package cli

import (
	"crypto/sha256"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"path/filepath"
	"time"

	"github.com/dustin/go-humanize"
	"github.com/multiformats/go-multihash"
	"github.com/spf13/afero"
	"github.com/urfave/cli/v2"
	"sneak.berlin/go/mfer/internal/log"
	"sneak.berlin/go/mfer/mfer"
)

const (
	// hashBufSize is the read buffer size used when hashing files.
	hashBufSize = 64 * 1024

	// scanProgressInterval is how many scanned files pass between
	// progress updates.
	scanProgressInterval = 100
)

// errEntryMissingMtime indicates a manifest entry that carries no
// modification time where one is required to carry it forward unchanged.
var errEntryMissingMtime = errors.New("manifest entry has no mtime")

// FreshenStatus contains progress information for the freshen operation.
type FreshenStatus struct {
	Phase        string        // "scan" or "hash"
	TotalFiles   int64         // Total files to process in current phase
	CurrentFiles int64         // Files processed so far
	TotalBytes   int64         // Total bytes to hash (hash phase only)
	CurrentBytes int64         // Bytes hashed so far
	BytesPerSec  float64       // Current throughput rate
	ETA          time.Duration // Estimated time to completion
}

// freshenEntry tracks a file's status during freshen
type freshenEntry struct {
	path      string
	size      int64
	mtime     time.Time
	needsHash bool             // true if new or changed
	existing  *mfer.MFFilePath // existing manifest entry if unchanged
}

// freshenScanner walks the filesystem and compares it against the
// entries of an existing manifest.
type freshenScanner struct {
	fs              afero.Fs
	absBase         string
	manifestBase    string
	includeDotfiles bool
	followSymlinks  bool
	showProgress    bool
	existingByPath  map[string]*mfer.MFFilePath

	entries   []*freshenEntry
	scanCount int64
	changed   int64
	added     int64
	unchanged int64
}

// resolveSymlink resolves a symlink to its target's FileInfo. The
// second return value is false when the entry should be skipped.
func (s *freshenScanner) resolveSymlink(path string) (fs.FileInfo, bool) {
	if !s.followSymlinks {
		return nil, false
	}

	realPath, err := filepath.EvalSymlinks(path)
	if err != nil {
		return nil, false // Skip broken symlinks
	}

	realInfo, err := s.fs.Stat(realPath)
	if err != nil || realInfo.IsDir() {
		return nil, false
	}

	return realInfo, true
}

// recordEntry classifies a scanned file as changed, unchanged, or added
// relative to the existing manifest.
func (s *freshenScanner) recordEntry(relPath string, info fs.FileInfo) {
	existing, inManifest := s.existingByPath[relPath]
	if !inManifest {
		s.added++

		log.Verbosef("A %s", relPath)
		s.entries = append(s.entries, &freshenEntry{
			path:      relPath,
			size:      info.Size(),
			mtime:     info.ModTime(),
			needsHash: true,
		})

		return
	}

	// Check if changed (size or mtime). An entry with no recorded mtime
	// cannot be compared, so it counts as changed and gets re-hashed;
	// silently treating the absent mtime as the Unix epoch would classify
	// every such entry as changed without saying why.
	existingMtime, haveMtime := entryMtime(existing)
	if !haveMtime {
		log.Debugf("%s: manifest entry has no mtime, treating as changed",
			relPath)
	}

	if !haveMtime || existing.GetSize() != info.Size() ||
		!existingMtime.Equal(info.ModTime()) {
		s.changed++

		log.Verbosef("M %s", relPath)
		s.entries = append(s.entries, &freshenEntry{
			path:      relPath,
			size:      info.Size(),
			mtime:     info.ModTime(),
			needsHash: true,
		})
	} else {
		s.unchanged++

		s.entries = append(s.entries, &freshenEntry{
			path:      relPath,
			size:      info.Size(),
			mtime:     info.ModTime(),
			needsHash: false,
			existing:  existing,
		})
	}
	// Mark as seen
	delete(s.existingByPath, relPath)
}

// walk is the afero.Walk callback for the scan phase.
func (s *freshenScanner) walk(path string, info fs.FileInfo, walkErr error) error {
	if walkErr != nil {
		return walkErr
	}

	// Get relative path
	relPath, err := filepath.Rel(s.absBase, path)
	if err != nil {
		return fmt.Errorf(
			"freshen: failed to compute relative path for %s: %w", path, err)
	}

