package litestream

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

	"github.com/benbjohnson/litestream/internal"
)

// FileReplicaClientType is the client type for file replica clients.
const FileReplicaClientType = "file"

var _ ReplicaClient = (*FileReplicaClient)(nil)

// FileReplicaClient is a client for writing snapshots & WAL segments to disk.
type FileReplicaClient struct {
	path string // destination path

	// File info
	FileMode os.FileMode
	DirMode  os.FileMode
	Uid, Gid int
}

// NewFileReplicaClient returns a new instance of FileReplicaClient.
func NewFileReplicaClient(path string) *FileReplicaClient {
	return &FileReplicaClient{
		path: path,

		FileMode: 0600,
		DirMode:  0700,
	}
}

// Type returns "file" as the client type.
func (c *FileReplicaClient) Type() string {
	return FileReplicaClientType
}

// Path returns the destination path to replicate the database to.
func (c *FileReplicaClient) Path() string {
	return c.path
}

// GenerationsDir returns the path to a generation root directory.
func (c *FileReplicaClient) GenerationsDir() (string, error) {
	if c.path == "" {
		return "", fmt.Errorf("file replica path required")
	}
	return filepath.Join(c.path, "generations"), nil
}

// GenerationDir returns the path to a generation's root directory.
func (c *FileReplicaClient) GenerationDir(generation string) (string, error) {
	dir, err := c.GenerationsDir()
	if err != nil {
		return "", err
	} else if generation == "" {
		return "", fmt.Errorf("generation required")
	}
	return filepath.Join(dir, generation), nil
}

// SnapshotsDir returns the path to a generation's snapshot directory.
func (c *FileReplicaClient) SnapshotsDir(generation string) (string, error) {
	dir, err := c.GenerationDir(generation)
	if err != nil {
		return "", err
	}
	return filepath.Join(dir, "snapshots"), nil
}

// SnapshotPath returns the path to an uncompressed snapshot file.
func (c *FileReplicaClient) SnapshotPath(generation string, index int) (string, error) {
	dir, err := c.SnapshotsDir(generation)
	if err != nil {
		return "", err
	}
	return filepath.Join(dir, FormatIndex(index)+".snapshot.lz4"), nil
}

// WALDir returns the path to a generation's WAL directory
func (c *FileReplicaClient) WALDir(generation string) (string, error) {
	dir, err := c.GenerationDir(generation)
	if err != nil {
		return "", err
	}
	return filepath.Join(dir, "wal"), nil
}

// WALSegmentPath returns the path to a WAL segment file.
func (c *FileReplicaClient) WALSegmentPath(generation string, index int, offset int64) (string, error) {
	dir, err := c.WALDir(generation)
	if err != nil {
		return "", err
	}
	return filepath.Join(dir, FormatIndex(index), fmt.Sprintf("%s.wal.lz4", FormatOffset(offset))), nil
}

// Generations returns a list of available generation names.
func (c *FileReplicaClient) Generations(ctx context.Context) ([]string, error) {
	root, err := c.GenerationsDir()
	if err != nil {
		return nil, fmt.Errorf("cannot determine generations path: %w", err)
	}

	fis, err := os.ReadDir(root)
	if os.IsNotExist(err) {
		return nil, nil
	} else if err != nil {
		return nil, err
	}

	var generations []string
	for _, fi := range fis {
		if !IsGenerationName(fi.Name()) {
			continue
		} else if !fi.IsDir() {
			continue
		}
		generations = append(generations, fi.Name())
	}
	return generations, nil
}

// DeleteGeneration deletes all snapshots & WAL segments within a generation.
func (c *FileReplicaClient) DeleteGeneration(ctx context.Context, generation string) error {
	dir, err := c.GenerationDir(generation)
	if err != nil {
		return fmt.Errorf("cannot determine generation path: %w", err)
	}

	if err := os.RemoveAll(dir); err != nil && !os.IsNotExist(err) {
		return err
	}
	return nil
}

