package main

import (
	"bufio"
	"bytes"
	"encoding/json"
	"errors"
	"flag"
	"fmt"
	"io"
	"os"
	"os/exec"
	"path/filepath"
	"runtime"
	"sort"
	"strings"
	"sync"
	"time"
	"unicode/utf8"
)

const version = "0.4.0"

var defaultSkippedDirs = map[string]bool{
	".cache": true, ".claude": true, ".codex": true, ".git": true, ".next": true, ".pnpm-store": true,
	".terraform": true, ".venv": true, "build": true, "coverage": true,
	"dist": true, "node_modules": true, "target": true, "vendor": true,
}

type options struct {
	root          string
	json          bool
	yes           bool
	strict        bool
	jobs          int
	timeout       time.Duration
	exclusions    stringList
	createMissing bool
}

type stringList []string

func (s *stringList) String() string { return strings.Join(*s, ",") }
func (s *stringList) Set(v string) error {
	*s = append(*s, v)
	return nil
}

type repo struct {
	Path              string   `json:"path"`
	RelativePath      string   `json:"relativePath"`
	Branch            string   `json:"branch,omitempty"`
	RemoteCount       int      `json:"remoteCount"`
	Dirty             []string `json:"dirty,omitempty"`
	NestedRepoEntries []string `json:"nestedRepoEntries,omitempty"`
	BlockReason       string   `json:"blockReason,omitempty"`
	Eligible          bool     `json:"eligible"`
}

type syncResult struct {
	Path     string             `json:"path"`
	Success  bool               `json:"success"`
	Skipped  bool               `json:"skipped"`
	Message  string             `json:"message,omitempty"`
	Duration string             `json:"duration,omitempty"`
	Branches []branchSyncResult `json:"branches,omitempty"`
}

type syncProgressEvent struct {
	Path    string
	Started bool
	Result  syncResult
}

func main() {
	code := run(os.Args[1:], os.Stdin, os.Stdout, os.Stderr)
	os.Exit(code)
}

func run(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
	command := "scan"
	if len(args) > 0 && !strings.HasPrefix(args[0], "-") {
		command, args = args[0], args[1:]
	}
	if command == "help" {
		usage(stdout)
		return 0
	}
	if command == "version" {
		fmt.Fprintf(stdout, "stack-sync %s\n", version)
		return 0
	}
	if command != "scan" && command != "sync" && command != "tui" {
		fmt.Fprintf(stderr, "unknown command %q\n\n", command)
		usage(stderr)
		return 2
	}

	opts, err := parseFlags(command, args, stderr)
	if err != nil {
		return 2
	}
	root, err := filepath.Abs(opts.root)
	if err != nil {
		fmt.Fprintf(stderr, "resolve root: %v\n", err)
		return 1
	}
	root, err = filepath.EvalSymlinks(root)
	if err != nil {
		fmt.Fprintf(stderr, "resolve root: %v\n", err)
		return 1
	}

	paths, err := discover(root, opts.exclusions)
	if err != nil {
		fmt.Fprintf(stderr, "discover repositories: %v\n", err)
		return 1
	}
	if len(paths) == 0 {
		fmt.Fprintf(stderr, "no Git repositories found under %s\n", root)
		return 1
	}
	repos := inspectAll(root, paths, opts.jobs)
	if command == "tui" {
		if err := runTUI(root, repos, opts); err != nil {
			fmt.Fprintf(stderr, "run TUI: %v\n", err)
			return 1
		}
		return 0
	}

	if command == "scan" {
		if opts.json {
			writeJSON(stdout, repos)
		} else {
			printScan(stdout, root, repos)
		}
		return 0
	}

	if !opts.json {
		printScan(stdout, root, repos)
	}
	eligible := eligibleCount(repos)
	if eligible == 0 {
		fmt.Fprintln(stderr, "nothing to sync: every repository is blocked")
		return 3
	}
	strictUnsafe := strictUnsafeCount(repos)
	if opts.strict && strictUnsafe > 0 {
		fmt.Fprintf(stderr, "strict mode: refusing to sync because %d repositories are blocked or dirty\n", strictUnsafe)
		return 3
	}
	if !opts.yes {
		if opts.json || !isTerminal(os.Stdin) {
			fmt.Fprintln(stderr, "refusing to sync without confirmation; pass --yes in non-interactive use")
			return 2
		}
		fmt.Fprintf(stdout, "\nSynchronize branches in %d repositories? Merged branches whose upstream was deleted may be removed. [y/N] ", eligible)
		answer, _ := bufio.NewReader(stdin).ReadString('\n')
		answer = strings.ToLower(strings.TrimSpace(answer))
		if answer != "y" && answer != "yes" {
			fmt.Fprintln(stdout, "Cancelled; no repositories were changed.")
			return 0
		}
	}

