package state

import (
	"errors"
	"fmt"

	"github.com/tendermint/tendermint/version"
)

// Rollback overwrites the current Tendermint state (height n) with the most
// recent previous state (height n - 1).
// Note that this function does not affect application state.
func Rollback(bs BlockStore, ss Store) (int64, []byte, error) {
	invalidState, err := ss.Load()
	if err != nil {
		return -1, nil, err
	}
	if invalidState.IsEmpty() {
		return -1, nil, errors.New("no state found")
	}

	height := bs.Height()

	// NOTE: persistence of state and blocks don't happen atomically. Therefore it is possible that
	// when the user stopped the node the state wasn't updated but the blockstore was. In this situation
	// we don't need to rollback any state and can just return early
	if height == invalidState.LastBlockHeight+1 {
		return invalidState.LastBlockHeight, invalidState.AppHash, nil
	}

	// If the state store isn't one below nor equal to the blockstore height than this violates the
	// invariant
	if height != invalidState.LastBlockHeight {
		return -1, nil, fmt.Errorf("statestore height (%d) is not one below or equal to blockstore height (%d)",
			invalidState.LastBlockHeight, height)
	}

	// state store height is equal to blockstore height. We're good to proceed with rolling back state
	rollbackHeight := invalidState.LastBlockHeight - 1
	rollbackBlock := bs.LoadBlockMeta(rollbackHeight)
	if rollbackBlock == nil {
		return -1, nil, fmt.Errorf("block at height %d not found", rollbackHeight)
	}
	// we also need to retrieve the latest block because the app hash and last results hash is only agreed upon in the following block
	latestBlock := bs.LoadBlockMeta(invalidState.LastBlockHeight)
	if latestBlock == nil {
		return -1, nil, fmt.Errorf("block at height %d not found", invalidState.LastBlockHeight)
	}

	previousLastValidatorSet, err := ss.LoadValidators(rollbackHeight)
	if err != nil {
		return -1, nil, err
	}

	previousParams, err := ss.LoadConsensusParams(rollbackHeight + 1)
	if err != nil {
		return -1, nil, err
	}

	valChangeHeight := invalidState.LastHeightValidatorsChanged
	// this can only happen if the validator set changed since the last block
	if valChangeHeight > rollbackHeight {
		valChangeHeight = rollbackHeight + 1
	}

	paramsChangeHeight := invalidState.LastHeightConsensusParamsChanged
	// this can only happen if params changed from the last block
	if paramsChangeHeight > rollbackHeight {
		paramsChangeHeight = rollbackHeight + 1
	}

	// build the new state from the old state and the prior block
	rolledBackState := State{
		Version: Version{
			Consensus: version.Consensus{
				Block: version.BlockProtocol,
				App:   previousParams.Version.AppVersion,
			},
			Software: version.TMVersion,
		},
		// immutable fields
		ChainID:       invalidState.ChainID,
		InitialHeight: invalidState.InitialHeight,

		LastBlockHeight: rollbackBlock.Header.Height,
		LastBlockID:     rollbackBlock.BlockID,
		LastBlockTime:   rollbackBlock.Header.Time,

		NextValidators:              invalidState.Validators,
		Validators:                  invalidState.LastValidators,
		LastValidators:              previousLastValidatorSet,
		LastHeightValidatorsChanged: valChangeHeight,

		ConsensusParams:                  previousParams,
		LastHeightConsensusParamsChanged: paramsChangeHeight,

		LastResultsHash: latestBlock.Header.LastResultsHash,
		AppHash:         latestBlock.Header.AppHash,
	}

	// persist the new state. This overrides the invalid one. NOTE: this will also
	// persist the validator set and consensus params over the existing structures,
	// but both should be the same
	if err := ss.Save(rolledBackState); err != nil {
		return -1, nil, fmt.Errorf("failed to save rolled back state: %w", err)
	}

	return rolledBackState.LastBlockHeight, rolledBackState.AppHash, nil
}