// Snapshots returns an iterator over all available snapshots for a generation.
func (c *FileReplicaClient) Snapshots(ctx context.Context, generation string) (SnapshotIterator, error) {
	dir, err := c.SnapshotsDir(generation)
	if err != nil {
		return nil, err
	}

	f, err := os.Open(dir)
	if os.IsNotExist(err) {
		return NewSnapshotInfoSliceIterator(nil), nil
	} else if err != nil {
		return nil, err
	}
	defer f.Close()

	fis, err := f.Readdir(-1)
	if err != nil {
		return nil, err
	}

	// Iterate over every file and convert to metadata.
	infos := make([]SnapshotInfo, 0, len(fis))
	for _, fi := range fis {
		// Parse index from filename.
		index, err := internal.ParseSnapshotPath(filepath.Base(fi.Name()))
		if err != nil {
			continue
		}

		infos = append(infos, SnapshotInfo{
			Generation: generation,
			Index:      index,
			Size:       fi.Size(),
			CreatedAt:  fi.ModTime().UTC(),
		})
	}

	sort.Sort(SnapshotInfoSlice(infos))

	return NewSnapshotInfoSliceIterator(infos), nil
}

// WriteSnapshot writes LZ4 compressed data from rd into a file on disk.
func (c *FileReplicaClient) WriteSnapshot(ctx context.Context, generation string, index int, rd io.Reader) (info SnapshotInfo, err error) {
	filename, err := c.SnapshotPath(generation, index)
	if err != nil {
		return info, err
	}

	// Ensure parent directory exists.
	if err := internal.MkdirAll(filepath.Dir(filename), c.DirMode, c.Uid, c.Gid); err != nil {
		return info, err
	}

	// Write snapshot to temporary file next to destination path.
	f, err := internal.CreateFile(filename+".tmp", c.FileMode, c.Uid, c.Gid)
	if err != nil {
		return info, err
	}
	defer f.Close()

	if _, err := io.Copy(f, rd); err != nil {
		return info, err
	} else if err := f.Sync(); err != nil {
		return info, err
	} else if err := f.Close(); err != nil {
		return info, err
	}

	// Build metadata.
	fi, err := os.Stat(filename + ".tmp")
	if err != nil {
		return info, err
	}
	info = SnapshotInfo{
		Generation: generation,
		Index:      index,
		Size:       fi.Size(),
		CreatedAt:  fi.ModTime().UTC(),
	}

	// Move snapshot to final path when it has been fully written & synced to disk.
	if err := os.Rename(filename+".tmp", filename); err != nil {
		return info, err
	}

	return info, nil
}

// SnapshotReader returns a reader for snapshot data at the given generation/index.
// Returns os.ErrNotExist if no matching index is found.
func (c *FileReplicaClient) SnapshotReader(ctx context.Context, generation string, index int) (io.ReadCloser, error) {
	filename, err := c.SnapshotPath(generation, index)
	if err != nil {
		return nil, err
	}
	return os.Open(filename)
}

// DeleteSnapshot deletes a snapshot with the given generation & index.
func (c *FileReplicaClient) DeleteSnapshot(ctx context.Context, generation string, index int) error {
	filename, err := c.SnapshotPath(generation, index)
	if err != nil {
		return fmt.Errorf("cannot determine snapshot path: %w", err)
	}
	if err := os.Remove(filename); err != nil && !os.IsNotExist(err) {
		return err
	}
	return nil
}

// WALSegments returns an iterator over all available WAL files for a generation.
func (c *FileReplicaClient) WALSegments(ctx context.Context, generation string) (WALSegmentIterator, error) {
	dir, err := c.WALDir(generation)
	if err != nil {
		return nil, err
	}

	f, err := os.Open(dir)
	if os.IsNotExist(err) {
		return NewWALSegmentInfoSliceIterator(nil), nil
	} else if err != nil {
		return nil, err
	}
	defer f.Close()

	fis, err := f.Readdir(-1)
	if err != nil {
		return nil, err
	}

	// Iterate over every file and convert to metadata.
	indexes := make([]int, 0, len(fis))
	for _, fi := range fis {
		index, err := ParseIndex(fi.Name())
		if err != nil || !fi.IsDir() {
			continue
		}
		indexes = append(indexes, index)
	}

	sort.Ints(indexes)

	return NewFileWALSegmentIterator(dir, generation, indexes), nil
}

// WriteWALSegment writes LZ4 compressed data from rd into a file on disk.
func (c *FileReplicaClient) WriteWALSegment(ctx context.Context, pos Pos, rd io.Reader) (info WALSegmentInfo, err error) {
	filename, err := c.WALSegmentPath(pos.Generation, pos.Index, pos.Offset)
	if err != nil {
		return info, err
	}

