package attesto

import (
	"encoding/json"
	"fmt"
	"os"
	"path/filepath"
	"sync"
)

// HeadStore persists the last accepted (seqNo, eventHash) head per stream so the
// client can detect a rewound or divergent server history.
type HeadStore interface {
	Get(streamID string) (seqNo int64, eventHash string, ok bool)
	Set(streamID string, seqNo int64, eventHash string)
}

// ForkDetectedError reports that a receipt did not extend the last accepted head
// for its stream. The store is NOT advanced when this is returned.
type ForkDetectedError struct {
	StreamID     string
	ExpectedSeq  int64
	ExpectedHash string
	GotSeq       int64
	GotPrevHash  string
}

func (e *ForkDetectedError) Error() string {
	return fmt.Sprintf(
		"fork detected on stream %s: receipt (seq=%d prev=%s) does not extend "+
			"last accepted head (seq=%d hash=%s)",
		e.StreamID, e.GotSeq, e.GotPrevHash, e.ExpectedSeq, e.ExpectedHash)
}

// MemoryHeadStore keeps heads in process memory; safe for concurrent use.
type MemoryHeadStore struct {
	mu    sync.Mutex
	heads map[string][2]any
}

func NewMemoryHeadStore() *MemoryHeadStore {
	return &MemoryHeadStore{heads: map[string][2]any{}}
}

func (s *MemoryHeadStore) Get(streamID string) (int64, string, bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	entry, ok := s.heads[streamID]
	if !ok {
		return 0, "", false
	}
	return entry[0].(int64), entry[1].(string), true
}

func (s *MemoryHeadStore) Set(streamID string, seqNo int64, eventHash string) {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.heads[streamID] = [2]any{seqNo, eventHash}
}

// FileHeadStore persists heads to a JSON file (default ~/.attesto/heads.json),
// giving fork detection across separate process invocations. Writes are atomic.
type FileHeadStore struct {
	path string
	mu   sync.Mutex
}

// NewFileHeadStore uses the given path, or ~/.attesto/heads.json when empty.
func NewFileHeadStore(path string) *FileHeadStore {
	if path == "" {
		if home, err := os.UserHomeDir(); err == nil {
			path = filepath.Join(home, ".attesto", "heads.json")
		} else {
			path = ".attesto-heads.json"
		}
	}
	return &FileHeadStore{path: path}
}

func (s *FileHeadStore) load() map[string][2]json.RawMessage {
	raw, err := os.ReadFile(s.path)
	if err != nil {
		return map[string][2]json.RawMessage{}
	}
	out := map[string][2]json.RawMessage{}
	if err := json.Unmarshal(raw, &out); err != nil {
		return map[string][2]json.RawMessage{}
	}
	return out
}

func (s *FileHeadStore) Get(streamID string) (int64, string, bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	entry, ok := s.load()[streamID]
	if !ok {
		return 0, "", false
	}
	var seqNo int64
	var eventHash string
	if json.Unmarshal(entry[0], &seqNo) != nil || json.Unmarshal(entry[1], &eventHash) != nil {
		return 0, "", false
	}
	return seqNo, eventHash, true
}

func (s *FileHeadStore) Set(streamID string, seqNo int64, eventHash string) {
	s.mu.Lock()
	defer s.mu.Unlock()
	heads := map[string][2]any{}
	for key, entry := range s.load() {
		var n int64
		var h string
		_ = json.Unmarshal(entry[0], &n)
		_ = json.Unmarshal(entry[1], &h)
		heads[key] = [2]any{n, h}
	}
	heads[streamID] = [2]any{seqNo, eventHash}
	body, err := json.Marshal(heads)
	if err != nil {
		return
	}
	if err := os.MkdirAll(filepath.Dir(s.path), 0o700); err != nil {
		return
	}
	tmp, err := os.CreateTemp(filepath.Dir(s.path), ".heads-")
	if err != nil {
		return
	}
	tmpName := tmp.Name()
	_, writeErr := tmp.Write(body)
	closeErr := tmp.Close()
	if writeErr != nil || closeErr != nil {
		_ = os.Remove(tmpName)
		return
	}
	_ = os.Chmod(tmpName, 0o600)
	_ = os.Rename(tmpName, s.path)
}

// checkAndAdvanceHead verifies a receipt extends the stored head, then advances
// the store. Returns *ForkDetectedError (without advancing) when the receipt
// rewinds or collides on seqNo, or claims to be the immediate next event but
// does not chain. A forward gap is accepted and advances.
func checkAndAdvanceHead(store HeadStore, receipt EventReceipt) error {
	if storedSeq, storedHash, ok := store.Get(receipt.StreamID); ok {
		if receipt.SeqNo <= storedSeq ||
			(receipt.SeqNo == storedSeq+1 && receipt.PrevEventHash != storedHash) {
			return &ForkDetectedError{
				StreamID:     receipt.StreamID,
				ExpectedSeq:  storedSeq,
				ExpectedHash: storedHash,
				GotSeq:       receipt.SeqNo,
				GotPrevHash:  receipt.PrevEventHash,
			}
		}
	}
	store.Set(receipt.StreamID, receipt.SeqNo, receipt.EventHash)
	return nil
}