# Terminal Payment State Machine

## States
- IDLE
- SESSION_OPEN
- PAYMENT_REQUESTED
- WAITING_FOR_TERMINAL
- APPROVED
- DECLINED
- CANCELLED
- REVERSAL_PENDING
- REVERSED
- ERROR
- DESYNC

## Events
- open_session
- close_session
- send_payment
- terminal_ack
- terminal_approve
- terminal_decline
- terminal_timeout
- backend_timeout
- reconnect
- cancel_request
- reversal_request
- reversal_success
- reversal_fail
- duplicate_response
- invalid_command

## Transitions
| From | Event | To |
|------|-------|----|
| IDLE | open_session | SESSION_OPEN |
| SESSION_OPEN | send_payment | PAYMENT_REQUESTED |
| PAYMENT_REQUESTED | terminal_ack | WAITING_FOR_TERMINAL |
| WAITING_FOR_TERMINAL | terminal_approve | APPROVED |
| WAITING_FOR_TERMINAL | terminal_decline | DECLINED |
| WAITING_FOR_TERMINAL | terminal_timeout | DESYNC |
| WAITING_FOR_TERMINAL | cancel_request | CANCELLED |
| APPROVED | reversal_request | REVERSAL_PENDING |
| REVERSAL_PENDING | reversal_success | REVERSED |
| REVERSAL_PENDING | reversal_fail | ERROR |
| * | invalid_command | ERROR |
| * | backend_timeout | DESYNC |