the same arrangement, but with the playout-item watermark BLANK
    ///     rather than missing, must fall through and return the resolvable channel logo. Together the pair
    ///     shows the guard distinguishes blank from unresolvable, rather than both landing on None.
    /// </summary>
    /// <remarks>
    ///     Parameterized over all three blank forms because the guard is <c>IsNullOrWhiteSpace</c>: testing only
    ///     <c>"   "</c> would let a mutation to <c>image == "   "</c> pass while silently breaking fall-through
    ///     for <c>null</c> and <c>""</c> — and <c>null</c> is the form the API actually persists.
    /// </remarks>
    [TestCase(null)]
    [TestCase("")]
    [TestCase("   ")]
    public void Blank_Custom_Playout_Item_Watermark_Should_Fall_Through_To_A_Resolvable_Channel_Logo(string image)
    {
        ChannelWatermark playoutItemWatermark = Watermark(ChannelWatermarkImageSource.Custom, image);
        ChannelWatermark channelWatermark = Watermark(ChannelWatermarkImageSource.ChannelLogo);
        channelWatermark.Id = 8;
        Channel channel = ChannelWith(LogoStoredPath, channelWatermark);

        Option<WatermarkOptions> result = Selector(null, LogoCachePath)
            .GetWatermarkOptions(channel, playoutItemWatermark, Option<ChannelWatermark>.None);

        result.IsSome.ShouldBeTrue();
        result.IfNone(() => throw new InvalidOperationException()).ImagePath.ShouldBe(LogoCachePath);
    }

    /// <summary>
    ///     Pins the <c>ImageSource is Custom</c> half of the blank-image guard, which nothing else covers.
    /// </summary>
    /// <remarks>
    ///     A <c>ChannelLogo</c> watermark's <c>Image</c> is NORMALLY blank — the API persists `Image = null` for
    ///     every non-`Custom` source — so if the guard's `is Custom` discriminator were dropped, leaving only
    ///     `IsNullOrWhiteSpace(Image)`, every playout-item `ChannelLogo` watermark would fall through to
    ///     channel/global instead of resolving the channel's own logo. This test fails on that mutation: the
    ///     playout-item watermark carries a distinguishing Id, so falling through is observable even though both
    ///     levels would resolve to the same cached path.
    /// </remarks>
    [Test]
    public void Blank_Image_ChannelLogo_Playout_Item_Watermark_Should_Win_And_Not_Fall_Through()
    {
        // Image is left blank, exactly as the API stores a ChannelLogo watermark
        ChannelWatermark playoutItemWatermark = Watermark(ChannelWatermarkImageSource.ChannelLogo);
        playoutItemWatermark.Id = 42;

        ChannelWatermark channelWatermark = Watermark(ChannelWatermarkImageSource.ChannelLogo);
        channelWatermark.Id = 8;
        Channel channel = ChannelWith(LogoStoredPath, channelWatermark);

        Option<WatermarkOptions> result = Selector(null, LogoCachePath)
            .GetWatermarkOptions(channel, playoutItemWatermark, Option<ChannelWatermark>.None);

        result.IsSome.ShouldBeTrue();
        WatermarkOptions options = result.IfNone(() => throw new InvalidOperationException());
        options.ImagePath.ShouldBe(LogoCachePath);

        // the PLAYOUT-ITEM watermark won; a fall-through would have returned the channel's (Id 8)
        options.Watermark.Id.ShouldBe(42);
    }

    /// <summary>
    ///     `CreateWatermarkHandler`/`UpdateWatermarkHandler` write `Image = null` for every non-`Custom`
    ///     watermark, so an API-created `Resource` watermark hits `Path.Combine(folder, null)` — an
    ///     `ArgumentNullException` out of stream startup. Uses the persisted shape (null), not a hand-made
    ///     filename, which is what the rest of the fixture would otherwise assume.
    /// </summary>
    [TestCase(null)]
    [TestCase("")]
    public void Resource_Watermark_With_No_Image_Name_Should_Resolve_To_None_Not_Throw(string image)
    {
        ChannelWatermark channelWatermark = Watermark(ChannelWatermarkImageSource.Resource, image);
        Channel channel = ChannelWith(LogoStoredPath, channelWatermark);

        Option<WatermarkOptions> result = Should.NotThrow(
            () => Selector(null).GetWatermarkOptions(
                channel,
                Option<ChannelWatermark>.None,
                Option<ChannelWatermark>.None));

        result.IsNone.ShouldBeTrue();
    }

    /// <summary>
    ///     Dropping an unresolvable watermark shortens the list handed to
    ///     <c>CanUseFFmpegNativeWatermark</c>, whose predicate includes `Count == 1`. So this is also the pin on
    ///     the observable routing change: two attached permanent watermarks, one missing, now yield ONE option
    ///     (ffmpeg-native) where they previously yielded two (graphics engine).
    /// </summary>
    [Test]
    public void Deco_With_One_Valid_And_One_Missing_Watermark_Should_Return_Only_The_Valid_One()
    {
        ChannelWatermark valid = Watermark(ChannelWatermarkImageSource.ChannelLogo);
        ChannelWatermark missing = Watermark(ChannelWatermarkImageSource.Custom, CustomStoredPath);
        missing.Id = 8;

        var deco = new Deco
        {
            Id = 1,
            Name = "Test Deco",
            WatermarkMode = DecoMode.Override,
            UseWatermarkDuringFiller = true,
            DecoWatermarks =
            [
                new DecoWatermark { WatermarkId = valid.Id, Watermark = valid },
                new DecoWatermark { WatermarkId = missing.Id, Watermark = missing }
            ],
            Watermarks = []
        };

