package storage

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"io"
	"net/url"
	"os"
	"path/filepath"
	"time"
)

// ErrPresignNotSupported is returned when presigned URLs are requested from a
// backend that does not support them.
var ErrPresignNotSupported = errors.New("presigned URLs not supported by filesystem backend")

// Compile-time check: *FilesystemStore implements FileStore.
var _ FileStore = (*FilesystemStore)(nil)

// FilesystemStore stores objects as files under a root directory.
type FilesystemStore struct {
	root string // absolute path
}

// NewFilesystemStore creates a new filesystem-backed store rooted at root.
// The directory is created if it does not exist.
func NewFilesystemStore(root string) (*FilesystemStore, error) {
	abs, err := filepath.Abs(root)
	if err != nil {
		return nil, fmt.Errorf("resolving root path: %w", err)
	}
	if err := os.MkdirAll(abs, 0o755); err != nil {
		return nil, fmt.Errorf("creating root directory: %w", err)
	}
	return &FilesystemStore{root: abs}, nil
}

// path returns the absolute filesystem path for a storage key.
func (fs *FilesystemStore) path(key string) string {
	return filepath.Join(fs.root, filepath.FromSlash(key))
}

// Put writes reader to the file at key using atomic rename.
// SHA-256 checksum is computed during write and returned in PutResult.
func (fs *FilesystemStore) Put(_ context.Context, key string, reader io.Reader, _ int64, _ string) (*PutResult, error) {
	dest := fs.path(key)

	if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
		return nil, fmt.Errorf("creating directories: %w", err)
	}

	// Write to a temp file in the same directory so os.Rename is atomic.
	tmp, err := os.CreateTemp(filepath.Dir(dest), ".silo-tmp-*")
	if err != nil {
		return nil, fmt.Errorf("creating temp file: %w", err)
	}
	tmpPath := tmp.Name()
	defer func() {
		// Clean up temp file on any failure path.
		tmp.Close()
		os.Remove(tmpPath)
	}()

	h := sha256.New()
	w := io.MultiWriter(tmp, h)

	n, err := io.Copy(w, reader)
	if err != nil {
		return nil, fmt.Errorf("writing file: %w", err)
	}
	if err := tmp.Close(); err != nil {
		return nil, fmt.Errorf("closing temp file: %w", err)
	}

	if err := os.Rename(tmpPath, dest); err != nil {
		return nil, fmt.Errorf("renaming temp file: %w", err)
	}

	return &PutResult{
		Key:      key,
		Size:     n,
		Checksum: hex.EncodeToString(h.Sum(nil)),
	}, nil
}

// Get opens the file at key for reading.
func (fs *FilesystemStore) Get(_ context.Context, key string) (io.ReadCloser, error) {
	f, err := os.Open(fs.path(key))
	if err != nil {
		return nil, fmt.Errorf("opening file: %w", err)
	}
	return f, nil
}

// GetVersion delegates to Get — filesystem storage has no versioning.
func (fs *FilesystemStore) GetVersion(ctx context.Context, key string, _ string) (io.ReadCloser, error) {
	return fs.Get(ctx, key)
}

// Delete removes the file at key. No error if already absent.
func (fs *FilesystemStore) Delete(_ context.Context, key string) error {
	err := os.Remove(fs.path(key))
	if err != nil && !errors.Is(err, os.ErrNotExist) {
		return fmt.Errorf("removing file: %w", err)
	}
	return nil
}

// Exists reports whether the file at key exists.
func (fs *FilesystemStore) Exists(_ context.Context, key string) (bool, error) {
	_, err := os.Stat(fs.path(key))
	if err == nil {
		return true, nil
	}
	if errors.Is(err, os.ErrNotExist) {
		return false, nil
	}
	return false, fmt.Errorf("checking file: %w", err)
}

// Copy duplicates a file from srcKey to dstKey using atomic rename.
func (fs *FilesystemStore) Copy(_ context.Context, srcKey, dstKey string) error {
	srcPath := fs.path(srcKey)
	dstPath := fs.path(dstKey)

	src, err := os.Open(srcPath)
	if err != nil {
		return fmt.Errorf("opening source: %w", err)
	}
	defer src.Close()

	if err := os.MkdirAll(filepath.Dir(dstPath), 0o755); err != nil {
		return fmt.Errorf("creating directories: %w", err)
	}

	tmp, err := os.CreateTemp(filepath.Dir(dstPath), ".silo-tmp-*")
	if err != nil {
		return fmt.Errorf("creating temp file: %w", err)
	}
	tmpPath := tmp.Name()
	defer func() {
		tmp.Close()
		os.Remove(tmpPath)
	}()

	if _, err := io.Copy(tmp, src); err != nil {
		return fmt.Errorf("copying file: %w", err)
	}
	if err := tmp.Close(); err != nil {
		return fmt.Errorf("closing temp file: %w", err)
	}

	if err := os.Rename(tmpPath, dstPath); err != nil {
		return fmt.Errorf("renaming temp file: %w", err)
	}
	return nil
}

// PresignPut is not supported by the filesystem backend.
func (fs *FilesystemStore) PresignPut(_ context.Context, _ string, _ time.Duration) (*url.URL, error) {
	return nil, ErrPresignNotSupported
}

// Ping verifies the root directory is accessible and writable.
func (fs *FilesystemStore) Ping(_ context.Context) error {
	tmp, err := os.CreateTemp(fs.root, ".silo-ping-*")
	if err != nil {
		return fmt.Errorf("storage ping failed: %w", err)
	}
	name := tmp.Name()
	tmp.Close()
	os.Remove(name)
	return nil
}