using System.Net.Http.Headers;
using System.Runtime.InteropServices;
using System.Text.Json;
using CursorLang.Core.Models;

namespace CursorLang.Core.Services;

/// <summary>
/// Gitea releases.
/// </summary>
/// <remarks>
/// The service address is the address of the server itself — "https://git.example.com/":
/// the Gitea API lives on the same host as the repository pages.
/// </remarks>
internal sealed class GiteaReleaseFeed : IReleaseFeed
{
    /// <summary>
    /// How many releases to ask the server for. It returns them newest first, but the
    /// newest one may turn out to have no package — when the build has not been
    /// published yet, for instance — so a small reserve is taken.
    /// </summary>
    private const int PageSize = 10;

    /// <summary>
    /// A response with the release list is a few kilobytes of text. There is no point
    /// waiting longer: the check runs in the background, and a failed one bothers nobody.
    /// </summary>
    private static readonly TimeSpan RequestTimeout = TimeSpan.FromSeconds(20);

    private readonly HttpClient _client;
    private readonly UpdateOptions _options;

    public GiteaReleaseFeed(HttpClient client, UpdateOptions options)
    {
        _client = client;
        _options = options;
    }

    /// <summary>
    /// How the architecture is spelled in the file names built by
    /// <c>build-msix.ps1</c>: <c>CursorLang-1.0.0.0-x64.msix</c>.
    /// </summary>
    private static string ArchitectureName => RuntimeInformation.ProcessArchitecture == Architecture.Arm64
        ? "arm64"
        : "x64";

    public async Task<ReleaseInfo?> GetLatestAsync(CancellationToken cancellationToken)
    {
        using var timeout = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
        timeout.CancelAfter(RequestTimeout);

        using var request = new HttpRequestMessage(HttpMethod.Get, BuildReleasesUri());
        Authorize(request);

        using HttpResponseMessage response = await _client.SendAsync(
            request, HttpCompletionOption.ResponseHeadersRead, timeout.Token);
        response.EnsureSuccessStatusCode();

        await using Stream stream = await response.Content.ReadAsStreamAsync(timeout.Token);
        using JsonDocument document = await JsonDocument.ParseAsync(stream, cancellationToken: timeout.Token);

        if (document.RootElement.ValueKind != JsonValueKind.Array)
        {
            return null;
        }

        // The order of the releases is up to the server, while what we need is the
        // highest version number: a fix released for an old branch may well be the newest one
        return document.RootElement.EnumerateArray()
            .Select(Read)
            .OfType<ReleaseInfo>()
            .MaxBy(release => release.Version);
    }

    public void Authorize(HttpRequestMessage request)
    {
        if (!string.IsNullOrWhiteSpace(_options.AccessToken))
        {
            // "token" is the Gitea scheme of its own for access keys; "Bearer" is not
            // understood by every version, while this one has been there since the API appeared
            request.Headers.Authorization = new AuthenticationHeaderValue("token", _options.AccessToken);
        }
    }

    /// <summary>
    /// Parses a single release. <c>null</c> means the release will not do: a draft,
    /// a prerelease or a release without a package.
    /// </summary>
    private static ReleaseInfo? Read(JsonElement release)
    {
        // A draft is visible only to whoever created it, and the application does not
        // offer a prerelease: those are sought out deliberately
        if (ReadFlag(release, "draft") || ReadFlag(release, "prerelease"))
        {
            return null;
        }

        Version? version = ParseTag(ReadString(release, "tag_name"));
        if (version is null)
        {
            return null;
        }

        if (!release.TryGetProperty("assets", out JsonElement assets) || assets.ValueKind != JsonValueKind.Array)
        {
            return null;
        }

