state_machine.ov

module tcp_state_machine

// A simplified TCP-like connection state machine.
// Demonstrates: enum with bare variants, exhaustive match on tuple patterns.

enum ConnectionState {
    Closed,
    Listen,
    SynSent,
    SynReceived,
    Established,
    FinWait,
}

enum Event {
    ActiveOpen,
    PassiveOpen,
    SynAckReceived,
    AckReceived,
    Close,
}

@derive(Debug, Display)
enum TransitionError {
    InvalidTransition { from: ConnectionState, event: Event },
}

// Pure function — no effect marker.
// Exhaustive pattern matching: the compiler checks every (state, event)
// pair is handled, or that a wildcard arm covers the rest.
fn transition(state: ConnectionState, event: Event) -> Result<ConnectionState, TransitionError> {
    match (state, event) {
        (ConnectionState.Closed,      Event.ActiveOpen)     => Ok(ConnectionState.SynSent),
        (ConnectionState.Closed,      Event.PassiveOpen)    => Ok(ConnectionState.Listen),
        (ConnectionState.Listen,      Event.SynAckReceived) => Ok(ConnectionState.SynReceived),
        (ConnectionState.SynSent,     Event.SynAckReceived) => Ok(ConnectionState.Established),
        (ConnectionState.SynReceived, Event.AckReceived)    => Ok(ConnectionState.Established),
        (ConnectionState.Established, Event.Close)          => Ok(ConnectionState.FinWait),
        (ConnectionState.FinWait,     Event.AckReceived)    => Ok(ConnectionState.Closed),
        _                                                   => Err(TransitionError.InvalidTransition { from = state, event = event }),
    }
}