	// Skip the manifest file itself
	if relPath == s.manifestBase || relPath == "."+s.manifestBase {
		return nil
	}

	// Handle dotfiles
	if !s.includeDotfiles && mfer.IsHiddenPath(filepath.ToSlash(relPath)) {
		if info.IsDir() {
			return filepath.SkipDir
		}

		return nil
	}

	// Skip directories
	if info.IsDir() {
		return nil
	}

	// Handle symlinks
	if info.Mode()&fs.ModeSymlink != 0 {
		realInfo, keep := s.resolveSymlink(path)
		if !keep {
			return nil
		}

		info = realInfo
	}

	s.scanCount++

	// Check against existing manifest
	s.recordEntry(relPath, info)

	// Report scan progress
	if s.showProgress && s.scanCount%scanProgressInterval == 0 {
		log.Progressf("Scanning: %d files found", s.scanCount)
	}

	return nil
}

// resolveFreshenManifestPath determines the manifest path from the CLI
// arguments, searching directories for a manifest where needed.
func (mfa *CLIApp) resolveFreshenManifestPath(ctx *cli.Context) (string, error) {
	if ctx.Args().Len() == 0 {
		return findManifest(mfa.Fs, ".")
	}

	arg := ctx.Args().Get(0)

	info, statErr := mfa.Fs.Stat(arg)
	if statErr == nil && info.IsDir() {
		return findManifest(mfa.Fs, arg)
	}

	return arg, nil
}

// freshenHasher hashes changed and added files and feeds all entries to
// a manifest builder.
type freshenHasher struct {
	fs             afero.Fs
	absBase        string
	showProgress   bool
	totalHashBytes int64
	filesToHash    int64
	startHash      time.Time
	builder        *mfer.Builder

	hashedFiles int64
	hashedBytes int64
}

// reportProgress renders hashing progress for the current byte count.
func (h *freshenHasher) reportProgress(n int64) {
	if !h.showProgress {
		return
	}

	currentBytes := h.hashedBytes + n
	elapsed := time.Since(h.startHash)

	var (
		rate float64
		eta  time.Duration
	)

	if elapsed > 0 && currentBytes > 0 {
		rate = float64(currentBytes) / elapsed.Seconds()

		remaining := h.totalHashBytes - currentBytes
		if rate > 0 {
			eta = time.Duration(float64(remaining)/rate) * time.Second
		}
	}

	if eta > 0 {
		log.Progressf("Hashing: %d/%d files, %s/s, ETA %s",
			h.hashedFiles, h.filesToHash, humanize.IBytes(safeRateUint64(rate)),
			eta.Round(time.Second))
	} else {
		log.Progressf("Hashing: %d/%d files, %s/s",
			h.hashedFiles, h.filesToHash, humanize.IBytes(safeRateUint64(rate)))
	}
}

// processEntry hashes the entry if needed and adds it to the builder.
func (h *freshenHasher) processEntry(e *freshenEntry) error {
	if !e.needsHash {
		// Use existing entry
		err := addExistingToBuilder(h.builder, e.existing)
		if err != nil {
			return fmt.Errorf("failed to add %s: %w", e.path, err)
		}

		return nil
	}

	// Need to read and hash the file
	absPath := filepath.Join(h.absBase, e.path)

	f, err := h.fs.Open(absPath)
	if err != nil {
		return fmt.Errorf("failed to open %s: %w", e.path, err)
	}

	hash, bytesRead, err := hashFile(f, h.reportProgress)
	_ = f.Close()

	if err != nil {
		return fmt.Errorf("failed to hash %s: %w", e.path, err)
	}

	h.hashedBytes += bytesRead
	h.hashedFiles++

	// Add to builder with computed hash
	err = addFileToBuilder(h.builder, e.path, e.size, e.mtime, hash)
	if err != nil {
		return fmt.Errorf("failed to add %s: %w", e.path, err)
	}

	return nil
}

// writeFreshenedManifest writes the manifest atomically (write to a
// temp file, then rename over the target).
func writeFreshenedManifest(
	afs afero.Fs, builder *mfer.Builder, manifestPath string,
) error {
	tmpPath := manifestPath + ".tmp"

	outFile, err := afs.Create(tmpPath)
	if err != nil {
		return fmt.Errorf("failed to create temp file: %w", err)
	}

	err = builder.Build(outFile)
	_ = outFile.Close()

	if err != nil {
		_ = afs.Remove(tmpPath)

		return fmt.Errorf("failed to write manifest: %w", err)
	}

	// Rename temp to final
	err = afs.Rename(tmpPath, manifestPath)
	if err != nil {
		_ = afs.Remove(tmpPath)

		return fmt.Errorf("failed to rename manifest: %w", err)
	}

	return nil
}

// newFreshenBuilder constructs the manifest builder configured from CLI
// flags.
func newFreshenBuilder(ctx *cli.Context) *mfer.Builder {
	builder := mfer.NewBuilder()
	if ctx.Bool("include-timestamps") {
		builder.SetIncludeTimestamps(true)
	}

