package iter

// Iterators - composable high-level abstraction to iterate over. It's more high-level than kv.Cursor and provides less controll, less features, but enough to build an app.
//
//	for s.HasNext() {
//		k, v, err := s.Next()
//		if err != nil {
//			return err
//		}
//	}
//
//   - K, V are valid only until next .Next() call (TODO: extend it to whole Tx lifetime?)
//   - No `Close` method: all streams produced by TemporalTx will be closed inside `tx.Rollback()` (by casting to `kv.Closer`)
//   - automatically checks cancelation of `ctx` passed to `db.Begin(ctx)`, can skip this
//     check in loops on stream. Dual has very limited API - user has no way to
//     terminate it - but user can specify more strict conditions when creating stream (then server knows better when to stop)

// Dual - return 2 items - usually called Key and Value (or `k` and `v`)
// Example:
//
//	for s.HasNext() {
//		k, v, err := s.Next()
//		if err != nil {
//			return err
//		}
//	}
type Dual[K, V any] interface {
	Next() (K, V, error)
	HasNext() bool
}

// Unary - return 1 item. Example:
//
//	for s.HasNext() {
//		v, err := s.Next()
//		if err != nil {
//			return err
//		}
//	}
type Unary[V any] interface {
	Next() (V, error)
	//NextBatch() ([]V, error)
	HasNext() bool
}

// KV - return 2 items of type []byte - usually called Key and Value (or `k` and `v`). Example:
//
//	for s.HasNext() {
//		k, v, err := s.Next()
//		if err != nil {
//			return err
//		}
//	}

// often used shortcuts
type (
	U64 Unary[uint64]
	KV  Dual[[]byte, []byte]
)

func ToU64Arr(s U64) ([]uint64, error)           { return ToArr[uint64](s) }
func ToKVArray(s KV) ([][]byte, [][]byte, error) { return ToDualArray[[]byte, []byte](s) }

func TransformKV(it KV, transform func(k, v []byte) ([]byte, []byte)) *TransformDualIter[[]byte, []byte] {
	return TransformDual[[]byte, []byte](it, transform)
}

// internal types
type (
	NextPageUnary[T any]   func(pageToken string) (arr []T, nextPageToken string, err error)
	NextPageDual[K, V any] func(pageToken string) (keys []K, values []V, nextPageToken string, err error)
)

func PaginateKV(f NextPageDual[[]byte, []byte]) *PaginatedDual[[]byte, []byte] {
	return PaginateDual[[]byte, []byte](f)
}
func PaginateU64(f NextPageUnary[uint64]) *Paginated[uint64] {
	return Paginate[uint64](f)
}