	// Ensure parent directory exists.
	if err := internal.MkdirAll(filepath.Dir(filename), c.DirMode, c.Uid, c.Gid); err != nil {
		return info, err
	}

	// Write WAL segment to temporary file next to destination path.
	f, err := internal.CreateFile(filename+".tmp", c.FileMode, c.Uid, c.Gid)
	if err != nil {
		return info, err
	}
	defer f.Close()

	if _, err := io.Copy(f, rd); err != nil {
		return info, err
	} else if err := f.Sync(); err != nil {
		return info, err
	} else if err := f.Close(); err != nil {
		return info, err
	}

	// Build metadata.
	fi, err := os.Stat(filename + ".tmp")
	if err != nil {
		return info, err
	}
	info = WALSegmentInfo{
		Generation: pos.Generation,
		Index:      pos.Index,
		Offset:     pos.Offset,
		Size:       fi.Size(),
		CreatedAt:  fi.ModTime().UTC(),
	}

	// Move WAL segment to final path when it has been written & synced to disk.
	if err := os.Rename(filename+".tmp", filename); err != nil {
		return info, err
	}

	return info, nil
}

// WALSegmentReader returns a reader for a section of WAL data at the given position.
// Returns os.ErrNotExist if no matching index/offset is found.
func (c *FileReplicaClient) WALSegmentReader(ctx context.Context, pos Pos) (io.ReadCloser, error) {
	filename, err := c.WALSegmentPath(pos.Generation, pos.Index, pos.Offset)
	if err != nil {
		return nil, err
	}
	return os.Open(filename)
}

// DeleteWALSegments deletes WAL segments at the given positions.
func (c *FileReplicaClient) DeleteWALSegments(ctx context.Context, a []Pos) error {
	for _, pos := range a {
		filename, err := c.WALSegmentPath(pos.Generation, pos.Index, pos.Offset)
		if err != nil {
			return err
		}
		if err := os.Remove(filename); err != nil && !os.IsNotExist(err) {
			return err
		}
	}
	return nil
}

type FileWALSegmentIterator struct {
	mu       sync.Mutex
	notifyCh chan struct{}

	dir        string
	generation string
	indexes    []int

	buffered bool
	infos    []WALSegmentInfo
	err      error
}

func NewFileWALSegmentIterator(dir, generation string, indexes []int) *FileWALSegmentIterator {
	return &FileWALSegmentIterator{
		dir:        dir,
		generation: generation,
		indexes:    indexes,

		notifyCh: make(chan struct{}, 1),
	}
}

func (itr *FileWALSegmentIterator) Close() (err error) {
	if e := itr.Err(); e != nil && err == nil {
		err = e
	}
	return err
}

func (itr *FileWALSegmentIterator) NotifyCh() <-chan struct{} {
	return itr.notifyCh
}

// Generation returns the generation this iterator was initialized with.
func (itr *FileWALSegmentIterator) Generation() string {
	return itr.generation
}

// Indexes returns the pending indexes. Only used for testing.
func (itr *FileWALSegmentIterator) Indexes() []int {
	itr.mu.Lock()
	defer itr.mu.Unlock()
	return itr.indexes
}

func (itr *FileWALSegmentIterator) Next() bool {
	itr.mu.Lock()
	defer itr.mu.Unlock()

	// Exit if an error has already occurred.
	if itr.err != nil {
		return false
	}

	// Read first info, if buffered.
	if itr.buffered {
		itr.buffered = false
		return true
	}

	for {
		// Move to the next segment in cache, if available.
		if len(itr.infos) > 1 {
			itr.infos = itr.infos[1:]
			return true
		}
		itr.infos = itr.infos[:0] // otherwise clear infos

		// If no indexes remain, stop iteration.
		if len(itr.indexes) == 0 {
			return false
		}

