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// Package fsm defines a finite state machine
package fsm
import "fmt"
// State represents a single node in the graph.
type State struct {
// Transitions indicates which out arrows this node has
Transitions []Transition
}
type (
// StateID represents the Identifier of the state.
StateID int8
// StateIDSlice represents the list of state identifiers.
StateIDSlice []StateID
// States is the map from state identifier to the state itself
States map[StateID]State
)
// Len is defined here such that we can sort the slice
func (v StateIDSlice) Len() int {
return len(v)
}
// Less is defined here such that we can sort the slice
func (v StateIDSlice) Less(i, j int) bool {
return v[i] < v[j]
}
// Swap is defined here such that we can sort the slice
func (v StateIDSlice) Swap(i, j int) {
v[i], v[j] = v[j], v[i]
}
// Transition indicates an arrow in the state graph.
type Transition struct {
// To represents the to-be-new state
To StateID
// Description is what type of message the arrow gets in the graph
Description string
}
// FSM represents the total graph.
type FSM struct {
// States is the map from state ID to states
States States
// Current is the current state represented by the identifier
Current StateID
// Name represents the descriptive name of this state machine
Name string
// StateCallback is the function ran when a transition occurs
// It takes the old state, the new state and the data and returns if this is handled by the client
StateCallback func(StateID, StateID, any) bool
// GetStateName gets the name of a state as a string
GetStateName func(StateID) string
// initial is the initial state that we can always go back to
initial StateID
}
// NewFSM creates a new finite state machine
func NewFSM(current StateID, states States, callback func(StateID, StateID, any) bool, nameGen func(StateID) string) FSM {
return FSM{
States: states,
Current: current,
StateCallback: callback,
GetStateName: nameGen,
initial: current,
}
}
// InState returns whether or not the state machine is in the given 'check' state.
func (fsm *FSM) InState(check StateID) bool {
return check == fsm.Current
}
// CheckTransition returns an error whether or not a transition to
// state `desired` is possible
func (fsm *FSM) CheckTransition(desired StateID) error {
// initial or begin state is fine
// 0 = deregistered
if desired == fsm.initial || desired == 0 {
return nil
}
for _, ts := range fsm.States[fsm.Current].Transitions {
if ts.To == desired {
return nil
}
}
return fmt.Errorf("fsm invalid transition attempt from '%s' to '%s'", fsm.GetStateName(fsm.Current), fsm.GetStateName(desired))
}
// GoTransitionRequired transitions the state machine to a new state with associated state data 'data'
// If this transition is not handled by the client, it returns an error.
func (fsm *FSM) GoTransitionRequired(newState StateID, data any) error {
oldState := fsm.Current
handled, err := fsm.GoTransitionWithData(newState, data)
// transition ios not possible
if err != nil {
return err
}
// transition is not handled
if !handled {
return fmt.Errorf("fsm failed transition from '%s' to '%s', is this required transition handled?", fsm.GetStateName(oldState), fsm.GetStateName(newState))
}
return nil
}
// GoTransitionWithData is a helper that transitions the state machine toward the 'newState' with associated state data 'data'
// It returns whether or not the transition is handled by the client.
func (fsm *FSM) GoTransitionWithData(newState StateID, data any) (bool, error) {
if err := fsm.CheckTransition(newState); err != nil {
return false, err
}
prev := fsm.Current
fsm.Current = newState
return fsm.StateCallback(prev, newState, data), nil
}
// GoTransition is an alias to call GoTransitionWithData but have an empty string as data.
func (fsm *FSM) GoTransition(newState StateID) (bool, error) {
// No data means the callback is never required
return fsm.GoTransitionWithData(newState, "")
}
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