        // only the channel logo's cached file exists; the custom watermark's does not
        List<WatermarkOptions> result = Selector(deco, LogoCachePath).SelectWatermarks(
            Option<ChannelWatermark>.None,
            ChannelWith(LogoStoredPath),
            PlayoutItem(),
            DateTimeOffset.Now);

        result.Count.ShouldBe(1);
        result[0].ImagePath.ShouldBe(LogoCachePath);

        // The routing claim itself, not just the filtering: call the real predicate. Asserting Count == 1 alone
        // would leave the decision record's "now routes ffmpeg-native" statement unpinned, since the decision
        // lives in FFmpegLibraryProcessService rather than in the selector.
        FFmpegLibraryProcessService.CanUseFFmpegNativeWatermark(0, result).ShouldBeTrue();
    }

    /// <summary>
    ///     Before #510 the global arm had no <c>Resource</c> case and hit <c>default: throw</c>.
    /// </summary>
    [Test]
    public void Global_Level_Resource_Watermark_Should_Resolve_Instead_Of_Throwing()
    {
        ChannelWatermark globalWatermark = Watermark(ChannelWatermarkImageSource.Resource, ResourceImage);
        Channel channel = ChannelWith(LogoStoredPath);

        Option<WatermarkOptions> result = Should.NotThrow(
            () => Selector(null, ResourcePath).GetWatermarkOptions(
                channel,
                Option<ChannelWatermark>.None,
                globalWatermark));

        result.IsSome.ShouldBeTrue();
        result.IfNone(() => throw new InvalidOperationException()).ImagePath.ShouldBe(ResourcePath);
    }

    // ---- the structural guard: deco and channel-level must agree, case for case ------------------

    private static IEnumerable<TestCaseData> ParityCases()
    {
        // (image source, watermark.Image, channel logo path, files that exist)
        yield return new TestCaseData(
                ChannelWatermarkImageSource.ChannelLogo, "", LogoStoredPath, new[] { LogoCachePath })
            .SetName("ChannelLogo, local file present");
        yield return new TestCaseData(
                ChannelWatermarkImageSource.ChannelLogo, "", LogoStoredPath, Array.Empty<string>())
            .SetName("ChannelLogo, local file missing");
        yield return new TestCaseData(
                ChannelWatermarkImageSource.ChannelLogo, "", ExternalLogoUrl, Array.Empty<string>())
            .SetName("ChannelLogo, external URL");
        yield return new TestCaseData(
                ChannelWatermarkImageSource.ChannelLogo, "", null, Array.Empty<string>())
            .SetName("ChannelLogo, no logo artwork");
        yield return new TestCaseData(
                ChannelWatermarkImageSource.Custom, CustomStoredPath, LogoStoredPath, new[] { CustomCachePath })
            .SetName("Custom, file present");
        yield return new TestCaseData(
                ChannelWatermarkImageSource.Custom, CustomStoredPath, LogoStoredPath, Array.Empty<string>())
            .SetName("Custom, file missing");
        // Both sides agree here by construction (each returns nothing), which is the point: it documents that
        // the blank-image fall-through asymmetry lives ONLY at the playout-item level -- covered by
        // Blank_Custom_Playout_Item_Watermark_Should_Fall_Through_To_Channel_Watermark -- rather than leaving
        // the omission looking like an evasion.
        yield return new TestCaseData(
                ChannelWatermarkImageSource.Custom, "   ", LogoStoredPath, Array.Empty<string>())
            .SetName("Custom, blank image");
        yield return new TestCaseData(
                ChannelWatermarkImageSource.Resource, ResourceImage, LogoStoredPath, new[] { ResourcePath })
            .SetName("Resource, file present");
        yield return new TestCaseData(
                ChannelWatermarkImageSource.Resource, ResourceImage, LogoStoredPath, Array.Empty<string>())
            .SetName("Resource, file missing");
    }

    [TestCaseSource(nameof(ParityCases))]
    public void Deco_And_Channel_Level_Should_Resolve_Identically(
        ChannelWatermarkImageSource source,
        string image,
        string logoPath,
        string[] existingFiles)
    {
        // deco path
        ChannelWatermark decoWatermark = Watermark(source, image);
        List<WatermarkOptions> viaDeco = SelectViaDeco(decoWatermark, ChannelWith(logoPath), existingFiles);

        // channel precedence level, same watermark definition and same channel
        ChannelWatermark channelWatermark = Watermark(source, image);
        Channel channel = ChannelWith(logoPath, channelWatermark);
        Option<WatermarkOptions> viaChannel = Selector(null, existingFiles)
            .GetWatermarkOptions(channel, Option<ChannelWatermark>.None, Option<ChannelWatermark>.None);

        List<string> decoPaths = viaDeco.Select(o => o.ImagePath).ToList();

        // built explicitly rather than via Option.ToList(), which yields a LanguageExt Lst<string>
        var channelPaths = new List<string>();
        viaChannel.IfSome(o => channelPaths.Add(o.ImagePath));

        decoPaths.ShouldBe(channelPaths);
    }
}