using System.Buffers.Binary;
using System.Collections.Immutable;
using System.Formats.Tar;
using System.IO.Compression;
using System.Security.Cryptography;
using System.Text;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace StellaOps.ExportCenter.RiskBundles;

public sealed class RiskBundleBuilder
{
    private const string ManifestVersion = "1";
    private static readonly UnixFileMode DefaultFileMode = UnixFileMode.UserRead | UnixFileMode.UserWrite | UnixFileMode.GroupRead | UnixFileMode.OtherRead;
    private static readonly DateTimeOffset FixedTimestamp = new(2024, 01, 01, 0, 0, 0, TimeSpan.Zero);

    private static readonly JsonSerializerOptions SerializerOptions = new()
    {
        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
        WriteIndented = false,
        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
    };

    public RiskBundleBuildResult Build(RiskBundleBuildRequest request, CancellationToken cancellationToken = default)
    {
        ArgumentNullException.ThrowIfNull(request);
        if (request.BundleId == Guid.Empty)
        {
            throw new ArgumentException("Bundle identifier must be provided.", nameof(request));
        }

        if (request.Providers is not { Count: > 0 })
        {
            throw new ArgumentException("At least one provider input is required.", nameof(request));
        }

        var providers = CollectProviders(request, cancellationToken);
        var manifest = BuildManifest(request.BundleId, providers);
        var manifestJson = JsonSerializer.Serialize(manifest, SerializerOptions);
        var rootHash = ComputeSha256(manifestJson);

        var bundleStream = CreateBundleStream(
            providers,
            request.ManifestFileName,
            manifestJson,
            request.BundleFileName ?? "risk-bundle.tar.gz");

        bundleStream.Position = 0;

        return new RiskBundleBuildResult(manifest, manifestJson, rootHash, bundleStream);
    }

    private static List<RiskBundleProviderEntry> CollectProviders(RiskBundleBuildRequest request, CancellationToken cancellationToken)
    {
        var entries = new List<RiskBundleProviderEntry>(request.Providers.Count);

        foreach (var provider in request.Providers.OrderBy(p => p.ProviderId, StringComparer.Ordinal))
        {
            cancellationToken.ThrowIfCancellationRequested();

            if (provider is null)
            {
                throw new ArgumentException("Provider list cannot contain null entries.", nameof(request));
            }

            if (string.IsNullOrWhiteSpace(provider.ProviderId))
            {
                throw new ArgumentException("ProviderId cannot be empty.", nameof(provider.ProviderId));
            }

            if (string.IsNullOrWhiteSpace(provider.SourcePath))
            {
                throw new ArgumentException("SourcePath cannot be empty.", nameof(provider.SourcePath));
            }

            var fullPath = Path.GetFullPath(provider.SourcePath);
            if (!File.Exists(fullPath))
            {
                if (provider.Optional && request.AllowMissingOptional)
                {
                    continue;
                }

                throw new FileNotFoundException($"Provider source file '{fullPath}' not found.", fullPath);
            }

            var sha256 = ComputeSha256FromFile(fullPath);
            var size = new FileInfo(fullPath).Length;
            var bundlePath = $"providers/{provider.ProviderId}/snapshot";

            entries.Add(new RiskBundleProviderEntry(
                provider.ProviderId,
                provider.Source,
                provider.SnapshotDate,
                sha256,
                size,
                provider.Optional,
                bundlePath,
                fullPath,
                SignaturePath: null));
        }

        if (entries.Count == 0)
        {
            throw new InvalidOperationException("No provider artefacts collected. Provide at least one valid provider input.");
        }

        return entries;
    }

    private static RiskBundleManifest BuildManifest(Guid bundleId, List<RiskBundleProviderEntry> providers)
    {
        var inputsHash = ComputeInputsHash(providers);
        return new RiskBundleManifest(
            ManifestVersion,
            bundleId,
            FixedTimestamp,
            providers.ToImmutableArray(),
            inputsHash);
    }

