// -----------------------------------------------------------------------------
// NpmPackageDownloader.cs
// Sprint: SPRINT_3700_0002_0001_vuln_surfaces_core (SURF-004)
// Description: Downloads npm packages from registry.npmjs.org for vulnerability
//              surface analysis.
// -----------------------------------------------------------------------------

using System;
using System.Diagnostics;
using System.IO;
using System.Net.Http;
using System.Text.Json;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using SharpCompress.Archives;
using SharpCompress.Archives.Tar;
using SharpCompress.Common;
using SharpCompress.Readers;

namespace StellaOps.Scanner.VulnSurfaces.Download;

/// <summary>
/// Downloads npm packages from registry.npmjs.org or custom registries.
/// npm packages are distributed as .tgz (gzipped tarball) files.
/// </summary>
public sealed class NpmPackageDownloader : IPackageDownloader
{
    private const string DefaultRegistryUrl = "https://registry.npmjs.org";

    private readonly HttpClient _httpClient;
    private readonly ILogger<NpmPackageDownloader> _logger;
    private readonly NpmDownloaderOptions _options;

    public NpmPackageDownloader(
        HttpClient httpClient,
        ILogger<NpmPackageDownloader> logger,
        IOptions<NpmDownloaderOptions> options)
    {
        _httpClient = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
        _options = options?.Value ?? new NpmDownloaderOptions();
    }

    /// <inheritdoc />
    public string Ecosystem => "npm";

    /// <inheritdoc />
    public async Task<PackageDownloadResult> DownloadAsync(
        PackageDownloadRequest request,
        CancellationToken cancellationToken = default)
    {
        ArgumentNullException.ThrowIfNull(request);

        var sw = Stopwatch.StartNew();

        try
        {
            // Normalize package name (npm uses lowercase, scoped packages have @scope/name)
            var packageName = request.PackageName;
            var safePackageName = GetSafeDirectoryName(packageName);
            var extractedDir = Path.Combine(request.OutputDirectory, $"{safePackageName}-{request.Version}");
            var archivePath = Path.Combine(request.OutputDirectory, $"{safePackageName}-{request.Version}.tgz");

            // Check cache first
            if (request.UseCache && Directory.Exists(extractedDir))
            {
                sw.Stop();
                _logger.LogDebug("Using cached npm package {Package} v{Version}", packageName, request.Version);
                return PackageDownloadResult.Ok(extractedDir, archivePath, sw.Elapsed, fromCache: true);
            }

            // Get package metadata to find tarball URL
            var registryUrl = request.RegistryUrl ?? _options.RegistryUrl ?? DefaultRegistryUrl;
            var tarballUrl = await GetTarballUrlAsync(registryUrl, packageName, request.Version, cancellationToken);

            if (tarballUrl is null)
            {
                sw.Stop();
                var error = $"Version {request.Version} not found for package {packageName}";
                _logger.LogWarning("npm package not found: {Error}", error);
                return PackageDownloadResult.Fail(error, sw.Elapsed);
            }

            _logger.LogDebug("Downloading npm package from {Url}", tarballUrl);

            // Download tarball
            Directory.CreateDirectory(request.OutputDirectory);

            using var response = await _httpClient.GetAsync(tarballUrl, cancellationToken);

            if (!response.IsSuccessStatusCode)
            {
                sw.Stop();
                var error = $"Failed to download: HTTP {(int)response.StatusCode} {response.ReasonPhrase}";
                _logger.LogWarning("npm download failed for {Package} v{Version}: {Error}",
                    packageName, request.Version, error);
                return PackageDownloadResult.Fail(error, sw.Elapsed);
            }

            // Save archive
            await using (var fs = File.Create(archivePath))
            {
                await response.Content.CopyToAsync(fs, cancellationToken);
            }

            // Extract .tgz (gzipped tarball)
            if (Directory.Exists(extractedDir))
            {
                Directory.Delete(extractedDir, recursive: true);
            }

