package executor

import (
	"bufio"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"log/slog"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"sync"
	"syscall"
	"time"

	"github.com/thepeterstone/claudomator/internal/storage"
	"github.com/thepeterstone/claudomator/internal/task"
)

// ClaudeRunner spawns the `claude` CLI in non-interactive mode.
type ClaudeRunner struct {
	BinaryPath string // defaults to "claude"
	Logger     *slog.Logger
	LogDir     string // base directory for execution logs
	APIURL     string // base URL of the Claudomator API, passed to subprocesses
}

// BlockedError is returned by Run when the agent wrote a question file and exited.
// The pool transitions the task to BLOCKED and stores the question for the user.
type BlockedError struct {
	QuestionJSON string // raw JSON from the question file
	SessionID    string // claude session to resume once the user answers
	SandboxDir   string // preserved sandbox path; resume must run here so Claude finds its session files
}

func (e *BlockedError) Error() string { return fmt.Sprintf("task blocked: %s", e.QuestionJSON) }

// ExecLogDir returns the log directory for the given execution ID.
// Implements LogPather so the pool can persist paths before execution starts.
func (r *ClaudeRunner) ExecLogDir(execID string) string {
	if r.LogDir == "" {
		return ""
	}
	return filepath.Join(r.LogDir, execID)
}

func (r *ClaudeRunner) binaryPath() string {
	if r.BinaryPath != "" {
		return r.BinaryPath
	}
	return "claude"
}

// Run executes a claude -p invocation, streaming output to log files.
// It retries up to 3 times on rate-limit errors using exponential backoff.
// If the agent writes a question file and exits, Run returns *BlockedError.
//
// When project_dir is set and this is not a resume execution, Run clones the
// project into a temp sandbox, runs the agent there, then merges committed
// changes back to project_dir. On failure the sandbox is preserved and its
// path is included in the error.
func (r *ClaudeRunner) Run(ctx context.Context, t *task.Task, e *storage.Execution) error {
	projectDir := t.Agent.ProjectDir

	// Validate project_dir exists when set.
	if projectDir != "" {
		if _, err := os.Stat(projectDir); err != nil {
			return fmt.Errorf("project_dir %q: %w", projectDir, err)
		}
	}

	// Setup log directory once; retries overwrite the log files.
	logDir := r.ExecLogDir(e.ID)
	if logDir == "" {
		logDir = e.ID // fallback for tests without LogDir set
	}
	if err := os.MkdirAll(logDir, 0700); err != nil {
		return fmt.Errorf("creating log dir: %w", err)
	}
	if e.StdoutPath == "" {
		e.StdoutPath = filepath.Join(logDir, "stdout.log")
		e.StderrPath = filepath.Join(logDir, "stderr.log")
		e.ArtifactDir = logDir
	}

	// Pre-assign session ID so we can resume after a BLOCKED state.
	// For resume executions, the claude session continues under the original
	// session ID (the one passed to --resume). Using the new exec's own UUID
	// would cause a second block-and-resume cycle to pass the wrong --resume
	// argument.
	if e.SessionID == "" {
		if e.ResumeSessionID != "" {
			e.SessionID = e.ResumeSessionID
		} else {
			e.SessionID = e.ID // reuse execution UUID as session UUID (both are UUIDs)
		}
	}

	// For new (non-resume) executions with a project_dir, clone into a sandbox.
	// Resume executions run in the preserved sandbox (e.SandboxDir) so Claude
	// finds its session files under the same project slug. If no sandbox was
	// preserved (e.g. task had no project_dir), fall back to project_dir.
	var sandboxDir string
	var startHEAD string
	effectiveWorkingDir := projectDir
	if e.ResumeSessionID != "" {
		if e.SandboxDir != "" {
			if _, statErr := os.Stat(e.SandboxDir); statErr == nil {
				effectiveWorkingDir = e.SandboxDir
			} else {
				// Preserved sandbox was cleaned up (e.g. /tmp purge after reboot).
				// Clone a fresh sandbox so the task can run rather than fail immediately.
				r.Logger.Warn("preserved sandbox missing, cloning fresh", "sandbox", e.SandboxDir, "project_dir", projectDir)
				e.SandboxDir = ""
				if projectDir != "" {
					var err error
					sandboxDir, err = setupSandbox(projectDir)
					if err != nil {
						return fmt.Errorf("setting up sandbox: %w", err)
					}
					effectiveWorkingDir = sandboxDir
					r.Logger.Info("fresh sandbox created for resume", "sandbox", sandboxDir, "project_dir", projectDir)
				}
			}
		}
	} else if projectDir != "" {
		var err error
		sandboxDir, err = setupSandbox(projectDir)
		if err != nil {
			return fmt.Errorf("setting up sandbox: %w", err)
		}
		effectiveWorkingDir = sandboxDir
		r.Logger.Info("sandbox created", "sandbox", sandboxDir, "project_dir", projectDir)
	}