    private static string ComputeInputsHash(IEnumerable<RiskBundleProviderEntry> providers)
    {
        using var sha = SHA256.Create();
        var builder = new StringBuilder();
        foreach (var entry in providers.OrderBy(e => e.ProviderId, StringComparer.Ordinal))
        {
            builder.Append(entry.ProviderId)
                .Append('\0')
                .Append(entry.Sha256)
                .Append('\0')
                .Append(entry.Source)
                .Append('\0')
                .Append(entry.Optional ? '1' : '0')
                .Append('\0')
                .Append(entry.SnapshotDate?.ToString("yyyy-MM-dd") ?? string.Empty)
                .Append('\0');
        }

        var bytes = Encoding.UTF8.GetBytes(builder.ToString());
        var hash = sha.ComputeHash(bytes);
        return ToHex(hash);
    }

    private static string ComputeSha256(string content)
    {
        var bytes = Encoding.UTF8.GetBytes(content);
        using var sha = SHA256.Create();
        var hash = sha.ComputeHash(bytes);
        return ToHex(hash);
    }

    private static string ComputeSha256FromFile(string filePath)
    {
        using var stream = File.OpenRead(filePath);
        using var sha = SHA256.Create();
        var hash = sha.ComputeHash(stream);
        return ToHex(hash);
    }

    private static string ToHex(ReadOnlySpan<byte> bytes)
    {
        Span<byte> hex = stackalloc byte[bytes.Length * 2];
        for (var i = 0; i < bytes.Length; i++)
        {
            var b = bytes[i];
            hex[i * 2] = GetHexValue(b / 16);
            hex[i * 2 + 1] = GetHexValue(b % 16);
        }
        return Encoding.ASCII.GetString(hex);
    }

    private static byte GetHexValue(int i) => (byte)(i < 10 ? i + 48 : i - 10 + 97);

    private static MemoryStream CreateBundleStream(
        IReadOnlyList<RiskBundleProviderEntry> providers,
        string manifestFileName,
        string manifestJson,
        string bundleFileName)
    {
        var stream = new MemoryStream();
        using (var gzip = new GZipStream(stream, CompressionLevel.SmallestSize, leaveOpen: true))
        using (var tar = new TarWriter(gzip, TarEntryFormat.Pax, leaveOpen: true))
        {
            WriteTextEntry(tar, $"manifests/{manifestFileName}", manifestJson, DefaultFileMode);

            foreach (var provider in providers)
            {
                WriteProviderFile(tar, provider);
            }
        }

        ApplyDeterministicGzipHeader(stream);
        stream.Position = 0;
        return stream;
    }

    private static void WriteProviderFile(TarWriter writer, RiskBundleProviderEntry entry)
    {
        var filePath = entry.SourceFilePath ?? throw new InvalidOperationException("Source file path missing for provider entry.");
        using var dataStream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read, 128 * 1024, FileOptions.SequentialScan);
        var tarEntry = new PaxTarEntry(TarEntryType.RegularFile, entry.BundlePath)
        {
            Mode = DefaultFileMode,
            ModificationTime = FixedTimestamp,
            DataStream = dataStream
        };
        writer.WriteEntry(tarEntry);
    }

    private static void WriteTextEntry(TarWriter writer, string path, string content, UnixFileMode mode)
    {
        var bytes = Encoding.UTF8.GetBytes(content);
        using var dataStream = new MemoryStream(bytes);
        var entry = new PaxTarEntry(TarEntryType.RegularFile, path)
        {
            Mode = mode,
            ModificationTime = FixedTimestamp,
            DataStream = dataStream
        };
        writer.WriteEntry(entry);
    }

    private static void ApplyDeterministicGzipHeader(MemoryStream stream)
    {
        if (stream.Length < 10)
        {
            throw new InvalidOperationException("GZip header not fully written for risk bundle.");
        }

        var seconds = checked((int)(FixedTimestamp - DateTimeOffset.UnixEpoch).TotalSeconds);
        Span<byte> buffer = stackalloc byte[4];
        BinaryPrimitives.WriteInt32LittleEndian(buffer, seconds);

        var originalPosition = stream.Position;
        stream.Position = 4;
        stream.Write(buffer);
        stream.Position = originalPosition;
    }
}