            Directory.CreateDirectory(extractedDir);
            ExtractTgz(archivePath, extractedDir);

            sw.Stop();
            _logger.LogDebug("Downloaded and extracted npm {Package} v{Version} in {Duration}ms",
                packageName, request.Version, sw.ElapsedMilliseconds);

            return PackageDownloadResult.Ok(extractedDir, archivePath, sw.Elapsed);
        }
        catch (Exception ex)
        {
            sw.Stop();
            _logger.LogWarning(ex, "Failed to download npm package {Package} v{Version}",
                request.PackageName, request.Version);
            return PackageDownloadResult.Fail(ex.Message, sw.Elapsed);
        }
    }

    /// <summary>
    /// Gets the tarball URL from the npm registry metadata.
    /// </summary>
    private async Task<string?> GetTarballUrlAsync(
        string registryUrl,
        string packageName,
        string version,
        CancellationToken cancellationToken)
    {
        // Encode scoped packages (@scope/name → @scope%2fname)
        var encodedName = Uri.EscapeDataString(packageName).Replace("%40", "@");
        var metadataUrl = $"{registryUrl}/{encodedName}";

        using var response = await _httpClient.GetAsync(metadataUrl, cancellationToken);

        if (!response.IsSuccessStatusCode)
        {
            _logger.LogDebug("Failed to fetch npm metadata for {Package}: HTTP {StatusCode}",
                packageName, (int)response.StatusCode);
            return null;
        }

        await using var stream = await response.Content.ReadAsStreamAsync(cancellationToken);
        using var doc = await JsonDocument.ParseAsync(stream, cancellationToken: cancellationToken);

        // Look for versions.<version>.dist.tarball
        if (doc.RootElement.TryGetProperty("versions", out var versions) &&
            versions.TryGetProperty(version, out var versionObj) &&
            versionObj.TryGetProperty("dist", out var dist) &&
            dist.TryGetProperty("tarball", out var tarball))
        {
            return tarball.GetString();
        }

        return null;
    }

    /// <summary>
    /// Extracts a .tgz file (gzipped tarball) to the specified directory.
    /// </summary>
    private static void ExtractTgz(string tgzPath, string destinationDir)
    {
        using var archive = ArchiveFactory.Open(tgzPath);

        foreach (var entry in archive.Entries)
        {
            if (entry.IsDirectory)
            {
                continue;
            }

            // npm packages have a "package/" prefix in the tarball
            var entryPath = entry.Key ?? string.Empty;
            if (entryPath.StartsWith("package/", StringComparison.OrdinalIgnoreCase))
            {
                entryPath = entryPath["package/".Length..];
            }

            var destPath = Path.Combine(destinationDir, entryPath);
            var destDir = Path.GetDirectoryName(destPath);

            if (!string.IsNullOrEmpty(destDir))
            {
                Directory.CreateDirectory(destDir);
            }

            entry.WriteToFile(destPath, new ExtractionOptions
            {
                ExtractFullPath = false,
                Overwrite = true
            });
        }
    }

    /// <summary>
    /// Converts a package name to a safe directory name.
    /// Handles scoped packages like @scope/name → scope-name
    /// </summary>
    private static string GetSafeDirectoryName(string packageName)
    {
        return packageName
            .Replace("@", string.Empty)
            .Replace("/", "-")
            .Replace("\\", "-");
    }
}

/// <summary>
/// Options for npm package downloader.
/// </summary>
public sealed class NpmDownloaderOptions
{
    /// <summary>
    /// Custom registry URL (null for registry.npmjs.org).
    /// </summary>
    public string? RegistryUrl { get; set; }

    /// <summary>
    /// Cache directory for downloaded packages.
    /// </summary>
    public string? CacheDirectory { get; set; }

    /// <summary>
    /// Maximum package size in bytes (0 for unlimited).
    /// </summary>
    public long MaxPackageSize { get; set; }
}