package appexport

import (
	"archive/tar"
	"compress/gzip"
	"context"
	"fmt"
	"io"
	"log"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"sync"
	"time"

	"gitea.dooplex.hu/admin/felhom-controller/internal/backup"
)

// Step tracks one step of an export/import operation.
type Step struct {
	Label  string `json:"label"`
	Status string `json:"status"` // "pending", "running", "done", "failed"
	Error  string `json:"error,omitempty"`
}

// Job tracks an in-progress export or import operation.
type Job struct {
	mu sync.RWMutex

	StackName   string `json:"stack_name"`
	DisplayName string `json:"display_name"`
	Steps       []Step `json:"steps"`
	Running     bool   `json:"running"`
	Done        bool   `json:"done"`
	Error       string `json:"error,omitempty"`
	OutputPath  string `json:"output_path,omitempty"`
	OutputSize  string `json:"output_size,omitempty"`
	JobType     string `json:"job_type"` // "export" or "import"
}

// Snapshot returns a thread-safe copy for JSON serialization.
func (j *Job) Snapshot() map[string]interface{} {
	j.mu.RLock()
	defer j.mu.RUnlock()
	steps := make([]Step, len(j.Steps))
	copy(steps, j.Steps)
	return map[string]interface{}{
		"ok":           true,
		"running":      j.Running,
		"done":         j.Done,
		"error":        j.Error,
		"steps":        steps,
		"output_path":  j.OutputPath,
		"output_size":  j.OutputSize,
		"stack_name":   j.StackName,
		"display_name": j.DisplayName,
		"job_type":     j.JobType,
	}
}

func (j *Job) setStep(idx int, status, errMsg string) {
	j.mu.Lock()
	defer j.mu.Unlock()
	if idx < len(j.Steps) {
		j.Steps[idx].Status = status
		j.Steps[idx].Error = errMsg
	}
}

// ExportRequest holds user-provided parameters for an export.
type ExportRequest struct {
	StackName string
	DestDrive string // drive mount path (e.g., "/mnt/hdd_1")
	Password  string // empty = no encryption
	StopApp   bool   // stop app before export
}

// Exporter manages app export/import operations.
type Exporter struct {
	provider ExportStackProvider
	logger   *log.Logger
	version  string
	debug    bool

	mu        sync.Mutex
	activeJob *Job
}

// NewExporter creates a new export/import engine.
func NewExporter(provider ExportStackProvider, logger *log.Logger, version string) *Exporter {
	return &Exporter{
		provider: provider,
		logger:   logger,
		version:  version,
	}
}

// SetDebug enables or disables verbose debug logging.
func (e *Exporter) SetDebug(debug bool) {
	e.debug = debug
}

// debugf logs a message only when debug mode is enabled.
func (e *Exporter) debugf(format string, args ...interface{}) {
	if e.debug {
		e.logger.Printf("[DEBUG] [appexport] "+format, args...)
	}
}

// IsRunning returns true if an export/import is in progress.
func (e *Exporter) IsRunning() bool {
	e.mu.Lock()
	defer e.mu.Unlock()
	return e.activeJob != nil && e.activeJob.Running
}

// GetActiveJob returns the current job (for status polling).
func (e *Exporter) GetActiveJob() *Job {
	e.mu.Lock()
	defer e.mu.Unlock()
	return e.activeJob
}

// StartExport validates and starts an async export. Returns error if blocked.
func (e *Exporter) StartExport(req ExportRequest) error {
	e.mu.Lock()
	if e.activeJob != nil && e.activeJob.Running {
		e.mu.Unlock()
		e.debugf("StartExport rejected: another job is already running")
		return fmt.Errorf("export or import already in progress")
	}

	if !e.provider.IsStackDeployed(req.StackName) {
		e.mu.Unlock()
		e.debugf("StartExport rejected: stack %q is not deployed", req.StackName)
		return fmt.Errorf("stack %q is not deployed", req.StackName)
	}

	e.debugf("StartExport: stack=%q dest=%q password=%v stopApp=%v",
		req.StackName, req.DestDrive, req.Password != "", req.StopApp)

	steps := []Step{
		{Label: "Előkészítés", Status: "pending"},
		{Label: "Konfiguráció mentése", Status: "pending"},
		{Label: "Adatbázis mentése", Status: "pending"},
		{Label: "Felhasználói adatok", Status: "pending"},
		{Label: "Csomag készítése", Status: "pending"},
	}
	if req.Password != "" {
		steps = append(steps, Step{Label: "Titkosítás", Status: "pending"})
	}