		// Read segments into a cache for the current index.
		index := itr.indexes[0]
		itr.indexes = itr.indexes[1:]
		f, err := os.Open(filepath.Join(itr.dir, FormatIndex(index)))
		if err != nil {
			itr.err = err
			return false
		}
		defer f.Close()

		fis, err := f.Readdir(-1)
		if err != nil {
			itr.err = err
			return false
		} else if err := f.Close(); err != nil {
			itr.err = err
			return false
		}

		for _, fi := range fis {
			filename := filepath.Base(fi.Name())
			if fi.IsDir() {
				continue
			}

			offset, err := ParseOffset(strings.TrimSuffix(filename, ".wal.lz4"))
			if err != nil {
				continue
			}

			itr.infos = append(itr.infos, WALSegmentInfo{
				Generation: itr.generation,
				Index:      index,
				Offset:     offset,
				Size:       fi.Size(),
				CreatedAt:  fi.ModTime().UTC(),
			})
		}

		// Ensure segments are sorted within index.
		sort.Sort(WALSegmentInfoSlice(itr.infos))

		if len(itr.infos) > 0 {
			return true
		}
	}
}

// SetErr sets the error on the iterator and notifies it of the change.
func (itr *FileWALSegmentIterator) SetErr(err error) {
	itr.mu.Lock()
	defer itr.mu.Unlock()
	if itr.err == nil {
		itr.err = err
	}

	select {
	case itr.notifyCh <- struct{}{}:
	default:
	}
}

// Err returns the first error that occurs on the iterator.
func (itr *FileWALSegmentIterator) Err() error {
	itr.mu.Lock()
	defer itr.mu.Unlock()
	return itr.err
}

func (itr *FileWALSegmentIterator) WALSegment() WALSegmentInfo {
	itr.mu.Lock()
	defer itr.mu.Unlock()

	if len(itr.infos) == 0 {
		return WALSegmentInfo{}
	}
	return itr.infos[0]
}

// Append add an additional WAL segment to the end of the iterator. This
// function expects that info will always be later than all previous infos
// that the iterator has or has seen.
func (itr *FileWALSegmentIterator) Append(info WALSegmentInfo) error {
	itr.mu.Lock()
	defer itr.mu.Unlock()

	if itr.err != nil {
		return itr.err
	} else if itr.generation != info.Generation {
		return fmt.Errorf("generation mismatch")
	}

	// If the info has an index that is still waiting to be read from disk into
	// the cache then simply append it to the end of the indices.
	//
	// If we have no pending indices, then append to the end of the infos. If
	// we don't have either then just append to the infos and avoid validation.
	if len(itr.indexes) > 0 {
		maxIndex := itr.indexes[len(itr.indexes)-1]

		if info.Index < maxIndex {
			return fmt.Errorf("appended index %q below max index %q", FormatIndex(info.Index), FormatIndex(maxIndex))
		} else if info.Index > maxIndex+1 {
			return fmt.Errorf("appended index %q skips index %q", FormatIndex(info.Index), FormatIndex(maxIndex+1))
		} else if info.Index == maxIndex+1 {
			itr.indexes = append(itr.indexes, info.Index)
		}
		// NOTE: no-op if segment index matches the current last index

	} else if len(itr.infos) > 0 {
		lastInfo := itr.infos[len(itr.infos)-1]
		if info.Index < lastInfo.Index {
			return fmt.Errorf("appended index %q below current index %q", FormatIndex(info.Index), FormatIndex(lastInfo.Index))
		} else if info.Index > lastInfo.Index+1 {
			return fmt.Errorf("appended index %q skips next index %q", FormatIndex(info.Index), FormatIndex(lastInfo.Index+1))
		} else if info.Index == lastInfo.Index+1 {
			itr.indexes = append(itr.indexes, info.Index)
		} else {
			// If the index matches the current infos, verify its offset and append.
			if info.Offset < lastInfo.Offset {
				return fmt.Errorf("appended offset %s/%s before last offset %s/%s", FormatIndex(info.Index), FormatOffset(info.Offset), FormatIndex(lastInfo.Index), FormatOffset(lastInfo.Offset))
			} else if info.Offset == lastInfo.Offset {
				return fmt.Errorf("duplicate offset %s/%s appended", FormatIndex(info.Index), FormatOffset(info.Offset))
			}
			itr.infos = append(itr.infos, info)
		}
	} else {
		itr.buffered = true
		itr.infos = append(itr.infos, info)
	}

	// Signal that a new segment is available.
	select {
	case itr.notifyCh <- struct{}{}:
	default:
	}

	return nil
}