	// Set up signing options if sign-key is provided
	if signKey := ctx.String("sign-key"); signKey != "" {
		builder.SetSigningOptions(&mfer.SigningOptions{
			KeyID: mfer.GPGKeyID(signKey),
		})
		log.Infof("signing manifest with GPG key: %s", signKey)
	}

	return builder
}

// freshenScan runs the scan phase against the loaded manifest entries
// and returns the populated scanner and the count of removed files.
func (mfa *CLIApp) freshenScan(
	ctx *cli.Context, manifestPath, absBase string,
	existingByPath map[string]*mfer.MFFilePath,
) (*freshenScanner, int64, error) {
	log.Infof("scanning filesystem...")

	startScan := time.Now()
	showProgress := ctx.Bool("progress")

	scanner := &freshenScanner{
		fs:              mfa.Fs,
		absBase:         absBase,
		manifestBase:    filepath.Base(manifestPath),
		includeDotfiles: ctx.Bool("include-dotfiles"),
		followSymlinks:  ctx.Bool("follow-symlinks"),
		showProgress:    showProgress,
		existingByPath:  existingByPath,
	}

	err := afero.Walk(mfa.Fs, absBase, scanner.walk)

	if showProgress {
		log.ProgressDone()
	}

	if err != nil {
		return nil, 0, fmt.Errorf("failed to scan filesystem: %w", err)
	}

	// Remaining entries in existingByPath are removed files
	removed := int64(len(existingByPath))
	for path := range existingByPath {
		log.Verbosef("D %s", path)
	}

	scanDuration := time.Since(startScan)
	log.Infof("scan complete in %s: %d unchanged, %d changed, %d added, %d removed",
		scanDuration.Round(time.Millisecond), scanner.unchanged, scanner.changed,
		scanner.added, removed)

	return scanner, removed, nil
}

// hashTotals returns the total byte count and file count of entries
// that need hashing.
func hashTotals(entries []*freshenEntry) (int64, int64) {
	var (
		totalHashBytes int64
		filesToHash    int64
	)

	for _, e := range entries {
		if e.needsHash {
			totalHashBytes += e.size
			filesToHash++
		}
	}

	return totalHashBytes, filesToHash
}

// runFreshenHash processes every entry through the hasher, aborting if
// the context is canceled.
func runFreshenHash(
	ctx *cli.Context, hasher *freshenHasher, entries []*freshenEntry,
) error {
	for _, e := range entries {
		select {
		case <-ctx.Done():
			return ctx.Err()
		default:
		}

		err := hasher.processEntry(e)
		if err != nil {
			return err
		}
	}

	return nil
}

// loadExistingEntries loads the manifest and indexes its file entries
// by path.
func (mfa *CLIApp) loadExistingEntries(
	manifestPath string,
) (map[string]*mfer.MFFilePath, error) {
	log.Infof("loading manifest from %s", manifestPath)

	// Load existing manifest
	manifest, err := mfer.NewManifestFromFile(mfa.Fs, manifestPath)
	if err != nil {
		return nil, fmt.Errorf("failed to load manifest: %w", err)
	}

	existingFiles := manifest.Files()
	log.Infof("manifest contains %d files", len(existingFiles))

	// Build map of existing entries by path
	existingByPath := make(map[string]*mfer.MFFilePath, len(existingFiles))
	for _, f := range existingFiles {
		existingByPath[f.GetPath()] = f
	}

	return existingByPath, nil
}

func (mfa *CLIApp) freshenManifestOperation(ctx *cli.Context) error {
	log.Debug("freshenManifestOperation()")

	basePath := ctx.String("base")
	showProgress := ctx.Bool("progress")

