package swoosh

import "fmt"

// Replay deterministically replays WAL records on top of a snapshot and returns the resulting state.
func Replay(wal WALStore, snapshot Snapshot) (OrchestratorState, error) {
	base, err := cloneState(snapshot.State)
	if err != nil {
		return OrchestratorState{}, fmt.Errorf("clone snapshot state: %w", err)
	}

	if base.StateHash == "" {
		hash, err := computeStateHash(base)
		if err != nil {
			return OrchestratorState{}, fmt.Errorf("compute snapshot hash: %w", err)
		}
		base.StateHash = hash
	}

	start := snapshot.LastAppliedIndex + 1
	records, err := wal.Replay(start)
	if err != nil {
		return OrchestratorState{}, fmt.Errorf("replay wal: %w", err)
	}

	state := base
	for _, record := range records {
		if state.StateHash != record.StatePreHash {
			return OrchestratorState{}, fmt.Errorf("wal pre-hash mismatch at index %d", record.Index)
		}

		next, err := Reduce(state, record.Transition, record.Guard)
		if err != nil {
			return OrchestratorState{}, fmt.Errorf("reduce wal record %d: %w", record.Index, err)
		}

		if next.StateHash != record.StatePostHash {
			return OrchestratorState{}, fmt.Errorf("wal post-hash mismatch at index %d", record.Index)
		}

		state = next
	}

	return state, nil
}