	var liveMu sync.Mutex
	live := newLiveProgress(stdout, &liveMu, len(repos))
	results := syncAllWithLive(root, repos, opts.jobs, opts.timeout, stdout, opts.json, opts.strict, opts.createMissing, live)
	liveMu.Lock()
	live.clear()
	liveMu.Unlock()
	if opts.json {
		writeJSON(stdout, results)
	} else {
		printSummary(stdout, results)
	}
	for i, result := range results {
		if !result.Success && !result.Skipped {
			return 1
		}
		// A repository which passed the plan but failed its immediate safety
		// recheck is materially different from an already-planned safe skip.
		if result.Skipped && repos[i].Eligible {
			return 3
		}
	}
	return 0
}

func parseFlags(command string, args []string, stderr io.Writer) (options, error) {
	opts := options{root: ".", jobs: min(4, runtime.NumCPU()), timeout: 5 * time.Minute}
	fs := flag.NewFlagSet(command, flag.ContinueOnError)
	fs.SetOutput(stderr)
	fs.StringVar(&opts.root, "root", opts.root, "workspace root to search")
	fs.BoolVar(&opts.json, "json", false, "emit machine-readable JSON")
	fs.IntVar(&opts.jobs, "jobs", opts.jobs, "maximum parallel inspections/syncs")
	fs.Var(&opts.exclusions, "exclude", "directory name or root-relative path to skip (repeatable)")
	if command == "sync" {
		fs.BoolVar(&opts.yes, "yes", false, "apply without an interactive confirmation")
		fs.BoolVar(&opts.strict, "strict", false, "abort the entire run if any repository is blocked or dirty")
	}
	if command == "sync" || command == "tui" {
		fs.DurationVar(&opts.timeout, "timeout", opts.timeout, "timeout for each repository sync")
		fs.BoolVar(&opts.createMissing, "create-missing", false, "create local branches that exist only on the remote")
	}
	fs.Usage = func() {
		usage(stderr)
		// PrintDefaults was dropped along with the stock usage function, which
		// left "use <command> -h for command options" promising flags that
		// were never actually listed.
		fmt.Fprintf(stderr, "\nOptions for %s:\n", command)
		fs.PrintDefaults()
	}
	if err := fs.Parse(args); err != nil {
		return opts, err
	}
	if fs.NArg() != 0 {
		err := fmt.Errorf("unexpected arguments: %s", strings.Join(fs.Args(), " "))
		fmt.Fprintln(stderr, err)
		return opts, err
	}
	if opts.jobs < 1 {
		err := errors.New("--jobs must be at least 1")
		fmt.Fprintln(stderr, err)
		return opts, err
	}
	if opts.timeout <= 0 {
		err := errors.New("--timeout must be positive")
		fmt.Fprintln(stderr, err)
		return opts, err
	}
	return opts, nil
}

func usage(w io.Writer) {
	fmt.Fprintln(w, `stack-sync safely synchronizes Git branches across a directory tree.

Usage:
  stack-sync [scan] [options]
  stack-sync sync [options]
  stack-sync tui [options]
  stack-sync version

Commands:
  scan   Discover repositories and report whether each is safe to sync (default)
  sync   Scan, confirm, re-check safety, then sync branches in eligible repositories
  tui    Interactively review, select, and sync repositories

Safety:
  Dirty worktrees are reported but remain eligible: their checked-out branch is
  protected while safe inactive branches are synchronized. Detached HEADs,
  repositories without remotes, and in-progress Git operations remain blocked.
  stack-sync never stashes, resets, cleans, or commits.

Branch handling:
  Outdated local branches are fast-forwarded, and a local branch whose upstream
  was deleted is removed once it is merged into the remote default branch.
  A local branch with no upstream and no matching remote branch cannot be
  fast-forwarded, so it is reported as UNTRACKED and left alone. Pass
  --create-missing to also create branches that exist only on the remote.

