package storage

import (
	"context"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"strings"

	"github.com/spf13/afero"
)

// FileStorer implements Storer using the local filesystem.
// It mirrors the S3 path structure for consistency.
type FileStorer struct {
	fs       afero.Fs
	basePath string
}

// NewFileStorer creates a new filesystem storage backend.
//
// Construction is intentionally cheap and does not touch the filesystem.
// The basePath is recorded; the directory is created lazily on first
// write. Reads (Get/Stat/List) tolerate a missing basePath — a missing
// or unmounted destination during `snapshot list` should NOT block the
// command, it should degrade to "no remote snapshots reachable" with a
// warning. Write operations (Put/PutWithProgress) call MkdirAll for the
// per-blob parent directory, which also covers basePath on first use.
//
// Uses the real OS filesystem by default; call SetFilesystem to
// override for testing.
func NewFileStorer(basePath string) (*FileStorer, error) {
	return &FileStorer{
		fs:       afero.NewOsFs(),
		basePath: basePath,
	}, nil
}

// SetFilesystem overrides the filesystem for testing.
func (f *FileStorer) SetFilesystem(fs afero.Fs) {
	f.fs = fs
}

// storageDirPerm is the mode used for directories created under the
// storage base path.
const storageDirPerm = 0o755

// tempSuffix marks a partially written object. writeAtomic streams into a
// temp file carrying this suffix and only renames it onto the real key once
// the whole object is on disk, so an interrupted write can never leave a
// truncated object at the key a later run would Stat and trust as a complete
// blob. List and ListStream skip these files, so a leftover from an
// interrupted write is never listed or trusted as a blob; it is otherwise
// harmless and is overwritten when the same key is written again.
const tempSuffix = ".partial"

// Put stores data at the specified key.
func (f *FileStorer) Put(_ context.Context, key string, data io.Reader) error {
	return f.writeAtomic(key, data, nil)
}

// PutWithProgress stores data with progress reporting.
func (f *FileStorer) PutWithProgress(
	_ context.Context, key string, data io.Reader,
	_ int64, progress ProgressCallback,
) error {
	return f.writeAtomic(key, data, progress)
}

// Get retrieves data from the specified key.
func (f *FileStorer) Get(_ context.Context, key string) (io.ReadCloser, error) {
	path := f.fullPath(key)

	file, err := f.fs.Open(path)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, ErrNotFound
		}

		return nil, fmt.Errorf("opening file: %w", err)
	}

	return file, nil
}

// Stat returns metadata about an object without retrieving its contents.
func (f *FileStorer) Stat(_ context.Context, key string) (*ObjectInfo, error) {
	path := f.fullPath(key)

	info, err := f.fs.Stat(path)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, ErrNotFound
		}

		return nil, fmt.Errorf("stat file: %w", err)
	}

	return &ObjectInfo{
		Key:  key,
		Size: info.Size(),
	}, nil
}

// Delete removes an object.
func (f *FileStorer) Delete(_ context.Context, key string) error {
	path := f.fullPath(key)

	err := f.fs.Remove(path)
	if os.IsNotExist(err) {
		return nil // Match S3 behavior: no error if doesn't exist
	}

	if err != nil {
		return fmt.Errorf("removing file: %w", err)
	}

	return nil
}

// List returns all keys with the given prefix.
func (f *FileStorer) List(ctx context.Context, prefix string) ([]string, error) {
	var keys []string

	basePath := f.fullPath(prefix)

	// Check if base path exists
	exists, err := afero.Exists(f.fs, basePath)
	if err != nil {
		return nil, fmt.Errorf("checking path: %w", err)
	}

	if !exists {
		return keys, nil // Empty list for non-existent prefix
	}

	err = afero.Walk(f.fs, basePath, func(path string, info os.FileInfo, err error) error {
		if err != nil {
			return err
		}

		// Check context cancellation
		select {
		case <-ctx.Done():
			return ctx.Err()
		default:
		}

		if !info.IsDir() && !strings.HasSuffix(info.Name(), tempSuffix) {
			// Convert back to key (relative path from basePath)
			relPath, err := filepath.Rel(f.basePath, path)
			if err != nil {
				return fmt.Errorf("computing relative path: %w", err)
			}
			// Normalize path separators to forward slashes for consistency
			relPath = strings.ReplaceAll(relPath, string(filepath.Separator), "/")
			keys = append(keys, relPath)
		}

		return nil
	})
	if err != nil {
		return nil, fmt.Errorf("walking directory: %w", err)
	}

	return keys, nil
}

// ListStream returns a channel of ObjectInfo for large result sets.
func (f *FileStorer) ListStream(ctx context.Context, prefix string) <-chan ObjectInfo {
	ch := make(chan ObjectInfo)

	go func() {
		defer close(ch)

		basePath := f.fullPath(prefix)

