using System.Text.Json;

namespace Quorate.Oci;

internal sealed record OciBlob(OciDigest Digest, long Size, string Path);

internal sealed record OciManifest(OciDigest Digest, string MediaType, byte[] Content);

internal sealed record OciPublicationPlan(
    OciDigest RootDigest,
    string RootMediaType,
    byte[] RootContent,
    IReadOnlyList<OciBlob> Blobs,
    IReadOnlyList<OciManifest> Manifests);

internal static class OciLayout
{
    private const string RefName = "org.opencontainers.image.ref.name";

    public static async Task<OciPublicationPlan> LoadAsync(
        string layout,
        OciTag sourceTag,
        CancellationToken cancellationToken)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(layout);
        string root = Path.GetFullPath(layout);
        string blobRoot = Path.Combine(root, "blobs", "sha256");
        byte[] indexContent = await File.ReadAllBytesAsync(
            Path.Combine(root, "index.json"),
            cancellationToken).ConfigureAwait(false);
        OciDocumentModel index = Deserialize(indexContent, "OCI index");
        OciDescriptorModel rootDescriptor = SelectRoot(index, sourceTag);
        var blobs = new Dictionary<OciDigest, OciBlob>();
        var manifests = new List<OciManifest>();
        var complete = new HashSet<OciDigest>();
        var active = new HashSet<OciDigest>();

        await VisitAsync(
            blobRoot,
            rootDescriptor,
            blobs,
            manifests,
            complete,
            active,
            cancellationToken).ConfigureAwait(false);

        OciManifest rootManifest = manifests[^1];
        return new OciPublicationPlan(
            rootManifest.Digest,
            rootManifest.MediaType,
            rootManifest.Content,
            blobs.Values.OrderBy(item => item.Digest.ToString(), StringComparer.Ordinal).ToArray(),
            manifests);
    }

    private static async Task VisitAsync(
        string blobRoot,
        OciDescriptorModel descriptor,
        Dictionary<OciDigest, OciBlob> blobs,
        List<OciManifest> manifests,
        HashSet<OciDigest> complete,
        HashSet<OciDigest> active,
        CancellationToken cancellationToken)
    {
        OciDigest digest = OciDigest.Parse(descriptor.Digest);

        if (complete.Contains(digest))
        {
            return;
        }

        if (!active.Add(digest))
        {
            throw new InvalidDataException($"OCI manifest graph contains a cycle at {digest}.");
        }

        string mediaType = RequireManifestType(descriptor.MediaType);
        string path = BlobPath(blobRoot, digest);
        byte[] content = await File.ReadAllBytesAsync(path, cancellationToken).ConfigureAwait(false);
        ValidateSize(descriptor.Size, content.LongLength, digest);
        OciDigest actual = OciDigest.Compute(content);

        if (actual != digest)
        {
            throw new InvalidDataException($"Manifest digest {actual} does not match {digest}.");
        }

        OciDocumentModel document = Deserialize(content, $"OCI manifest {digest}");

        if (IsIndex(mediaType))
        {
            OciDescriptorModel[] children = document.Manifests
                ?? throw new InvalidDataException($"OCI index {digest} has no manifests.");

            foreach (OciDescriptorModel child in children)
            {
                await VisitAsync(
                    blobRoot,
                    child,
                    blobs,
                    manifests,
                    complete,
                    active,
                    cancellationToken).ConfigureAwait(false);
            }
        }
        else
        {
            AddBlob(blobRoot, document.Config, blobs, digest, "config");
            OciDescriptorModel[] layers = document.Layers
                ?? throw new InvalidDataException($"OCI manifest {digest} has no layers.");

            foreach (OciDescriptorModel layer in layers)
            {
                AddBlob(blobRoot, layer, blobs, digest, "layer");
            }
        }

        active.Remove(digest);
        complete.Add(digest);
        manifests.Add(new OciManifest(digest, mediaType, content));
    }

    private static void AddBlob(
        string blobRoot,
        OciDescriptorModel? descriptor,
        Dictionary<OciDigest, OciBlob> blobs,
        OciDigest manifest,
        string kind)
    {
        if (descriptor is null)
        {
            throw new InvalidDataException($"OCI manifest {manifest} has no {kind} descriptor.");
        }

        OciDigest digest = OciDigest.Parse(descriptor.Digest);
        var blob = new OciBlob(digest, descriptor.Size, BlobPath(blobRoot, digest));

        if (blobs.TryGetValue(digest, out OciBlob? existing) && existing.Size != blob.Size)
        {
            throw new InvalidDataException($"OCI blob {digest} has conflicting sizes.");
        }

        blobs.TryAdd(digest, blob);
    }

    private static OciDescriptorModel SelectRoot(OciDocumentModel index, OciTag tag)
    {
        OciDescriptorModel[] manifests = index.Manifests
            ?? throw new InvalidDataException("OCI index has no manifests.");
        OciDescriptorModel[] named = manifests.Where(item =>
            item.Annotations?.TryGetValue(RefName, out string? name) == true &&
            StringComparer.Ordinal.Equals(name, tag.ToString())).ToArray();

        return named.Length switch
        {
            1 => named[0],
            0 when manifests.Length == 1 => manifests[0],
            _ => throw new InvalidDataException($"OCI layout does not contain one root for {tag}.")
        };
    }

    private static OciDocumentModel Deserialize(byte[] content, string name)
    {
        try
        {
            OciDocumentModel value = JsonSerializer.Deserialize(
                content,
                OciJsonContext.Default.OciDocumentModel)
                ?? throw new InvalidDataException($"{name} is empty.");

            if (value.SchemaVersion != 2)
            {
                throw new InvalidDataException($"{name} has unsupported schema version {value.SchemaVersion}.");
            }

            return value;
        }
        catch (JsonException error)
        {
            throw new InvalidDataException($"{name} is invalid.", error);
        }
    }

    private static string RequireManifestType(string? mediaType) =>
        mediaType switch
        {
            "application/vnd.docker.distribution.manifest.v2+json" => mediaType,
            "application/vnd.docker.distribution.manifest.list.v2+json" => mediaType,
            "application/vnd.oci.image.manifest.v1+json" => mediaType,
            "application/vnd.oci.image.index.v1+json" => mediaType,
            null => throw new InvalidDataException("Manifest media type is unavailable."),
            _ => throw new InvalidDataException($"Unsupported manifest media type {mediaType}.")
        };

    private static bool IsIndex(string mediaType) =>
        mediaType is "application/vnd.docker.distribution.manifest.list.v2+json" or
            "application/vnd.oci.image.index.v1+json";

    private static void ValidateSize(long expected, long actual, OciDigest digest)
    {
        if (expected < 0 || expected != actual)
        {
            throw new InvalidDataException($"OCI content {digest} has size {actual}, expected {expected}.");
        }
    }

    private static string BlobPath(string root, OciDigest digest) => Path.Combine(root, digest.Hex);
}