	job := &Job{
		StackName:   req.StackName,
		DisplayName: e.provider.GetStackDisplayName(req.StackName),
		Steps:       steps,
		Running:     true,
		JobType:     "export",
	}
	e.activeJob = job
	e.mu.Unlock()

	go e.executeExport(req, job)
	return nil
}

func (e *Exporter) executeExport(req ExportRequest, job *Job) {
	exportStart := time.Now()
	e.debugf("=== EXPORT START: stack=%q dest=%q encrypted=%v stopApp=%v ===",
		req.StackName, req.DestDrive, req.Password != "", req.StopApp)

	defer func() {
		job.mu.Lock()
		job.Running = false
		job.Done = true
		job.mu.Unlock()
		e.debugf("=== EXPORT END: stack=%q elapsed=%v ===", req.StackName, time.Since(exportStart))
	}()

	step := 0

	// --- Step 0: Preparation ---
	job.setStep(step, "running", "")
	stepStart := time.Now()

	destDir := ExportDir(req.DestDrive)
	e.debugf("export dest dir: %s", destDir)
	if err := os.MkdirAll(destDir, 0755); err != nil {
		e.failJob(job, step, fmt.Sprintf("Nem sikerült létrehozni az export könyvtárat: %v", err))
		return
	}

	// Check free space
	est, err := e.EstimateExport(req.StackName, req.DestDrive)
	if err != nil {
		e.debugf("estimate error (non-fatal): %v", err)
	} else {
		e.debugf("estimate: config=%s data=%s total=%s destFree=%s fits=%v",
			est.ConfigSizeHuman, est.DataSizeHuman, est.TotalSizeHuman, est.DestFreeHuman, est.FitsOnDest)
		if !est.FitsOnDest {
			e.failJob(job, step, fmt.Sprintf("Nincs elég hely: szükséges ~%s, szabad %s",
				est.TotalSizeHuman, est.DestFreeHuman))
			return
		}
	}

	// Optionally stop the app
	wasRunning := false
	if req.StopApp && e.provider.IsStackRunning(req.StackName) {
		wasRunning = true
		e.logger.Printf("[INFO] Export: stopping %s", req.StackName)
		e.debugf("stopping stack %s before export", req.StackName)
		if err := e.provider.StopStack(req.StackName); err != nil {
			e.logger.Printf("[WARN] Export: could not stop %s: %v", req.StackName, err)
		} else {
			e.debugf("stack %s stopped successfully", req.StackName)
		}
	} else {
		e.debugf("skip stop: stopApp=%v isRunning=%v", req.StopApp, e.provider.IsStackRunning(req.StackName))
	}
	// Always restart after export if we stopped it
	if wasRunning {
		defer func() {
			e.logger.Printf("[INFO] Export: restarting %s", req.StackName)
			e.debugf("restarting stack %s after export", req.StackName)
			if err := e.provider.StartStack(req.StackName); err != nil {
				e.logger.Printf("[WARN] Export: could not restart %s: %v", req.StackName, err)
			} else {
				e.debugf("stack %s restarted successfully", req.StackName)
			}
		}()
	}

	e.debugf("step 0 (preparation) done in %v", time.Since(stepStart))
	job.setStep(step, "done", "")
	step++

	// --- Step 1: Config files ---
	job.setStep(step, "running", "")
	stepStart = time.Now()

	tmpDir, err := os.MkdirTemp("", "felhom-export-*")
	if err != nil {
		e.failJob(job, step, fmt.Sprintf("Temp könyvtár hiba: %v", err))
		return
	}
	e.debugf("temp dir: %s", tmpDir)
	defer os.RemoveAll(tmpDir)

	configDir := filepath.Join(tmpDir, "config")
	if err := os.MkdirAll(configDir, 0755); err != nil {
		e.failJob(job, step, err.Error())
		return
	}

	stackDir, ok := e.provider.GetStackDir(req.StackName)
	if !ok {
		e.failJob(job, step, "Stack könyvtár nem található")
		return
	}
	e.debugf("stack dir: %s", stackDir)