		// Check if base path exists
		exists, err := afero.Exists(f.fs, basePath)
		if err != nil {
			ch <- ObjectInfo{Err: fmt.Errorf("checking path: %w", err)}

			return
		}

		if !exists {
			return // Empty channel for non-existent prefix
		}

		_ = afero.Walk(f.fs, basePath, func(path string, info os.FileInfo, err error) error {
			// Check context cancellation
			select {
			case <-ctx.Done():
				ch <- ObjectInfo{Err: ctx.Err()}

				return ctx.Err()
			default:
			}

			if err != nil {
				ch <- ObjectInfo{Err: err}

				return nil //nolint:nilerr // continue walking despite errors
			}

			if !info.IsDir() && !strings.HasSuffix(info.Name(), tempSuffix) {
				relPath, err := filepath.Rel(f.basePath, path)
				if err != nil {
					ch <- ObjectInfo{Err: fmt.Errorf("computing relative path: %w", err)}

					return nil
				}
				// Normalize path separators
				relPath = strings.ReplaceAll(relPath, string(filepath.Separator), "/")
				ch <- ObjectInfo{
					Key:  relPath,
					Size: info.Size(),
				}
			}

			return nil
		})
	}()

	return ch
}

// Info returns human-readable storage location information.
func (f *FileStorer) Info() Info {
	return Info{
		Type:     schemeFile,
		Location: f.basePath,
	}
}

// writeAtomic streams data into a temp file in the destination directory,
// fsyncs it, and renames it onto the final key. The key therefore appears
// only once the whole object has been durably written; a failure part-way
// leaves a temp file (removed here on the failing path) rather than a
// truncated object at the key.
func (f *FileStorer) writeAtomic(
	key string, data io.Reader, progress ProgressCallback,
) error {
	path := f.fullPath(key)
	dir := filepath.Dir(path)

	err := f.fs.MkdirAll(dir, storageDirPerm)
	if err != nil {
		return fmt.Errorf("creating directories: %w", err)
	}

	tmp, err := afero.TempFile(f.fs, dir, filepath.Base(path)+"-*"+tempSuffix)
	if err != nil {
		return fmt.Errorf("creating temp file: %w", err)
	}

	tmpPath := tmp.Name()

	// Remove the temp file unless the rename below claims it. On the success
	// path renamed is true, so the deferred Close and Remove are harmless
	// no-ops on a name that no longer exists.
	renamed := false

	defer func() {
		_ = tmp.Close()

		if !renamed {
			_ = f.fs.Remove(tmpPath)
		}
	}()

	var w io.Writer = tmp
	if progress != nil {
		w = &progressWriter{writer: tmp, callback: progress}
	}

	_, err = io.Copy(w, data)
	if err != nil {
		return fmt.Errorf("writing file: %w", err)
	}

	err = tmp.Sync()
	if err != nil {
		return fmt.Errorf("syncing temp file: %w", err)
	}

	err = tmp.Close()
	if err != nil {
		return fmt.Errorf("closing temp file: %w", err)
	}

	err = f.fs.Rename(tmpPath, path)
	if err != nil {
		return fmt.Errorf("renaming temp file: %w", err)
	}

	renamed = true

	return nil
}

// fullPath returns the full filesystem path for a key.
func (f *FileStorer) fullPath(key string) string {
	return filepath.Join(f.basePath, key)
}

// progressWriter wraps an io.Writer to track write progress.
type progressWriter struct {
	writer   io.Writer
	written  int64
	callback ProgressCallback
}

func (pw *progressWriter) Write(p []byte) (int, error) {
	n, err := pw.writer.Write(p)
	if n > 0 {
		pw.written += int64(n)
		if pw.callback != nil {
			callbackErr := pw.callback(pw.written)
			if callbackErr != nil {
				return n, callbackErr
			}
		}
	}

	return n, err
}