	if effectiveWorkingDir != "" {
		// Capture the initial HEAD so we can identify new commits later.
		headOut, _ := exec.Command("git", gitSafe("-C", effectiveWorkingDir, "rev-parse", "HEAD")...).Output()
		startHEAD = strings.TrimSpace(string(headOut))
	}

	questionFile := filepath.Join(logDir, "question.json")
	args := r.buildArgs(t, e, questionFile)

	attempt := 0
	err := runWithBackoff(ctx, 3, 5*time.Second, func() error {
		if attempt > 0 {
			delay := 5 * time.Second * (1 << (attempt - 1))
			r.Logger.Warn("rate-limited by Claude API, retrying",
				"attempt", attempt,
				"delay", delay,
			)
		}
		attempt++
		return r.execOnce(ctx, args, effectiveWorkingDir, projectDir, e)
	})
	if err != nil {
		if sandboxDir != "" {
			return fmt.Errorf("%w (sandbox preserved at %s)", err, sandboxDir)
		}
		return err
	}

	// Check whether the agent left a question before exiting.
	data, readErr := os.ReadFile(questionFile)
	if readErr == nil {
		os.Remove(questionFile) // consumed
		questionJSON := strings.TrimSpace(string(data))
		// If the agent wrote a completion report instead of a real question,
		// extract the text as the summary and fall through to normal completion.
		if isCompletionReport(questionJSON) {
			r.Logger.Info("treating question file as completion report", "taskID", e.TaskID)
			e.Summary = extractQuestionText(questionJSON)
		} else {
			// Preserve sandbox on BLOCKED — agent may have partial work and its
			// Claude session files are stored under the sandbox's project slug.
			// The resume execution must run in the same directory.
			return &BlockedError{QuestionJSON: questionJSON, SessionID: e.SessionID, SandboxDir: sandboxDir}
		}
	}

	// Read agent summary if written.
	summaryFile := filepath.Join(logDir, "summary.txt")
	if summaryData, readErr := os.ReadFile(summaryFile); readErr == nil {
		os.Remove(summaryFile) // consumed
		e.Summary = strings.TrimSpace(string(summaryData))
	}

	// Merge sandbox back to project_dir and clean up.
	if sandboxDir != "" {
		if mergeErr := teardownSandbox(projectDir, sandboxDir, startHEAD, r.Logger, e); mergeErr != nil {
			return fmt.Errorf("sandbox teardown: %w (sandbox preserved at %s)", mergeErr, sandboxDir)
		}
	}
	return nil
}

// isCompletionReport returns true when a question-file JSON looks like a
// completion report rather than a real user question. Heuristic: no options
// (or empty options) and no "?" anywhere in the text.
func isCompletionReport(questionJSON string) bool {
	var q struct {
		Text    string   `json:"text"`
		Options []string `json:"options"`
	}
	if err := json.Unmarshal([]byte(questionJSON), &q); err != nil {
		return false
	}
	return len(q.Options) == 0 && !strings.Contains(q.Text, "?")
}

// extractQuestionText returns the "text" field from a question-file JSON, or
// the raw string if parsing fails.
func extractQuestionText(questionJSON string) string {
	var q struct {
		Text string `json:"text"`
	}
	if err := json.Unmarshal([]byte(questionJSON), &q); err != nil {
		return questionJSON
	}
	return strings.TrimSpace(q.Text)
}

// gitSafe returns git arguments that prepend "-c safe.directory=*" so that
// commands succeed regardless of the repository owner. This is needed when
// claudomator operates on project directories owned by a different OS user.
func gitSafe(args ...string) []string {
	return append([]string{"-c", "safe.directory=*"}, args...)
}

