Fairlanes Tour

Appearance

Party Loop ​

Fairlanes uses boost_ext/sml as its FSM library.

One of the main state machines is PartyLoop.

Current transition table ​

cpp
return make_transition_table(
    *state<Idle> + sml::on_entry<_> / enter_idle,
    state<Farming> + sml::on_entry<_> / enter_farming,
    state<Farming> + sml::on_exit<_> / exit_farming,
    state<Fixing> + sml::on_entry<_> / enter_fixing,

    state<Idle> + event<NextEvent> = state<Farming>,
    state<Farming> + event<NextEvent>[needs_town] = state<Fixing>,
    state<Farming> + event<NextEvent>[in_combat] / combat_tick,
    state<Farming> + event<NextEvent> = state<CombatIdle>,
    state<CombatIdle> + event<NextEvent> = state<Farming>,
    state<Fixing> + event<NextEvent> = state<Idle>);

How to read this ​

SML transition lines generally look like this:

cpp
state<From> + event<Event>[guard] / action = state<To>

Not every part is required.

  • state<From>: the current state
  • event<Event>: the event being processed
  • [guard]: optional condition that must be true
  • / action: optional function to run
  • = state<To>: optional state transition target

So a line can mean:

  • enter a state and run setup code
  • handle an event and stay in the same state
  • handle an event and transition to another state
  • handle an event conditionally

English breakdown ​

Entry and exit hooks ​

cpp
*state<Idle> + sml::on_entry<_> / enter_idle,
  • Idle is the starting state
  • when the machine enters Idle, run enter_idle
cpp
state<Farming> + sml::on_entry<_> / enter_farming,
  • when the machine enters Farming, run enter_farming
cpp
state<Farming> + sml::on_exit<_> / exit_farming,
  • when the machine leaves Farming, run exit_farming
cpp
state<Fixing> + sml::on_entry<_> / enter_fixing,
  • when the machine enters Fixing, run enter_fixing

High-level flow ​

Party Loop ​

In English, the loop is roughly:

  • start in Idle
  • next tick moves to Farming
  • while farming:
    • if town work is needed, go to Fixing
    • if combat is happening, process a combat tick
    • otherwise move into CombatIdle
  • CombatIdle moves back to Farming
  • Fixing moves back to Idle

What to notice ​

  • NextEvent appears to act like the machine's general “advance the loop” event
  • not every event handler causes a state transition
  • guards matter a lot, especially in Farming
  • the ordering of transitions is important for understanding intent
  • combat_tick is a behavior triggered inside the FSM, not a separate polling loop

Why this matters ​

This state machine is one of the places where Fairlanes makes game flow explicit.

Instead of scattering “if farming, maybe combat, maybe town” logic across the codebase, the allowed flow is written down as a transition table.

That gives us:

  • readable control flow
  • constrained state progression
  • a single place to reason about the party loop

Fairlanes take ​

We use FSMs when the flow itself is important enough to deserve first-class structure.

PartyLoop is not just a pile of conditionals. It is a declared model of how the party advances.

Farming is the most interesting state here because it can either redirect the loop, process combat in place, or fall through to the next state depending on the current conditions.