	configFiles, err := copyStackConfig(stackDir, configDir, req.StackName, e.provider)
	if err != nil {
		e.failJob(job, step, fmt.Sprintf("Konfiguráció mentése sikertelen: %v", err))
		return
	}
	e.debugf("config files copied: %v (%d files)", configFiles, len(configFiles))
	e.debugf("step 1 (config) done in %v", time.Since(stepStart))

	job.setStep(step, "done", "")
	step++

	// --- Step 2: Database dump ---
	job.setStep(step, "running", "")
	stepStart = time.Now()

	dbDir := filepath.Join(tmpDir, "database")
	os.MkdirAll(dbDir, 0755)

	manifest := &Manifest{
		Version:       ManifestVersion,
		AppName:       req.StackName,
		DisplayName:   e.provider.GetStackDisplayName(req.StackName),
		ExportedAt:    time.Now().UTC(),
		ControllerVer: e.version,
		NeedsHDD:      e.provider.GetStackNeedsHDD(req.StackName),
		Encrypted:     req.Password != "",
		ConfigFiles:   configFiles,
	}
	e.debugf("manifest: app=%s display=%s needsHDD=%v encrypted=%v",
		manifest.AppName, manifest.DisplayName, manifest.NeedsHDD, manifest.Encrypted)

	dbDumped := e.dumpDatabase(req.StackName, dbDir, manifest)
	if !dbDumped {
		e.debugf("no database found for %s — skipping DB step", req.StackName)
		os.Remove(dbDir)
	} else {
		e.debugf("database dumped: type=%s", manifest.DBType)
		// Log the dump file size
		entries, _ := os.ReadDir(dbDir)
		for _, entry := range entries {
			if info, err := entry.Info(); err == nil {
				e.debugf("  db dump file: %s (%s)", entry.Name(), humanizeBytes(info.Size()))
			}
		}
	}
	e.debugf("step 2 (database) done in %v", time.Since(stepStart))

	job.setStep(step, "done", "")
	step++

	// --- Step 3: User data ---
	job.setStep(step, "running", "")
	stepStart = time.Now()

	dataDir := filepath.Join(tmpDir, "data")
	os.MkdirAll(dataDir, 0755)

	if e.provider.GetStackNeedsHDD(req.StackName) {
		e.debugf("exporting HDD data for %s", req.StackName)
		e.exportHDDData(req.StackName, dataDir, manifest)
		e.debugf("HDD data exported: subdirs=%v hasData=%v", manifest.HDDSubdirs, manifest.HasHDDData)
	} else {
		e.debugf("exporting Docker volumes for %s", req.StackName)
		e.exportVolumeData(req.StackName, dataDir, manifest)
		e.debugf("volume data exported: volumes=%v hasData=%v", manifest.VolumeNames, manifest.HasVolumeData)
	}
	e.debugf("step 3 (user data) done in %v", time.Since(stepStart))

	job.setStep(step, "done", "")
	step++

	// --- Step 4: Create .fab bundle ---
	job.setStep(step, "running", "")
	stepStart = time.Now()

	// Calculate total size
	manifest.TotalSizeBytes = calcDirSize(tmpDir)
	e.debugf("total bundle content size: %s (%d bytes)", humanizeBytes(manifest.TotalSizeBytes), manifest.TotalSizeBytes)

	// Write manifest.json to tmpDir root
	manifestData, err := manifest.Marshal()
	if err != nil {
		e.failJob(job, step, fmt.Sprintf("Manifest hiba: %v", err))
		return
	}
	e.debugf("manifest JSON: %d bytes", len(manifestData))
	if err := os.WriteFile(filepath.Join(tmpDir, "manifest.json"), manifestData, 0644); err != nil {
		e.failJob(job, step, err.Error())
		return
	}

	timestamp := time.Now().Format("20060102-150405")
	fabName := fmt.Sprintf("%s_%s.fab", req.StackName, timestamp)
	fabPath := filepath.Join(destDir, fabName)
	e.debugf("target .fab path: %s", fabPath)

