package blobstore

import (
	"context"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"os"
	"path/filepath"
	"strings"
)

// LocalFS stores blobs as files beneath a root directory.
type LocalFS struct {
	root string
}

var _ BlobStore = (*LocalFS)(nil)

// NewLocalFS returns a blob store rooted at dir, creating it if necessary.
func NewLocalFS(dir string) (*LocalFS, error) {
	abs, err := filepath.Abs(dir)
	if err != nil {
		return nil, fmt.Errorf("blobstore: resolve root %q: %w", dir, err)
	}
	if err := os.MkdirAll(abs, 0o750); err != nil {
		return nil, fmt.Errorf("blobstore: create root %q: %w", abs, err)
	}
	return &LocalFS{root: abs}, nil
}

// Root returns the absolute directory holding the blobs.
func (l *LocalFS) Root() string { return l.root }

// Put writes the stream to a temporary file and renames it into place, so a
// failed or oversized upload never leaves a partial object visible.
func (l *LocalFS) Put(ctx context.Context, key string, r io.Reader, maxBytes int64) (int64, error) {
	path, err := l.resolve(key)
	if err != nil {
		return 0, err
	}
	if err := os.MkdirAll(filepath.Dir(path), 0o750); err != nil {
		return 0, fmt.Errorf("blobstore: create parent of %q: %w", key, err)
	}

	tmp, err := os.CreateTemp(filepath.Dir(path), ".upload-*")
	if err != nil {
		return 0, fmt.Errorf("blobstore: create temp file: %w", err)
	}
	tmpName := tmp.Name()
	defer func() {
		tmp.Close()
		os.Remove(tmpName) // no-op once the rename has succeeded
	}()

	// Copy one byte past the limit so an oversized payload is detectable.
	written, err := io.Copy(tmp, io.LimitReader(r, maxBytes+1))
	if err != nil {
		return 0, fmt.Errorf("blobstore: write %q: %w", key, err)
	}
	if written > maxBytes {
		return written, fmt.Errorf("%w: limit is %d bytes", ErrTooLarge, maxBytes)
	}
	if err := tmp.Sync(); err != nil {
		return 0, fmt.Errorf("blobstore: sync %q: %w", key, err)
	}
	if err := tmp.Close(); err != nil {
		return 0, fmt.Errorf("blobstore: close %q: %w", key, err)
	}
	if err := ctx.Err(); err != nil {
		return written, err
	}
	if err := os.Rename(tmpName, path); err != nil {
		return written, fmt.Errorf("blobstore: commit %q: %w", key, err)
	}
	return written, nil
}

// Get opens the stored file.
func (l *LocalFS) Get(ctx context.Context, key string) (io.ReadCloser, error) {
	path, err := l.resolve(key)
	if err != nil {
		return nil, err
	}
	f, err := os.Open(path)
	if errors.Is(err, fs.ErrNotExist) {
		return nil, fmt.Errorf("%w: %s", ErrNotFound, key)
	}
	if err != nil {
		return nil, fmt.Errorf("blobstore: open %q: %w", key, err)
	}
	return f, nil
}

// Delete removes the stored file, ignoring a missing one.
func (l *LocalFS) Delete(ctx context.Context, key string) error {
	path, err := l.resolve(key)
	if err != nil {
		return err
	}
	if err := os.Remove(path); err != nil && !errors.Is(err, fs.ErrNotExist) {
		return fmt.Errorf("blobstore: delete %q: %w", key, err)
	}
	return nil
}

// resolve maps a key to a path inside the root, rejecting any key that escapes
// it. Keys reach this code from upload filenames, so traversal has to fail
// closed rather than being normalised away.
func (l *LocalFS) resolve(key string) (string, error) {
	if key == "" {
		return "", errors.New("blobstore: empty key")
	}
	if filepath.IsAbs(key) {
		return "", fmt.Errorf("blobstore: key %q must be relative", key)
	}

	clean := filepath.Clean("/" + key)
	// filepath.Clean on an absolute path collapses any ".." segments.
	rel := strings.TrimPrefix(clean, "/")
	if rel == "" {
		return "", fmt.Errorf("blobstore: key %q resolves to the root", key)
	}

	path := filepath.Join(l.root, rel)
	// Defence in depth: confirm the result really is under the root.
	if path != l.root && !strings.HasPrefix(path, l.root+string(os.PathSeparator)) {
		return "", fmt.Errorf("blobstore: key %q escapes the store root", key)
	}
	return path, nil
}