using System.Diagnostics;
using System.Text;

namespace w4c_workflows.Services.Execution;

/// <summary>Raw subprocess output before JSON interpretation.</summary>
public sealed record ProcessOutput(int ExitCode, string Stdout, string Stderr);

/// <summary>
/// Runs a subprocess with stdin/stdout/stderr redirection, an optional JSON
/// stdin payload, extra environment variables, and a hard timeout. On timeout
/// or external cancellation the whole process tree is always killed, and the
/// captured output is bounded so a chatty script cannot exhaust memory.
/// </summary>
public static class ProcessRunner
{
    /// <summary>Per-stream capture ceiling. Extra output is drained and discarded.</summary>
    internal const int MaxCapturedChars = 1024 * 1024;

    public static async Task<ProcessOutput> RunAsync(
        string executable,
        IReadOnlyList<string> args,
        string workingDir,
        IReadOnlyDictionary<string, string>? env,
        string? stdin,
        TimeSpan timeout,
        CancellationToken ct)
    {
        var psi = new ProcessStartInfo
        {
            FileName = executable,
            WorkingDirectory = workingDir,
            RedirectStandardInput = true,
            RedirectStandardOutput = true,
            RedirectStandardError = true,
            UseShellExecute = false,
        };
        foreach (var arg in args)
            psi.ArgumentList.Add(arg);

        if (env != null)
        {
            foreach (var (key, value) in env)
                psi.Environment[key] = value;
        }

        using var process = new Process { StartInfo = psi };
        if (!process.Start())
            throw new InvalidOperationException($"failed to start '{executable}'");

        // Start draining stdout/stderr before writing stdin so a chatty child
        // never blocks on a full pipe. Both readers drain without bound but keep
        // only MaxCapturedChars in memory.
        var stdoutTask = ReadCappedAsync(process.StandardOutput, MaxCapturedChars);
        var stderrTask = ReadCappedAsync(process.StandardError, MaxCapturedChars);

        if (!string.IsNullOrEmpty(stdin))
        {
            try
            {
                await process.StandardInput.WriteAsync(stdin);
            }
            catch (IOException)
            {
                // The child exited before reading stdin (e.g. a script that ignores
                // its input) — a normal condition, not a failure.
            }
        }

        try
        {
            process.StandardInput.Close();
        }
        catch (IOException)
        {
            // The child exited between the write and the close, so the pipe is
            // already broken — likewise normal, not a failure.
        }

        using var timeoutCts = CancellationTokenSource.CreateLinkedTokenSource(ct);
        timeoutCts.CancelAfter(timeout);

        try
        {
            await process.WaitForExitAsync(timeoutCts.Token);
        }
        catch (OperationCanceledException) when (!ct.IsCancellationRequested)
        {
            throw new TimeoutException($"process exceeded the {timeout.TotalSeconds:0}s time limit");
        }
        finally
        {
            // The wait throws on timeout AND on external cancellation; without
            // this the child tree survived as an orphan whenever a run was
            // cancelled or the host shut down. Reap it unconditionally, then
            // drain the readers so they are never left unobserved.
            if (!process.HasExited)
            {
                try { process.Kill(entireProcessTree: true); } catch { /* already gone */ }
                // Reap the killed child so it does not linger as a zombie; bounded
                // so a pathological kill cannot wedge the runner.
                try { await process.WaitForExitAsync(CancellationToken.None).WaitAsync(TimeSpan.FromSeconds(5)); }
                catch { /* best effort */ }
            }

            await IgnoreFailureAsync(stdoutTask);
            await IgnoreFailureAsync(stderrTask);
        }

        var stdout = await stdoutTask;
        var stderr = await stderrTask;
        return new ProcessOutput(process.ExitCode, stdout, stderr);
    }

    /// <summary>
    /// Reads <paramref name="reader"/> to the end while capturing at most
    /// <paramref name="maxChars"/> characters. The remainder is still drained so
    /// the child never blocks on a full pipe, but it is not retained.
    /// </summary>
    public static async Task<string> ReadCappedAsync(TextReader reader, int maxChars, CancellationToken ct = default)
    {
        var buffer = new char[8192];
        var captured = new StringBuilder(Math.Min(maxChars, 64 * 1024));

        while (true)
        {
            var read = await reader.ReadAsync(buffer.AsMemory(0, buffer.Length), ct);
            if (read == 0)
                break;

            if (captured.Length >= maxChars)
                continue; // over the cap — keep draining, discard

            var take = Math.Min(read, maxChars - captured.Length);
            captured.Append(buffer, 0, take);
        }

        return captured.ToString();
    }

    private static async Task IgnoreFailureAsync(Task<string> task)
    {
        try { await task; } catch { /* best-effort drain */ }
    }

    /// <summary>
    /// Resolves an executable name against PATH (or a literal path) without
    /// invoking it — a cheap availability probe.
    /// </summary>
    public static string? FindInPath(string executable)
    {
        if (executable.Contains(Path.DirectorySeparatorChar) || executable.Contains('/'))
            return File.Exists(executable) ? Path.GetFullPath(executable) : null;

        var path = Environment.GetEnvironmentVariable("PATH") ?? string.Empty;
        foreach (var dir in path.Split(Path.PathSeparator, StringSplitOptions.RemoveEmptyEntries))
        {
            var full = Path.Combine(dir, executable);
            if (File.Exists(full))
                return full;
        }

        return null;
    }
}