	// Build tar.gz (to .tmp if encrypting, to final path if not)
	targetPath := fabPath
	if req.Password != "" {
		targetPath = fabPath + ".tgz.tmp"
	} else {
		targetPath = fabPath + ".tmp"
	}

	e.debugf("creating tar.gz: %s", targetPath)
	tgzStart := time.Now()
	if err := createTarGz(targetPath, tmpDir); err != nil {
		os.Remove(targetPath)
		e.failJob(job, step, fmt.Sprintf("Csomag készítése sikertelen: %v", err))
		return
	}
	if tgzInfo, err := os.Stat(targetPath); err == nil {
		e.debugf("tar.gz created: %s (%s) in %v", targetPath, humanizeBytes(tgzInfo.Size()), time.Since(tgzStart))
	}

	job.setStep(step, "done", "")
	step++

	// --- Step 5: Encrypt (optional) ---
	if req.Password != "" {
		job.setStep(step, "running", "")
		encStart := time.Now()

		encPath := fabPath + ".tmp"
		e.debugf("encrypting: %s → %s", targetPath, encPath)
		if err := EncryptFile(targetPath, encPath, req.Password); err != nil {
			os.Remove(targetPath)
			e.failJob(job, step, fmt.Sprintf("Titkosítás sikertelen: %v", err))
			return
		}
		os.Remove(targetPath)
		targetPath = encPath
		if encInfo, err := os.Stat(encPath); err == nil {
			e.debugf("encryption done: %s in %v", humanizeBytes(encInfo.Size()), time.Since(encStart))
		}

		job.setStep(step, "done", "")
	}

	// Atomic rename to final path
	e.debugf("atomic rename: %s → %s", targetPath, fabPath)
	if err := os.Rename(targetPath, fabPath); err != nil {
		e.failJob(job, step, fmt.Sprintf("Fájl átnevezés sikertelen: %v", err))
		return
	}

	// Record result
	stat, _ := os.Stat(fabPath)
	job.mu.Lock()
	job.OutputPath = fabPath
	if stat != nil {
		job.OutputSize = humanizeBytes(stat.Size())
	}
	job.mu.Unlock()

	e.logger.Printf("[INFO] Export completed: %s → %s (%s) in %v", req.StackName, fabPath, job.OutputSize, time.Since(exportStart))
}

func (e *Exporter) failJob(job *Job, stepIdx int, msg string) {
	job.setStep(stepIdx, "failed", msg)
	job.mu.Lock()
	job.Error = msg
	job.mu.Unlock()
	e.logger.Printf("[ERROR] Export/import failed at step %d: %s", stepIdx, msg)
	e.debugf("FAIL at step %d: %s", stepIdx, msg)
}

// GetDebugInfo returns diagnostic information about the exporter state.
func (e *Exporter) GetDebugInfo() map[string]interface{} {
	e.mu.Lock()
	defer e.mu.Unlock()

	info := map[string]interface{}{
		"debug_enabled": e.debug,
		"version":       e.version,
		"has_active_job": e.activeJob != nil,
	}

	if e.activeJob != nil {
		info["active_job"] = e.activeJob.Snapshot()
	}

	return info
}

// copyStackConfig copies all relevant config files from the stack dir.
// app.yaml is saved with decrypted (plaintext) secrets for portability.
func copyStackConfig(stackDir, configDir, stackName string, provider ExportStackProvider) ([]string, error) {
	var copied []string

	entries, err := os.ReadDir(stackDir)
	if err != nil {
		return nil, err
	}

	for _, entry := range entries {
		if entry.IsDir() {
			continue
		}
		name := entry.Name()
		// Skip temp files
		if strings.HasSuffix(name, ".tmp") {
			continue
		}

		src := filepath.Join(stackDir, name)
		dst := filepath.Join(configDir, name)

		// app.yaml: save with plaintext secrets
		if name == "app.yaml" {
			env := provider.GetDecryptedEnv(stackName)
			if env != nil {
				if err := writeDecryptedAppYaml(dst, env); err != nil {
					return nil, fmt.Errorf("writing decrypted app.yaml: %w", err)
				}
				copied = append(copied, name)
				continue
			}
		}

		// Copy file as-is
		if err := copyFile(src, dst); err != nil {
			return nil, fmt.Errorf("copying %s: %w", name, err)
		}
		copied = append(copied, name)
	}

	return copied, nil
}

// writeDecryptedAppYaml writes a plaintext app.yaml with the given env map.
func writeDecryptedAppYaml(dst string, env map[string]string) error {
	var sb strings.Builder
	sb.WriteString("# Exported by felhom-controller — plaintext secrets\n")
	sb.WriteString("deployed: true\n")
	sb.WriteString("env:\n")
	for k, v := range env {
		// YAML-safe: quote values
		sb.WriteString(fmt.Sprintf("  %s: %q\n", k, v))
	}
	return os.WriteFile(dst, []byte(sb.String()), 0644)
}