// sandboxCloneSource returns the URL to clone the sandbox from. It prefers a
// remote named "local" (a local bare repo that accepts pushes cleanly), then
// falls back to "origin", then to the working copy path itself.
func sandboxCloneSource(projectDir string) string {
	for _, remote := range []string{"local", "origin"} {
		out, err := exec.Command("git", gitSafe("-C", projectDir, "remote", "get-url", remote)...).Output()
		if err == nil && len(strings.TrimSpace(string(out))) > 0 {
			return strings.TrimSpace(string(out))
		}
	}
	return projectDir
}

// setupSandbox prepares a temporary git clone of projectDir.
// If projectDir is not a git repo it is initialised with an initial commit first.
func setupSandbox(projectDir string) (string, error) {
	// Ensure projectDir is a git repo; initialise if not.
	if err := exec.Command("git", gitSafe("-C", projectDir, "rev-parse", "--git-dir")...).Run(); err != nil {
		cmds := [][]string{
			gitSafe("-C", projectDir, "init"),
			gitSafe("-C", projectDir, "add", "-A"),
			gitSafe("-C", projectDir, "commit", "--allow-empty", "-m", "chore: initial commit"),
		}
		for _, args := range cmds {
			if out, err := exec.Command("git", args...).CombinedOutput(); err != nil { //nolint:gosec
				return "", fmt.Errorf("git init %s: %w\n%s", projectDir, err, out)
			}
		}
	}

	src := sandboxCloneSource(projectDir)

	tempDir, err := os.MkdirTemp("", "claudomator-sandbox-*")
	if err != nil {
		return "", fmt.Errorf("creating sandbox dir: %w", err)
	}
	// git clone requires the target to not exist; remove the placeholder first.
	if err := os.Remove(tempDir); err != nil {
		return "", fmt.Errorf("removing temp dir placeholder: %w", err)
	}
	out, err := exec.Command("git", gitSafe("clone", "--no-hardlinks", src, tempDir)...).CombinedOutput()
	if err != nil {
		return "", fmt.Errorf("git clone: %w\n%s", err, out)
	}
	return tempDir, nil
}

// teardownSandbox verifies the sandbox is clean and pushes new commits to the
// canonical bare repo. If the push is rejected because another task pushed
// concurrently, it fetches and rebases then retries once.
//
// The working copy (projectDir) is NOT updated automatically — it is the
// developer's workspace and is pulled manually. This avoids permission errors
// from mixed-owner .git/objects directories.
func teardownSandbox(projectDir, sandboxDir, startHEAD string, logger *slog.Logger, execRecord *storage.Execution) error {
	// Automatically commit uncommitted changes.
	out, err := exec.Command("git", "-C", sandboxDir, "status", "--porcelain").Output()
	if err != nil {
		return fmt.Errorf("git status: %w", err)
	}
	if len(strings.TrimSpace(string(out))) > 0 {
		logger.Info("autocommitting uncommitted changes", "sandbox", sandboxDir)
		cmds := [][]string{
			gitSafe("-C", sandboxDir, "add", "-A"),
			gitSafe("-C", sandboxDir, "commit", "-m", "chore: autocommit uncommitted changes"),
		}
		for _, args := range cmds {
			if out, err := exec.Command("git", args...).CombinedOutput(); err != nil {
				return fmt.Errorf("autocommit failed (%v): %w\n%s", args, err, out)
			}
		}
	}

	// Capture commits before pushing/deleting.
	// Use startHEAD..HEAD to find all commits made during this execution.
	logRange := "origin/HEAD..HEAD"
	if startHEAD != "" && startHEAD != "HEAD" {
		logRange = startHEAD + "..HEAD"
	}

	logCmd := exec.Command("git", gitSafe("-C", sandboxDir, "log", logRange, "--pretty=format:%H|%s")...)
	logOut, logErr := logCmd.CombinedOutput()
	if logErr == nil {
		lines := strings.Split(strings.TrimSpace(string(logOut)), "\n")
		logger.Debug("captured commits", "count", len(lines), "range", logRange)
		for _, line := range lines {
			if line == "" {
				continue
			}
			parts := strings.SplitN(line, "|", 2)
			if len(parts) == 2 {
				execRecord.Commits = append(execRecord.Commits, task.GitCommit{
					Hash:    parts[0],
					Message: parts[1],
				})
			}
		}
	} else {
		logger.Warn("failed to capture commits", "err", logErr, "range", logRange, "output", string(logOut))
	}

