using System.Collections.Concurrent;
using System.Diagnostics;

namespace w4c_workflows.Services;

/// <summary>Result of a single <c>git</c> invocation.</summary>
public sealed record GitResult(int ExitCode, string StdOut, string StdErr, bool TimedOut)
{
    /// <summary>True when git ran to completion with exit code 0.</summary>
    public bool Succeeded => !TimedOut && ExitCode == 0;

    /// <summary>Trimmed stdout when the call succeeded, otherwise <c>null</c>.</summary>
    public string? TrimmedStdOut => Succeeded ? StdOut.Trim() : null;
}

/// <summary>
/// The single entry point for invoking the <c>git</c> CLI. It replaces the four
/// copy-pasted synchronous helpers (workflow source, per-tenant source, workflow
/// file controller, Forgejo service) that each drained only stdout and blocked on
/// <c>WaitForExit(5000)</c>: a full stderr pipe could deadlock the request and the
/// child was never reaped on timeout.
///
/// <see cref="RunAsync"/> drains both pipes concurrently, waits asynchronously
/// under a wall-clock timeout (and the caller token) and kills the whole process
/// tree on timeout/cancellation. <see cref="GetStateAsync"/> adds a short-TTL
/// cache so a workflow list does not shell out to git on every request.
/// </summary>
public sealed class GitRunner
{
    private static readonly TimeSpan DefaultTimeout = TimeSpan.FromSeconds(5);

    private readonly ILogger<GitRunner> _logger;
    private readonly TimeSpan _stateTtl;
    private readonly ConcurrentDictionary<string, CachedState> _stateCache = new(StringComparer.Ordinal);

    private sealed record CachedState(WorkflowSourceState State, DateTime At);

    public GitRunner(IConfiguration config, ILogger<GitRunner> logger)
    {
        _logger = logger;
        // Cache duration for GetStateAsync. 0 disables caching (always shell out).
        _stateTtl = int.TryParse(config["WorkflowSource:GitStateCacheSeconds"], out var seconds)
            ? TimeSpan.FromSeconds(Math.Max(0, seconds))
            : TimeSpan.FromSeconds(15);
    }

    /// <summary>
    /// Runs git in <paramref name="workDir"/>. Both output pipes are drained
    /// concurrently, the wait is asynchronous and bounded by
    /// <paramref name="timeout"/> (default 5s) plus <paramref name="ct"/>, and the
    /// process tree is killed if either fires. Failures (missing git, bad working
    /// directory) are reported as a result with exit code -1 rather than thrown.
    /// </summary>
    public async Task<GitResult> RunAsync(
        string workDir,
        IReadOnlyList<string> args,
        CancellationToken ct = default,
        TimeSpan? timeout = null,
        IReadOnlyDictionary<string, string>? environment = null)
    {
        Process? process = null;
        try
        {
            var psi = new ProcessStartInfo("git")
            {
                WorkingDirectory = workDir,
                RedirectStandardOutput = true,
                RedirectStandardError = true,
                UseShellExecute = false,
                CreateNoWindow = true,
            };
            foreach (var arg in args)
                psi.ArgumentList.Add(arg);
            if (environment != null)
                foreach (var (key, value) in environment)
                    psi.Environment[key] = value;

            process = Process.Start(psi);
            if (process == null)
                return new GitResult(-1, string.Empty, "git failed to start", TimedOut: false);

            using var linked = CancellationTokenSource.CreateLinkedTokenSource(ct);
            linked.CancelAfter(timeout ?? DefaultTimeout);

            var stdoutTask = process.StandardOutput.ReadToEndAsync();
            var stderrTask = process.StandardError.ReadToEndAsync();

            try
            {
                await process.WaitForExitAsync(linked.Token);
            }
            catch (OperationCanceledException)
            {
                TryKill(process);
                // Distinguish a wall-clock timeout (report it) from the caller's
                // request being aborted (propagate — do not mask cancellation).
                if (ct.IsCancellationRequested)
                    throw;
                return new GitResult(-1, Pending(stdoutTask), Pending(stderrTask), TimedOut: true);
            }

            return new GitResult(process.ExitCode, await stdoutTask, await stderrTask, TimedOut: false);
        }
        catch (Exception ex)
        {
            _logger.LogDebug(ex, "git invocation failed in {Dir}", workDir);
            return new GitResult(-1, string.Empty, ex.Message, TimedOut: false);
        }
        finally
        {
            process?.Dispose();
        }
    }

    /// <summary>
    /// HEAD sha + dirty flag for a working tree, computed with a single
    /// <c>git status --porcelain=v2 --branch</c> call (instead of the previous
    /// <c>rev-parse HEAD</c> + <c>status --porcelain</c> pair) and cached for
    /// <c>WorkflowSource:GitStateCacheSeconds</c> (default 15s).
    /// </summary>
    public async Task<WorkflowSourceState> GetStateAsync(string workDir, CancellationToken ct = default)
    {
        if (_stateTtl > TimeSpan.Zero
            && _stateCache.TryGetValue(workDir, out var cached)
            && DateTime.UtcNow - cached.At < _stateTtl)
            return cached.State;

        var result = await RunAsync(
            workDir,
            new[] { "status", "--porcelain=v2", "--branch", "--untracked-files=normal" },
            ct);

        var state = ParseState(result);
        _stateCache[workDir] = new CachedState(state, DateTime.UtcNow);
        return state;
    }

    /// <summary>Drops the cached state for a working tree (e.g. after a commit/pull).</summary>
    public void InvalidateState(string workDir) => _stateCache.TryRemove(workDir, out _);

    /// <summary>
    /// Parses <c>git status --porcelain=v2 --branch</c> output into a
    /// <see cref="WorkflowSourceState"/>. Lines starting with <c>#</c> are branch
    /// headers; <c># branch.oid</c> is the HEAD sha and any other line (changed or
    /// untracked entry) marks the tree dirty. A failed call yields an empty state.
    /// </summary>
    public static WorkflowSourceState ParseState(GitResult result)
    {
        if (!result.Succeeded)
            return new WorkflowSourceState(null, false);

        string? head = null;
        var dirty = false;
        foreach (var raw in result.StdOut.Split('\n'))
        {
            var line = raw.TrimEnd('\r');
            if (line.Length == 0)
                continue;
            if (line.StartsWith("# branch.oid ", StringComparison.Ordinal))
                head = line["# branch.oid ".Length..].Trim();
            else if (line[0] != '#')
                dirty = true;
        }

        // An unborn branch reports the sentinel "(initial)" instead of a sha.
        if (head is "(initial)" or "")
            head = null;

        return new WorkflowSourceState(head, dirty);
    }

    private static string Pending(Task<string> task)
        => task.IsCompletedSuccessfully ? task.Result : string.Empty;

    private static void TryKill(Process process)
    {
        try { process.Kill(entireProcessTree: true); }
        catch { /* already exited or inaccessible */ }
    }
}