// dumpDatabase discovers and dumps the database for a stack.
func (e *Exporter) dumpDatabase(stackName, dbDir string, manifest *Manifest) bool {
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
	defer cancel()

	e.debugf("discovering databases (looking for stack %s)...", stackName)
	dbs, err := backup.DiscoverDatabases(ctx, e.logger, e.debug)
	if err != nil {
		e.logger.Printf("[WARN] Export: DB discovery error: %v", err)
		return false
	}
	e.debugf("found %d databases total", len(dbs))
	for i := range dbs {
		e.debugf("  db[%d]: stack=%s container=%s type=%s", i, dbs[i].StackName, dbs[i].ContainerName, dbs[i].DBType)
	}

	var stackDB *backup.DiscoveredDB
	for i := range dbs {
		if dbs[i].StackName == stackName {
			stackDB = &dbs[i]
			break
		}
	}
	if stackDB == nil {
		e.debugf("no database container found for stack %s", stackName)
		return false
	}
	e.debugf("matched DB: container=%s type=%s", stackDB.ContainerName, stackDB.DBType)

	dumpStart := time.Now()
	result := backup.DumpOne(ctx, *stackDB, dbDir, e.logger, e.debug)
	if result.Error != nil {
		e.logger.Printf("[WARN] Export: DB dump failed for %s: %v", stackName, result.Error)
		return false
	}
	e.debugf("DB dump completed in %v: %s", time.Since(dumpStart), result.FilePath)

	// Gzip the dump
	if result.FilePath != "" {
		gzPath := result.FilePath + ".gz"
		gzStart := time.Now()
		if err := gzipFile(result.FilePath, gzPath); err != nil {
			e.logger.Printf("[WARN] Export: gzip dump failed: %v", err)
		} else {
			if origInfo, _ := os.Stat(result.FilePath); origInfo != nil {
				if gzInfo, _ := os.Stat(gzPath); gzInfo != nil {
					e.debugf("gzip: %s → %s (ratio %.1f%%) in %v",
						humanizeBytes(origInfo.Size()), humanizeBytes(gzInfo.Size()),
						float64(gzInfo.Size())/float64(origInfo.Size())*100, time.Since(gzStart))
				}
			}
			os.Remove(result.FilePath)
		}
	}

	manifest.HasDatabase = true
	manifest.DBType = string(stackDB.DBType)

	return true
}

// exportHDDData copies HDD bind mount data for the export.
func (e *Exporter) exportHDDData(stackName, dataDir string, manifest *Manifest) {
	hddDir := filepath.Join(dataDir, "hdd")
	os.MkdirAll(hddDir, 0755)

	mounts := e.provider.GetStackHDDMounts(stackName)
	e.debugf("HDD mounts for %s: %v (%d total)", stackName, mounts, len(mounts))
	if len(mounts) == 0 {
		e.debugf("no HDD mounts — skipping HDD data export")
		return
	}

	for _, mount := range mounts {
		if _, err := os.Stat(mount); os.IsNotExist(err) {
			e.debugf("HDD mount %s does not exist — skipping", mount)
			continue
		}
		subdir := filepath.Base(mount)
		manifest.HDDSubdirs = append(manifest.HDDSubdirs, subdir)

		tarPath := filepath.Join(hddDir, subdir+".tar")
		e.debugf("tarring HDD mount: %s → %s", mount, tarPath)
		tarStart := time.Now()
		if err := tarDirectory(mount, tarPath); err != nil {
			e.logger.Printf("[WARN] Export: failed to tar %s: %v", mount, err)
		} else {
			if info, _ := os.Stat(tarPath); info != nil {
				e.debugf("HDD tar complete: %s (%s) in %v", subdir, humanizeBytes(info.Size()), time.Since(tarStart))
			}
		}
	}
	manifest.HasHDDData = len(manifest.HDDSubdirs) > 0
}

// exportVolumeData exports Docker named volumes for apps without HDD storage.
func (e *Exporter) exportVolumeData(stackName, dataDir string, manifest *Manifest) {
	volDir := filepath.Join(dataDir, "volumes")
	os.MkdirAll(volDir, 0755)

	volumes := e.provider.GetDockerVolumes(stackName)
	e.debugf("Docker volumes for %s: %v (%d total)", stackName, volumes, len(volumes))
	if len(volumes) == 0 {
		e.debugf("no Docker volumes — skipping volume data export")
		return
	}

	for _, volName := range volumes {
		tarPath := filepath.Join(volDir, volName+".tar")
		e.debugf("exporting volume %s via docker run alpine tar...", volName)
		volStart := time.Now()