	// Check whether there are any new commits to push.
	ahead, err := exec.Command("git", gitSafe("-C", sandboxDir, "rev-list", "--count", logRange)...).Output()
	if err != nil {
		logger.Warn("could not determine commits ahead of origin; proceeding", "err", err, "range", logRange)
	}
	if strings.TrimSpace(string(ahead)) == "0" {
		os.RemoveAll(sandboxDir)
		return nil
	}

	// Push from sandbox → bare repo (sandbox's origin is the bare repo).
	if out, err := exec.Command("git", "-C", sandboxDir, "push", "origin", "HEAD").CombinedOutput(); err != nil {
		// If rejected due to concurrent push, fetch+rebase and retry once.
		if strings.Contains(string(out), "fetch first") || strings.Contains(string(out), "non-fast-forward") {
			logger.Info("push rejected (concurrent task); rebasing and retrying", "sandbox", sandboxDir)
			if out2, err2 := exec.Command("git", "-C", sandboxDir, "pull", "--rebase", "origin", "master").CombinedOutput(); err2 != nil {
				return fmt.Errorf("git rebase before retry push: %w\n%s", err2, out2)
			}
			// Re-capture commits after rebase (hashes might have changed)
			execRecord.Commits = nil
			logOut, logErr = exec.Command("git", "-C", sandboxDir, "log", logRange, "--pretty=format:%H|%s").Output()
			if logErr == nil {
				lines := strings.Split(strings.TrimSpace(string(logOut)), "\n")
				for _, line := range lines {
					parts := strings.SplitN(line, "|", 2)
					if len(parts) == 2 {
						execRecord.Commits = append(execRecord.Commits, task.GitCommit{
							Hash:    parts[0],
							Message: parts[1],
						})
					}
				}
			}

			if out3, err3 := exec.Command("git", "-C", sandboxDir, "push", "origin", "HEAD").CombinedOutput(); err3 != nil {
				return fmt.Errorf("git push to origin (after rebase): %w\n%s", err3, out3)
			}
		} else {
			return fmt.Errorf("git push to origin: %w\n%s", err, out)
		}
	}

	logger.Info("sandbox pushed to bare repo", "sandbox", sandboxDir)
	os.RemoveAll(sandboxDir)
	return nil
}

// execOnce runs the claude subprocess once, streaming output to e's log paths.
func (r *ClaudeRunner) execOnce(ctx context.Context, args []string, workingDir, projectDir string, e *storage.Execution) error {
	cmd := exec.CommandContext(ctx, r.binaryPath(), args...)
	cmd.Env = append(os.Environ(),
		"CLAUDOMATOR_API_URL="+r.APIURL,
		"CLAUDOMATOR_TASK_ID="+e.TaskID,
		"CLAUDOMATOR_PROJECT_DIR="+projectDir,
		"CLAUDOMATOR_QUESTION_FILE="+filepath.Join(e.ArtifactDir, "question.json"),
		"CLAUDOMATOR_SUMMARY_FILE="+filepath.Join(e.ArtifactDir, "summary.txt"),
	)
	// Put the subprocess in its own process group so we can SIGKILL the entire
	// group (MCP servers, bash children, etc.) on cancellation.
	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
	if workingDir != "" {
		cmd.Dir = workingDir
	}

	stdoutFile, err := os.Create(e.StdoutPath)
	if err != nil {
		return fmt.Errorf("creating stdout log: %w", err)
	}
	defer stdoutFile.Close()

	stderrFile, err := os.Create(e.StderrPath)
	if err != nil {
		return fmt.Errorf("creating stderr log: %w", err)
	}
	defer stderrFile.Close()

	// Use os.Pipe for stdout so we own the read-end lifetime.
	// cmd.StdoutPipe() would add the read-end to closeAfterWait, causing
	// cmd.Wait() to close it before our goroutine finishes reading.
	stdoutR, stdoutW, err := os.Pipe()
	if err != nil {
		return fmt.Errorf("creating stdout pipe: %w", err)
	}
	cmd.Stdout = stdoutW // *os.File — not added to closeAfterStart/Wait
	cmd.Stderr = stderrFile

	if err := cmd.Start(); err != nil {
		stdoutW.Close()
		stdoutR.Close()
		return fmt.Errorf("starting claude: %w", err)
	}
	// Close our write-end immediately; the subprocess holds its own copy.
	// The goroutine below gets EOF when the subprocess exits.
	stdoutW.Close()