Use "stack-sync <command> -h" for command options.`)
}

func discover(root string, exclusions []string) ([]string, error) {
	excluded := make(map[string]bool, len(exclusions))
	for _, item := range exclusions {
		item = filepath.Clean(item)
		excluded[item] = true
	}
	var repos []string
	err := filepath.WalkDir(root, func(path string, entry os.DirEntry, walkErr error) error {
		if walkErr != nil {
			return walkErr
		}
		if !entry.IsDir() {
			return nil
		}
		if path != root {
			rel, _ := filepath.Rel(root, path)
			if defaultSkippedDirs[entry.Name()] || excluded[entry.Name()] || excluded[rel] {
				return filepath.SkipDir
			}
		}
		marker := filepath.Join(path, ".git")
		if info, err := os.Stat(marker); err == nil && (info.IsDir() || info.Mode().IsRegular()) {
			repos = append(repos, path)
			// Continue: unrelated repositories may intentionally be nested here.
		}
		return nil
	})
	if err != nil {
		return nil, err
	}
	sort.Strings(repos)
	return repos, nil
}

func inspectAll(root string, paths []string, jobs int) []repo {
	type item struct {
		index int
		path  string
	}
	work := make(chan item)
	results := make([]repo, len(paths))
	var wg sync.WaitGroup
	for range min(jobs, len(paths)) {
		wg.Add(1)
		go func() {
			defer wg.Done()
			for next := range work {
				results[next.index] = inspect(root, next.path, paths)
			}
		}()
	}
	for i, path := range paths {
		work <- item{i, path}
	}
	close(work)
	wg.Wait()
	return results
}

func inspect(root, path string, allRepos []string) repo {
	rel, _ := filepath.Rel(root, path)
	if rel == "." {
		rel = filepath.Base(root)
	}
	r := repo{Path: path, RelativePath: rel}

	branch, err := gitOutput(path, "symbolic-ref", "--quiet", "--short", "HEAD")
	if err != nil {
		r.BlockReason = "detached HEAD or unborn branch"
	} else {
		r.Branch = strings.TrimSpace(branch)
	}
	remotes, remoteErr := gitOutput(path, "remote")
	if remoteErr != nil {
		setBlock(&r, "cannot list remotes")
	} else if strings.TrimSpace(remotes) == "" {
		setBlock(&r, "no remotes")
	} else {
		r.RemoteCount = len(strings.Fields(remotes))
	}
	if operation := gitOperation(path); operation != "" {
		setBlock(&r, operation+" in progress")
	}

	status, err := gitBytes(path, "status", "--porcelain=v1", "-z", "--untracked-files=all", "--ignore-submodules=none")
	if err != nil {
		setBlock(&r, "cannot read worktree status")
		return r
	}
	nested := nestedPaths(path, allRepos)
	for _, change := range parsePorcelain(status) {
		if change.code == "??" && belongsToNestedRepo(change.path, nested) {
			r.NestedRepoEntries = append(r.NestedRepoEntries, change.path)
			continue
		}
		r.Dirty = append(r.Dirty, change.code+" "+change.path)
	}
	r.Eligible = r.BlockReason == ""
	return r
}

func setBlock(r *repo, reason string) {
	if r.BlockReason == "" {
		r.BlockReason = reason
	}
}

type change struct{ code, path string }

func parsePorcelain(data []byte) []change {
	parts := bytes.Split(data, []byte{0})
	changes := make([]change, 0, len(parts))
	for i := 0; i < len(parts); i++ {
		part := parts[i]
		if len(part) < 4 {
			continue
		}
		code, path := string(part[:2]), string(part[3:])
		changes = append(changes, change{code, filepath.Clean(path)})
		if (strings.Contains(code, "R") || strings.Contains(code, "C")) && i+1 < len(parts) {
			i++ // porcelain v1 -z includes the original path as the next field
		}
	}
	return changes
}

func nestedPaths(parent string, allRepos []string) []string {
	var nested []string
	prefix := parent + string(filepath.Separator)
	for _, candidate := range allRepos {
		if strings.HasPrefix(candidate, prefix) {
			rel, _ := filepath.Rel(parent, candidate)
			nested = append(nested, filepath.Clean(rel))
		}
	}
	sort.Slice(nested, func(i, j int) bool { return len(nested[i]) < len(nested[j]) })
	return nested
}

