using System.Diagnostics;
using System.Net.Http.Json;
using System.Text.Json;

namespace w4c_workflows.Services.Execution;

/// <summary>
/// Executes a workflow task on a <em>remote</em> managed server instead of a local
/// subprocess (S8). When a task carries a <c>server</c> reference, this executor:
///   <list type="number">
///   <item>reads the task's entry script from the local worker working dir;</item>
///   <item>builds a shell command that writes the script to the target (base64, so the
///   script can never inject shell metacharacters) and runs it with the language's
///   interpreter;</item>
///   <item>POSTs it to w4c-webapi <c>/api/serverconsole/exec</c>, which resolves the
///   server via <c>ServerConnectionRegistry</c> and runs it over SSH with
///   <c>SshRunner</c> — the same path the /server console uses;</item>
///   <item>maps the remote stdout/stderr back into the standard
///   <see cref="ExecutionResult"/> contract.</item>
///   </list>
///
/// The local subprocess path stays the default: this executor is only used when
/// <see cref="TaskInvocation.Server"/> is non-empty. The w4c-webapi endpoint is
/// authenticated with the run's tenant id (<c>X-Tenant-Id</c>, the dev bypass) plus an
/// optional service token configured via <c>Workflows:ServerApiToken</c>.
/// </summary>
public sealed class RemoteServerExecutor
{
    private readonly IHttpClientFactory _httpFactory;
    private readonly string _serverApiUrl;
    private readonly string _serverApiToken;
    private readonly int _timeoutSeconds;
    private readonly ILogger<RemoteServerExecutor> _logger;

    private static readonly JsonSerializerOptions Json = new(JsonSerializerDefaults.Web);

    public RemoteServerExecutor(IHttpClientFactory httpFactory, IConfiguration config, ILogger<RemoteServerExecutor> logger)
    {
        _httpFactory = httpFactory;
        _serverApiUrl = (config["Workflows:ServerApiUrl"] ?? string.Empty).TrimEnd('/');
        _serverApiToken = config["Workflows:ServerApiToken"] ?? string.Empty;
        _timeoutSeconds = ParseInt(config["Workflows:ServerTaskTimeoutSeconds"], 120);
        _logger = logger;
    }

    /// <summary>True when the w4c-webapi server-console URL is configured; otherwise remote execution is unavailable.</summary>
    public bool IsAvailable() => !string.IsNullOrWhiteSpace(_serverApiUrl);

    public async Task<ExecutionResult> ExecuteAsync(TaskInvocation invocation, CancellationToken ct)
    {
        var sw = Stopwatch.StartNew();
        try
        {
            if (!IsAvailable())
                return Fail($"remote execution is not configured (Workflows:ServerApiUrl is empty)", sw.Elapsed);

            var entryFile = ResolveLocalScriptPath(invocation);
            if (!File.Exists(entryFile))
                return Fail($"entry script not found locally: {entryFile}", sw.Elapsed);

            var script = await File.ReadAllTextAsync(entryFile, ct);
            var command = BuildRemoteCommand(invocation, script, out var remoteErr);
            if (remoteErr != null)
                return Fail(remoteErr, sw.Elapsed);

            var env = ExecutionHelpers.BuildEnvironment(invocation);
            var request = new ServerExecRequestDto
            {
                ServerId = invocation.Server!,
                Cwd = ExecutionHelpers.ResolveWorkingDir(invocation),
                Command = command,
                Env = env.ToDictionary(kv => kv.Key, kv => kv.Value),
                TimeoutSeconds = _timeoutSeconds,
            };

            var body = JsonSerializer.Serialize(request, Json);
            using var client = _httpFactory.CreateClient();
            using var httpReq = new HttpRequestMessage(HttpMethod.Post, $"{_serverApiUrl}/api/serverconsole/exec")
            {
                Content = new StringContent(body, System.Text.Encoding.UTF8, "application/json"),
            };
            httpReq.Headers.TryAddWithoutValidation("X-Tenant-Id", invocation.TenantId);
            if (!string.IsNullOrWhiteSpace(_serverApiToken))
                httpReq.Headers.TryAddWithoutValidation("Authorization", $"Bearer {_serverApiToken}");

            using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
            cts.CancelAfter(TimeSpan.FromSeconds(_timeoutSeconds + 15));
            using var response = await client.SendAsync(httpReq, HttpCompletionOption.ResponseContentRead, cts.Token);

            if (!response.IsSuccessStatusCode)
            {
                var errText = await response.Content.ReadAsStringAsync(ct);
                return Fail($"server {invocation.Server} exec returned {(int)response.StatusCode}: {Truncate(errText)}", sw.Elapsed);
            }

            var result = await response.Content.ReadFromJsonAsync<ServerExecResultDto>(Json, ct)
                ?? new ServerExecResultDto { ExitCode = -1, Stderr = "empty exec response" };