        ReleaseAsset? package = PickPackage(assets.EnumerateArray().Select(ReadAsset).OfType<ReleaseAsset>());
        if (package is null)
        {
            return null;
        }

        return new ReleaseInfo(
            version,
            ReadString(release, "tag_name") ?? version.ToString(),
            ReadUri(release, "html_url"),
            package);
    }

    private static ReleaseAsset? ReadAsset(JsonElement asset)
    {
        string? name = ReadString(asset, "name");
        Uri? url = ReadUri(asset, "browser_download_url");

        if (string.IsNullOrWhiteSpace(name) || url is null)
        {
            return null;
        }

        long size = asset.TryGetProperty("size", out JsonElement value) && value.TryGetInt64(out long bytes)
            ? bytes
            : 0;

        return new ReleaseAsset(name, url, size);
    }

    /// <summary>
    /// The version from a tag. "1.2.3" and "v1.2.3" are understood; a tag with
    /// anything besides numbers — "v1.2.3-beta" — counts as a prerelease and is
    /// skipped: the application does not offer such versions on its own.
    /// </summary>
    private static Version? ParseTag(string? tag)
    {
        if (string.IsNullOrWhiteSpace(tag))
        {
            return null;
        }

        ReadOnlySpan<char> numbers = tag.AsSpan().Trim().TrimStart("vV");

        foreach (char symbol in numbers)
        {
            if (!char.IsAsciiDigit(symbol) && symbol != '.')
            {
                return null;
            }
        }

        if (!Version.TryParse(numbers, out Version? version))
        {
            return null;
        }

        // In a tag such as "v1.2" the lower parts are not set at all, yet comparing
        // them with the version of the installed package calls for zeros
        return new Version(
            version.Major,
            version.Minor,
            Math.Max(version.Build, 0),
            Math.Max(version.Revision, 0));
    }

    /// <summary>
    /// Picks the attached file the application updates itself with.
    /// </summary>
    private static ReleaseAsset? PickPackage(IEnumerable<ReleaseAsset> assets)
    {
        // An unencrypted connection is out right away: Windows will check the package
        // signature by itself, but a substituted file is not even worth downloading
        ReleaseAsset[] packages = [.. assets.Where(asset => asset.Url.Scheme == Uri.UriSchemeHttps)];

        ReleaseAsset? bundle = packages.FirstOrDefault(
            asset => asset.FileName.EndsWith(".msixbundle", StringComparison.OrdinalIgnoreCase));

        if (bundle is not null)
        {
            // A bundle carries both architectures, so there is nothing to choose between
            return bundle;
        }

        ReleaseAsset[] single = [.. packages.Where(
            asset => asset.FileName.EndsWith(".msix", StringComparison.OrdinalIgnoreCase))];

        ReleaseAsset? matching = single.FirstOrDefault(
            asset => asset.FileName.Contains(ArchitectureName, StringComparison.OrdinalIgnoreCase));

        // A package without an architecture in its name will do only when it is the
        // only one: otherwise it is unclear which of them is for this machine
        return matching ?? (single.Length == 1 ? single[0] : null);
    }

    private static string? ReadString(JsonElement element, string name) =>
        element.TryGetProperty(name, out JsonElement value) && value.ValueKind == JsonValueKind.String
            ? value.GetString()
            : null;

    private static Uri? ReadUri(JsonElement element, string name) =>
        Uri.TryCreate(ReadString(element, name), UriKind.Absolute, out Uri? uri) ? uri : null;

    private static bool ReadFlag(JsonElement element, string name) =>
        element.TryGetProperty(name, out JsonElement value) && value.ValueKind == JsonValueKind.True;

    /// <summary>
    /// The address the server returns the release list at. The trailing slash matters:
    /// without it <c>Uri</c> drops the last part of the address, and
    /// "https://host/gitea" would have turned into "https://host/api/...".
    /// </summary>
    private Uri BuildReleasesUri()
    {
        string service = _options.ServiceUri.AbsoluteUri;
        string path = $"api/v1/repos/{_options.Project.Trim('/')}/releases?limit={PageSize}";

        return new Uri(service.EndsWith('/') ? service + path : $"{service}/{path}");
    }
}