func belongsToNestedRepo(path string, nested []string) bool {
	path = strings.TrimSuffix(filepath.Clean(path), string(filepath.Separator))
	for _, child := range nested {
		if path == child || strings.HasPrefix(path, child+string(filepath.Separator)) {
			return true
		}
	}
	return false
}

func gitOperation(path string) string {
	checks := []struct{ marker, label string }{
		{"MERGE_HEAD", "merge"}, {"CHERRY_PICK_HEAD", "cherry-pick"}, {"REVERT_HEAD", "revert"},
		{"BISECT_LOG", "bisect"}, {"rebase-merge", "rebase"}, {"rebase-apply", "rebase"},
	}
	for _, check := range checks {
		marker, err := gitOutput(path, "rev-parse", "--git-path", check.marker)
		if err != nil {
			continue
		}
		marker = strings.TrimSpace(marker)
		if !filepath.IsAbs(marker) {
			marker = filepath.Join(path, marker)
		}
		if _, err := os.Stat(marker); err == nil {
			return check.label
		}
	}
	return ""
}

func gitOutput(path string, args ...string) (string, error) {
	data, err := gitBytes(path, args...)
	return string(data), err
}

func gitBytes(path string, args ...string) ([]byte, error) {
	cmd := exec.Command("git", append([]string{"-C", path}, args...)...)
	return cmd.Output()
}

func syncAll(root string, repos []repo, jobs int, timeout time.Duration, stdout io.Writer, quiet, strict, createMissing bool) []syncResult {
	return syncSelected(root, repos, repos, jobs, timeout, stdout, quiet, strict, createMissing, nil)
}

// syncAllWithLive is syncAll with the in-place progress line attached.
func syncAllWithLive(root string, repos []repo, jobs int, timeout time.Duration, stdout io.Writer, quiet, strict, createMissing bool, live *liveProgress) []syncResult {
	return syncSelected(root, repos, repos, jobs, timeout, stdout, quiet, strict, createMissing, live)
}

func syncSelected(root string, repos, workspaceRepos []repo, jobs int, timeout time.Duration, stdout io.Writer, quiet, strict, createMissing bool, live *liveProgress) []syncResult {
	return syncSelectedWithProgress(root, repos, workspaceRepos, jobs, timeout, stdout, quiet, strict, createMissing, live, nil)
}

func syncSelectedWithProgress(root string, repos, workspaceRepos []repo, jobs int, timeout time.Duration, stdout io.Writer, quiet, strict, createMissing bool, live *liveProgress, progress func(syncProgressEvent)) []syncResult {
	type item struct {
		index int
		repo  repo
	}
	work := make(chan item)
	results := make([]syncResult, len(repos))
	var wg sync.WaitGroup
	var outputMu sync.Mutex
	allPaths := make([]string, 0, len(workspaceRepos))
	for _, r := range workspaceRepos {
		allPaths = append(allPaths, r.Path)
	}
	record := func(index int, result syncResult) {
		results[index] = result
		if progress != nil {
			progress(syncProgressEvent{Path: result.Path, Result: result})
		}
		live.update(func() { live.finished(result); live.draw() })
	}

	for range min(jobs, len(repos)) {
		wg.Add(1)
		go func() {
			defer wg.Done()
			for next := range work {
				r := next.repo
				if !r.Eligible {
					record(next.index, syncResult{Path: r.RelativePath, Skipped: true, Message: r.BlockReason})
					continue
				}
				// Close the scan/apply race: inspect again immediately before mutation.
				fresh := inspect(root, r.Path, allPaths)
				if !fresh.Eligible {
					record(next.index, syncResult{Path: r.RelativePath, Skipped: true, Message: "pre-sync recheck: " + fresh.BlockReason})
					continue
				}
				if strict && len(fresh.Dirty) > 0 {
					record(next.index, syncResult{Path: r.RelativePath, Skipped: true, Message: fmt.Sprintf("pre-sync recheck: dirty worktree (%d changes)", len(fresh.Dirty))})
					continue
				}
				if progress != nil {
					progress(syncProgressEvent{Path: r.RelativePath, Started: true})
				}
				if !quiet {
					outputMu.Lock()
					live.clear()
					fmt.Fprintf(stdout, "\nSTART  %s (%s)\n", r.RelativePath, r.Branch)
					outputMu.Unlock()
				}
				live.update(func() { live.started(); live.draw() })
				started := time.Now()
				report, err := syncRepositoryWith(r.Path, timeout, len(fresh.Dirty) > 0, fresh.NestedRepoEntries, syncOptions{CreateMissing: createMissing})
				message := report.Message
				duration := time.Since(started).Round(time.Millisecond).String()
				if !quiet {
					outputMu.Lock()
					live.clear()
					state := "DONE"
					if err != nil {
						state = "FAIL"
					}
					fmt.Fprintf(stdout, "%s   %s (%s)\n", state, r.RelativePath, duration)
					if message != "" {
						fmt.Fprintln(stdout, indent(message, "    "))
					}
					outputMu.Unlock()
				}
				if err != nil && message == "" {
					message = err.Error()
				}
				record(next.index, syncResult{Path: r.RelativePath, Success: err == nil, Message: message, Duration: duration, Branches: report.Branches})
			}
		}()
	}
	for i, r := range repos {
		work <- item{i, r}
	}
	close(work)
	wg.Wait()
	return results
}