            // Map the remote result into the same ProcessOutput -> ExecutionResult
            // contract the local executors use (last non-empty stdout line => JSON output).
            var output = new ProcessOutput(result.ExitCode, result.Stdout ?? string.Empty, result.Stderr ?? string.Empty);
            var mapped = ExecutionHelpers.ToResult(output, sw.Elapsed);
            if (result.TimedOut)
                return new ExecutionResult(false, result.ExitCode, output.Stdout, output.Stderr, null,
                    $"remote exec timed out after {_timeoutSeconds}s", sw.Elapsed);
            return mapped;
        }
        catch (Exception ex) when (ex is not OperationCanceledException)
        {
            sw.Stop();
            return Fail(ex.Message, sw.Elapsed);
        }
        finally
        {
            sw.Stop();
        }
    }

    /// <summary>
    /// Builds the remote shell command: write the script to the target via base64
    /// (so content can never inject metacharacters) and run it with the interpreter.
    /// </summary>
    private static string BuildRemoteCommand(TaskInvocation invocation, string script, out string? error)
    {
        error = null;
        var interpreter = RemoteInterpreter(invocation.Language);
        if (interpreter == null)
        {
            error = $"language '{invocation.Language}' is not supported for remote (SSH) execution";
            return string.Empty;
        }

        var workDir = ExecutionHelpers.ResolveWorkingDir(invocation);
        var safeWorkDir = ShQuote(workDir);
        var remoteFile = Path.Combine(workDir, $".w4c-run-{invocation.TaskId[..8]}-{invocation.Attempt}-{SafeName(invocation.EntryFile)}");
        var b64 = Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(script));

        // mkdir -p <dir> && echo '<b64>' | base64 -d > <file> && {
        //   <interpreter> <file>; rc=$?; rm -f <file>; exit $rc; }
        // The staged script is removed on every completion path (including a
        // non-zero interpreter exit) instead of being left as .w4c-run-* litter;
        // the exit code is still propagated. A hard timeout kills the ssh
        // connection before this runs, so the local startup sweep cannot reach it —
        // the next run overwrites the deterministic per-(task,attempt) name.
        return $"mkdir -p {safeWorkDir} "
             + $"&& echo {ShQuote(b64)} | base64 -d > {ShQuote(remoteFile)} "
             + $"&& {{ {interpreter} {ShQuote(remoteFile)}; rc=$?; rm -f {ShQuote(remoteFile)}; exit $rc; }}";
    }

    // Only languages whose entry file can be shipped to the target as-is are
    // mapped here. TypeScript is deliberately absent: the local path bundles the
    // entry file (and its imports) with esbuild before running node, so sending
    // raw .ts source to a remote `node` would always fail with a syntax error.
    // Returning null yields an honest "not supported for remote" instead.
    private static string? RemoteInterpreter(string language) => language switch
    {
        "shell" => "sh",
        "javascript" => "node",
        "python" => "python3",
        "csharp" => null, // requires a compiler + runtime not assumed present on a target
        _ => null,
    };

    private string ResolveLocalScriptPath(TaskInvocation invocation)
        => ExecutionHelpers.TryResolveEntryPath(invocation, out var path, out _) ? path : string.Empty;

    private ExecutionResult Fail(string message, TimeSpan elapsed)
    {
        _logger.LogWarning("Remote task execution failed: {Message}", message);
        return new ExecutionResult(false, -1, string.Empty, string.Empty, null, message, elapsed);
    }

    /// <summary>Single-quotes a value for the remote sh -c evaluation (same semantics as SshRunner.ShellQuote).</summary>
    private static string ShQuote(string value)
        => string.IsNullOrEmpty(value) ? "''" : "'" + value.Replace("'", "'\"'\"'") + "'";

    private static string SafeName(string path)
    {
        var name = Path.GetFileName(path);
        return string.IsNullOrWhiteSpace(name) ? "script" : name.Replace(' ', '_');
    }

    private static string Truncate(string text)
        => text.Length > 400 ? text[..400] + "…" : text;

    private static int ParseInt(string? text, int fallback)
        => int.TryParse(text, out var value) && value > 0 ? value : fallback;

    private sealed class ServerExecRequestDto
    {
        public string ServerId { get; set; } = string.Empty;
        public string? Cwd { get; set; }
        public string Command { get; set; } = string.Empty;
        public Dictionary<string, string>? Env { get; set; }
        public int? TimeoutSeconds { get; set; }
    }

    private sealed class ServerExecResultDto
    {
        public int ExitCode { get; set; }
        public string? Stdout { get; set; }
        public string? Stderr { get; set; }
        public bool TimedOut { get; set; }
    }
}