package transaction

import "fmt"

// State 是服务端更新事务的持久化状态。
type State string

const (
	StateCreated     State = "CREATED"
	StateValidating  State = "VALIDATING"
	StatePrepared    State = "PREPARED"
	StateStarting    State = "STARTING"
	StateSwitching   State = "SWITCHING"
	StateVerifying   State = "VERIFYING"
	StateDraining    State = "DRAINING"
	StateCommitted   State = "COMMITTED"
	StateRollingBack State = "ROLLING_BACK"
	StateRolledBack  State = "ROLLED_BACK"
	StateFailed      State = "FAILED"
)

var forwardTransitions = map[State]State{
	StateCreated:    StateValidating,
	StateValidating: StatePrepared,
	StatePrepared:   StateStarting,
	StateStarting:   StateSwitching,
	StateSwitching:  StateVerifying,
	StateVerifying:  StateDraining,
	StateDraining:   StateCommitted,
}

// Valid 报告状态是否属于当前状态机协议。
func (s State) Valid() bool {
	switch s {
	case StateCreated,
		StateValidating,
		StatePrepared,
		StateStarting,
		StateSwitching,
		StateVerifying,
		StateDraining,
		StateCommitted,
		StateRollingBack,
		StateRolledBack,
		StateFailed:
		return true
	default:
		return false
	}
}

// Terminal 报告事务是否已经不可再推进。
func (s State) Terminal() bool {
	return s == StateCommitted || s == StateRolledBack || s == StateFailed
}

// CanTransitionTo 校验正常推进、回滚和不可恢复失败三类转换。
func (s State) CanTransitionTo(next State) bool {
	if !s.Valid() || !next.Valid() || s.Terminal() {
		return false
	}
	if s == StateRollingBack {
		return next == StateRolledBack || next == StateFailed
	}
	if next == StateRollingBack || next == StateFailed {
		return true
	}
	return forwardTransitions[s] == next
}

// TransitionError 表示状态机拒绝了一次转换。
type TransitionError struct {
	From State
	To   State
}

func (e *TransitionError) Error() string {
	return fmt.Sprintf("transaction state transition is not allowed: %s -> %s", e.From, e.To)
}