func printScan(w io.Writer, root string, repos []repo) {
	fmt.Fprintf(w, "Workspace: %s\nRepositories: %d (%d ready, %d blocked)\n\n", root, len(repos), eligibleCount(repos), blockedCount(repos))
	width := len("REPOSITORY")
	for _, r := range repos {
		if len(r.RelativePath) > width {
			width = len(r.RelativePath)
		}
	}
	fmt.Fprintf(w, "%-*s  %-10s  %s\n", width, "REPOSITORY", "BRANCH", "STATUS")
	for _, r := range repos {
		status := "ready"
		if !r.Eligible {
			status = "BLOCKED: " + r.BlockReason
		} else if len(r.Dirty) > 0 {
			status = fmt.Sprintf("ready; dirty worktree (%d changes), checked-out branch protected", len(r.Dirty))
		}
		if len(r.NestedRepoEntries) > 0 {
			status += fmt.Sprintf("; %d nested repo path(s) ignored", len(r.NestedRepoEntries))
		}
		branch := r.Branch
		if branch == "" {
			branch = "-"
		}
		fmt.Fprintf(w, "%-*s  %-10s  %s\n", width, r.RelativePath, truncate(branch, 10), status)
	}
}

// actionOrder fixes the order actions are summarised in, so the same workspace
// always reads the same way from one run to the next.
var actionOrder = []string{"updated", "deleted", "created", "protected", "warning", "untracked"}

// liveProgress redraws a single line in place while repositories sync, so
// workspace totals are visible without waiting for the final summary. It is only
// enabled when stdout is a terminal: writing cursor control into a pipe or a
// --json document would corrupt the captured output, so callers get a nil
// *liveProgress in those cases and every method here is a no-op.
type liveProgress struct {
	w     io.Writer
	mu    *sync.Mutex
	total int

	done      int
	active    int
	changed   int
	failed    int
	skipped   int
	counts    map[string]int
	lastWidth int
}

func newLiveProgress(w io.Writer, mu *sync.Mutex, total int) *liveProgress {
	if !isTerminalWriter(w) {
		return nil
	}
	return &liveProgress{w: w, mu: mu, total: total, counts: make(map[string]int, len(actionOrder))}
}

// started records that a repository has begun syncing.
func (p *liveProgress) started() {
	if p == nil {
		return
	}
	p.active++
}

// finished folds a completed repository into the running totals.
func (p *liveProgress) finished(result syncResult) {
	if p == nil {
		return
	}
	p.done++
	p.active--
	for _, branch := range result.Branches {
		p.counts[branch.Action]++
	}
	switch {
	case result.Skipped:
		p.skipped++
	case !result.Success:
		p.failed++
	case repositoryChanged(result):
		p.changed++
	}
}

func (p *liveProgress) text() string {
	parts := []string{fmt.Sprintf("%d/%d", p.done, p.total)}
	if p.active > 0 {
		parts = append(parts, fmt.Sprintf("%d active", p.active))
	}
	parts = append(parts, fmt.Sprintf("%d changed", p.changed))
	if line := formatActionCounts(p.counts); line != "" {
		parts = append(parts, line)
	}
	if p.skipped > 0 {
		parts = append(parts, fmt.Sprintf("%d skipped", p.skipped))
	}
	if p.failed > 0 {
		parts = append(parts, fmt.Sprintf("%d FAILED", p.failed))
	}
	return "  " + strings.Join(parts, " · ")
}