	// Find manifest file
	manifestPath, err := mfa.resolveFreshenManifestPath(ctx)
	if err != nil {
		return fmt.Errorf("freshen: %w", err)
	}

	existingByPath, err := mfa.loadExistingEntries(manifestPath)
	if err != nil {
		return err
	}

	absBase, err := filepath.Abs(basePath)
	if err != nil {
		return fmt.Errorf("freshen: invalid base path: %w", err)
	}

	// Phase 1: Scan filesystem
	scanner, removed, err := mfa.freshenScan(ctx, manifestPath, absBase,
		existingByPath)
	if err != nil {
		return err
	}

	// Calculate total bytes to hash
	totalHashBytes, filesToHash := hashTotals(scanner.entries)

	// Phase 2: Hash changed and new files
	if filesToHash > 0 {
		log.Infof("hashing %d files (%s)...", filesToHash,
			humanize.IBytes(safeUint64(totalHashBytes)))
	}

	hasher := &freshenHasher{
		fs:             mfa.Fs,
		absBase:        absBase,
		showProgress:   showProgress,
		totalHashBytes: totalHashBytes,
		filesToHash:    filesToHash,
		startHash:      time.Now(),
		builder:        newFreshenBuilder(ctx),
	}

	err = runFreshenHash(ctx, hasher, scanner.entries)
	if err != nil {
		return err
	}

	if showProgress && filesToHash > 0 {
		log.ProgressDone()
	}

	// Print summary
	log.Infof("freshen complete: %d unchanged, %d changed, %d added, %d removed",
		scanner.unchanged, scanner.changed, scanner.added, removed)

	// Skip writing if nothing changed
	if scanner.changed == 0 && scanner.added == 0 && removed == 0 {
		log.Infof("manifest unchanged, skipping write")

		return nil
	}

	// Write updated manifest atomically (write to temp, then rename)
	err = writeFreshenedManifest(mfa.Fs, hasher.builder, manifestPath)
	if err != nil {
		return err
	}

	totalDuration := time.Since(mfa.startupTime)
	if hasher.hashedBytes > 0 {
		hashDuration := time.Since(hasher.startHash)
		hashRate := float64(hasher.hashedBytes) / hashDuration.Seconds()
		log.Infof("hashed %s in %.1fs (%s/s)",
			humanize.IBytes(safeUint64(hasher.hashedBytes)),
			totalDuration.Seconds(), humanize.IBytes(safeRateUint64(hashRate)))
	}

	log.Infof("wrote %d files to %s", len(scanner.entries), manifestPath)

	return nil
}

// hashFile reads a file and computes its SHA256 multihash.
// Progress callback is called with bytes read so far.
func hashFile(r io.Reader, progress func(int64)) ([]byte, int64, error) {
	h := sha256.New()
	buf := make([]byte, hashBufSize)

	var total int64

	for {
		n, err := r.Read(buf)
		if n > 0 {
			h.Write(buf[:n])

			total += int64(n)
			if progress != nil {
				progress(total)
			}
		}

		if err == io.EOF {
			break
		}

		// Returned unwrapped: the caller renders this as
		// "failed to hash <path>: <err>" and adding a second layer here
		// would change that message.
		if err != nil {
			return nil, total, err
		}
	}

	mh, err := multihash.Encode(h.Sum(nil), multihash.SHA2_256)
	if err != nil {
		return nil, total, err
	}

	return mh, total, nil
}

// addFileToBuilder adds a new file entry to the builder
func addFileToBuilder(
	b *mfer.Builder, path string, size int64, mtime time.Time, hash []byte,
) error {
	return b.AddFileWithHash(
		mfer.RelFilePath(path), mfer.FileSize(size), mfer.ModTime(mtime), hash)
}

// addExistingToBuilder adds an existing manifest entry to the builder.
//
// Entries reach this path only when recordEntry classified them as
// unchanged, which requires a recorded mtime, so an absent mtime here is
// an error rather than something to paper over with the Unix epoch.
func addExistingToBuilder(b *mfer.Builder, entry *mfer.MFFilePath) error {
	mtime, ok := entryMtime(entry)
	if !ok {
		return fmt.Errorf("%w: %s", errEntryMissingMtime, entry.GetPath())
	}

	if len(entry.GetHashes()) == 0 {
		return nil
	}

	return b.AddFileWithHash(mfer.RelFilePath(entry.GetPath()),
		mfer.FileSize(entry.GetSize()), mfer.ModTime(mtime),
		entry.GetHashes()[0].GetMultiHash())
}