	// killDone is closed when cmd.Wait() returns, stopping the pgid-kill goroutine.
	//
	// Safety: this goroutine cannot block indefinitely. The select has two arms:
	//   • ctx.Done() — fires if the caller cancels (e.g. timeout, user cancel).
	//     The goroutine sends SIGKILL and exits immediately.
	//   • killDone — closed by close(killDone) below, immediately after cmd.Wait()
	//     returns. This fires when the process exits for any reason (natural exit,
	//     SIGKILL from the ctx arm, or any other signal). The goroutine exits without
	//     doing anything.
	//
	// Therefore: for a task that completes normally with a long-lived (non-cancelled)
	// context, the killDone arm fires and the goroutine exits. There is no path where
	// this goroutine outlives execOnce().
	killDone := make(chan struct{})
	go func() {
		select {
		case <-ctx.Done():
			// SIGKILL the entire process group to reap orphan children.
			syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
		case <-killDone:
		}
	}()

	// Stream stdout to the log file and parse cost/errors.
	// wg ensures costUSD and streamErr are fully written before we read them after cmd.Wait().
	var costUSD float64
	var streamErr error
	var wg sync.WaitGroup
	wg.Add(1)
	go func() {
		defer wg.Done()
		costUSD, streamErr = parseStream(stdoutR, stdoutFile, r.Logger)
		stdoutR.Close()
	}()

	waitErr := cmd.Wait()
	close(killDone) // stop the pgid-kill goroutine
	wg.Wait()       // drain remaining stdout before reading costUSD/streamErr

	e.CostUSD = costUSD

	if waitErr != nil {
		if exitErr, ok := waitErr.(*exec.ExitError); ok {
			e.ExitCode = exitErr.ExitCode()
		}
		// If the stream captured a rate-limit or quota message, return it
		// so callers can distinguish it from a generic exit-status failure.
		if isRateLimitError(streamErr) || isQuotaExhausted(streamErr) {
			return streamErr
		}
		if tail := tailFile(e.StderrPath, 20); tail != "" {
			return fmt.Errorf("claude exited with error: %w\nstderr:\n%s", waitErr, tail)
		}
		return fmt.Errorf("claude exited with error: %w", waitErr)
	}

	e.ExitCode = 0
	if streamErr != nil {
		return streamErr
	}
	return nil
}

func (r *ClaudeRunner) buildArgs(t *task.Task, e *storage.Execution, questionFile string) []string {
	// Resume execution: the agent already has context; just deliver the answer.
	if e.ResumeSessionID != "" {
		args := []string{
			"-p", e.ResumeAnswer,
			"--resume", e.ResumeSessionID,
			"--output-format", "stream-json",
			"--verbose",
		}
		permMode := t.Agent.PermissionMode
		if permMode == "" {
			permMode = "bypassPermissions"
		}
		args = append(args, "--permission-mode", permMode)
		if t.Agent.Model != "" {
			args = append(args, "--model", t.Agent.Model)
		}
		return args
	}

	instructions := t.Agent.Instructions
	allowedTools := t.Agent.AllowedTools

	if !t.Agent.SkipPlanning {
		instructions = withPlanningPreamble(instructions)
		// Ensure Bash is available so the agent can POST subtasks and ask questions.
		hasBash := false
		for _, tool := range allowedTools {
			if tool == "Bash" {
				hasBash = true
				break
			}
		}
		if !hasBash {
			allowedTools = append(allowedTools, "Bash")
		}
	}

	args := []string{
		"-p", instructions,
		"--session-id", e.SessionID,
		"--output-format", "stream-json",
		"--verbose",
	}