// draw repaints the line. Callers must already hold p.mu when drawing.
func (p *liveProgress) draw() {
	if p == nil {
		return
	}
	line := p.text()
	fmt.Fprintf(p.w, "\r%s\r%s", strings.Repeat(" ", p.lastWidth), line)
	p.lastWidth = utf8.RuneCountInString(line)
}

// clear erases the line so ordinary output can be written on top of it.
func (p *liveProgress) clear() {
	if p == nil {
		return
	}
	p.lastWidth = 0
}

func (p *liveProgress) update(fn func()) {
	if p == nil {
		return
	}
	p.mu.Lock()
	defer p.mu.Unlock()
	fn()
}

func printSummary(w io.Writer, results []syncResult) {
	counts := make(map[string]int, len(actionOrder))
	var changed, unchanged, failed, skipped int
	for _, r := range results {
		for _, branch := range r.Branches {
			counts[branch.Action]++
		}
		switch {
		case r.Skipped:
			skipped++
		case !r.Success:
			failed++
		case repositoryChanged(r):
			changed++
		default:
			unchanged++
		}
	}
	// "synced" counted every repository that ran, including ones where nothing
	// was actually touched, so a run that changed nothing still looked busy.
	fmt.Fprintf(w, "\nSummary: %d changed, %d unchanged, %d failed, %d skipped\n", changed, unchanged, failed, skipped)
	if line := formatActionCounts(counts); line != "" {
		fmt.Fprintf(w, "  %s\n", line)
	}
	// Deletions and failures are destructive or unexpected, so they are listed
	// in full here rather than left to be found in the scrolling log above.
	if failed > 0 || counts["deleted"] > 0 {
		fmt.Fprintln(w, "\nNeeds review:")
		for _, r := range results {
			if !r.Success && !r.Skipped {
				fmt.Fprintf(w, "  FAILED   %s: %s\n", r.Path, firstLine(r.Message))
			}
			for _, branch := range r.Branches {
				if branch.Action == "deleted" {
					fmt.Fprintf(w, "  DELETED  %s / %s: %s\n", r.Path, branch.Branch, branch.Message)
				}
			}
		}
	}
}

func repositoryChanged(r syncResult) bool {
	for _, branch := range r.Branches {
		switch branch.Action {
		case "updated", "deleted", "created":
			return true
		}
	}
	return false
}

func formatActionCounts(counts map[string]int) string {
	parts := make([]string, 0, len(actionOrder))
	for _, action := range actionOrder {
		if n := counts[action]; n > 0 {
			parts = append(parts, fmt.Sprintf("%d %s", n, action))
		}
	}
	return strings.Join(parts, " · ")
}

func eligibleCount(repos []repo) int {
	n := 0
	for _, r := range repos {
		if r.Eligible {
			n++
		}
	}
	return n
}
func blockedCount(repos []repo) int { return len(repos) - eligibleCount(repos) }
func strictUnsafeCount(repos []repo) int {
	n := 0
	for _, r := range repos {
		if !r.Eligible || len(r.Dirty) > 0 {
			n++
		}
	}
	return n
}
func writeJSON(w io.Writer, value any) {
	enc := json.NewEncoder(w)
	enc.SetIndent("", "  ")
	_ = enc.Encode(value)
}
func truncate(s string, n int) string {
	if len(s) <= n {
		return s
	}
	return s[:n-1] + "…"
}
func firstLine(s string) string {
	if i := strings.IndexByte(s, '\n'); i >= 0 {
		return s[:i]
	}
	return s
}
func indent(s, prefix string) string {
	// Git's progress output uses carriage returns when it believes it has a
	// terminal. Normalize them so captured concurrent output cannot overwrite a
	// repository label or escape its indentation.
	s = strings.ReplaceAll(s, "\r\n", "\n")
	s = strings.ReplaceAll(s, "\r", "\n")
	return prefix + strings.ReplaceAll(s, "\n", "\n"+prefix)
}

func isTerminal(file *os.File) bool {
	info, err := file.Stat()
	return err == nil && info.Mode()&os.ModeCharDevice != 0
}

// isTerminalWriter reports whether w is a real terminal, so callers can avoid
// emitting cursor control sequences into redirected or buffered output.
func isTerminalWriter(w io.Writer) bool {
	file, ok := w.(*os.File)
	return ok && isTerminal(file)
}