		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Minute)
		cmd := exec.CommandContext(ctx, "docker", "run", "--rm",
			"-v", volName+":/vol:ro",
			"-v", volDir+":/out",
			"alpine", "tar", "cf", "/out/"+volName+".tar", "-C", "/vol", ".")
		out, err := cmd.CombinedOutput()
		cancel()

		if err != nil {
			e.logger.Printf("[WARN] Export: volume %s export failed: %s — %v",
				volName, strings.TrimSpace(string(out)), err)
			e.debugf("volume %s export failed: %s", volName, strings.TrimSpace(string(out)))
			os.Remove(tarPath)
			continue
		}
		if info, _ := os.Stat(tarPath); info != nil {
			e.debugf("volume %s exported: %s in %v", volName, humanizeBytes(info.Size()), time.Since(volStart))
		}
		manifest.VolumeNames = append(manifest.VolumeNames, volName)
	}
	manifest.HasVolumeData = len(manifest.VolumeNames) > 0
}

// createTarGz creates a gzipped tar archive of a directory.
func createTarGz(outputPath, sourceDir string) error {
	outFile, err := os.Create(outputPath)
	if err != nil {
		return err
	}
	defer outFile.Close()

	gw := gzip.NewWriter(outFile)
	defer gw.Close()

	tw := tar.NewWriter(gw)
	defer tw.Close()

	return filepath.Walk(sourceDir, func(path string, info os.FileInfo, err error) error {
		if err != nil {
			return err
		}

		// Get path relative to sourceDir
		relPath, err := filepath.Rel(sourceDir, path)
		if err != nil {
			return err
		}
		if relPath == "." {
			return nil
		}

		header, err := tar.FileInfoHeader(info, "")
		if err != nil {
			return err
		}
		header.Name = relPath

		if err := tw.WriteHeader(header); err != nil {
			return err
		}

		if info.IsDir() {
			return nil
		}

		f, err := os.Open(path)
		if err != nil {
			return err
		}
		defer f.Close()
		_, err = io.Copy(tw, f)
		return err
	})
}

// tarDirectory creates a tar (not gzipped) of a directory's contents.
func tarDirectory(sourceDir, outputPath string) error {
	outFile, err := os.Create(outputPath)
	if err != nil {
		return err
	}
	defer outFile.Close()

	tw := tar.NewWriter(outFile)
	defer tw.Close()

	return filepath.Walk(sourceDir, func(path string, info os.FileInfo, err error) error {
		if err != nil {
			return err
		}
		relPath, err := filepath.Rel(sourceDir, path)
		if err != nil {
			return err
		}
		if relPath == "." {
			return nil
		}

		header, err := tar.FileInfoHeader(info, "")
		if err != nil {
			return err
		}
		header.Name = relPath

		if err := tw.WriteHeader(header); err != nil {
			return err
		}

		if info.IsDir() {
			return nil
		}

		f, err := os.Open(path)
		if err != nil {
			return err
		}
		defer f.Close()
		_, err = io.Copy(tw, f)
		return err
	})
}

// gzipFile compresses a file with gzip.
func gzipFile(src, dst string) error {
	in, err := os.Open(src)
	if err != nil {
		return err
	}
	defer in.Close()

	out, err := os.Create(dst)
	if err != nil {
		return err
	}
	defer out.Close()

	gw := gzip.NewWriter(out)
	defer gw.Close()

	_, err = io.Copy(gw, in)
	return err
}

// copyFile copies a file from src to dst.
func copyFile(src, dst string) error {
	in, err := os.Open(src)
	if err != nil {
		return err
	}
	defer in.Close()

	out, err := os.Create(dst)
	if err != nil {
		return err
	}
	defer out.Close()

	if _, err := io.Copy(out, in); err != nil {
		return err
	}
	return out.Sync()
}

// calcDirSize recursively calculates total file size in a directory.
func calcDirSize(dir string) int64 {
	var total int64
	filepath.Walk(dir, func(_ string, info os.FileInfo, err error) error {
		if err != nil || info.IsDir() {
			return nil
		}
		total += info.Size()
		return nil
	})
	return total
}