	if t.Agent.Model != "" {
		args = append(args, "--model", t.Agent.Model)
	}
	if t.Agent.MaxBudgetUSD > 0 {
		args = append(args, "--max-budget-usd", fmt.Sprintf("%.2f", t.Agent.MaxBudgetUSD))
	}
	// Default to bypassPermissions — claudomator runs tasks unattended, so
	// prompting for write access would always stall execution. Tasks that need
	// a more restrictive mode can set permission_mode explicitly.
	permMode := t.Agent.PermissionMode
	if permMode == "" {
		permMode = "bypassPermissions"
	}
	args = append(args, "--permission-mode", permMode)
	if t.Agent.SystemPromptAppend != "" {
		args = append(args, "--append-system-prompt", t.Agent.SystemPromptAppend)
	}
	for _, tool := range allowedTools {
		args = append(args, "--allowedTools", tool)
	}
	for _, tool := range t.Agent.DisallowedTools {
		args = append(args, "--disallowedTools", tool)
	}
	for _, f := range t.Agent.ContextFiles {
		args = append(args, "--add-dir", f)
	}
	args = append(args, t.Agent.AdditionalArgs...)

	return args
}

// parseStream reads streaming JSON from claude, writes to w, and returns
// (costUSD, error). error is non-nil if the stream signals task failure:
// - result message has is_error:true
// - a tool_result was denied due to missing permissions
func parseStream(r io.Reader, w io.Writer, logger *slog.Logger) (float64, error) {
	tee := io.TeeReader(r, w)
	scanner := bufio.NewScanner(tee)
	scanner.Buffer(make([]byte, 1024*1024), 1024*1024) // 1MB buffer for large lines

	var totalCost float64
	var streamErr error

	for scanner.Scan() {
		line := scanner.Bytes()
		var msg map[string]interface{}
		if err := json.Unmarshal(line, &msg); err != nil {
			continue
		}

		msgType, _ := msg["type"].(string)
		switch msgType {
		case "rate_limit_event":
			if info, ok := msg["rate_limit_info"].(map[string]interface{}); ok {
				status, _ := info["status"].(string)
				if status == "rejected" {
					streamErr = fmt.Errorf("claude rate limit reached (rejected): %v", msg)
					// Immediately break since we can't continue anyway
					break
				}
			}
		case "assistant":
			if errStr, ok := msg["error"].(string); ok && errStr == "rate_limit" {
				streamErr = fmt.Errorf("claude rate limit reached: %v", msg)
			}
		case "result":
			if isErr, _ := msg["is_error"].(bool); isErr {
				result, _ := msg["result"].(string)
				if result != "" {
					streamErr = fmt.Errorf("claude task failed: %s", result)
				} else {
					streamErr = fmt.Errorf("claude task failed (is_error=true in result)")
				}
			}
			// Prefer total_cost_usd from result message; fall through to legacy check below.
			if cost, ok := msg["total_cost_usd"].(float64); ok {
				totalCost = cost
			}
		case "user":
			// Detect permission-denial tool_results. These occur when permission_mode
			// is not bypassPermissions and claude exits 0 without completing its task.
			if err := permissionDenialError(msg); err != nil && streamErr == nil {
				streamErr = err
			}
		}

		// Legacy cost field used by older claude versions.
		if cost, ok := msg["cost_usd"].(float64); ok {
			totalCost = cost
		}
	}

	return totalCost, streamErr
}

// permissionDenialError inspects a "user" stream message for tool_result entries
// that were denied due to missing permissions. Returns an error if found.
func permissionDenialError(msg map[string]interface{}) error {
	message, ok := msg["message"].(map[string]interface{})
	if !ok {
		return nil
	}
	content, ok := message["content"].([]interface{})
	if !ok {
		return nil
	}
	for _, item := range content {
		itemMap, ok := item.(map[string]interface{})
		if !ok {
			continue
		}
		if itemMap["type"] != "tool_result" {
			continue
		}
		if isErr, _ := itemMap["is_error"].(bool); !isErr {
			continue
		}
		text, _ := itemMap["content"].(string)
		if strings.Contains(text, "requested permissions") || strings.Contains(text, "haven't granted") {
			return fmt.Errorf("permission denied by host: %s", text)
		}
	}
	return nil
}

// tailFile returns the last n lines of the file at path, or empty string if
// the file cannot be read. Used to surface subprocess stderr on failure.
func tailFile(path string, n int) string {
	f, err := os.Open(path)
	if err != nil {
		return ""
	}
	defer f.Close()

	var lines []string
	scanner := bufio.NewScanner(f)
	for scanner.Scan() {
		lines = append(lines, scanner.Text())
		if len(lines) > n {
			lines = lines[1:]
		}
	}
	return strings.Join(lines, "\n")
}