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gstatemachine.h
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1//
2// Copyright (C) 2001-2021 Graeme Walker <graeme_walker@users.sourceforge.net>
3//
4// This program is free software: you can redistribute it and/or modify
5// it under the terms of the GNU General Public License as published by
6// the Free Software Foundation, either version 3 of the License, or
7// (at your option) any later version.
8//
9// This program is distributed in the hope that it will be useful,
10// but WITHOUT ANY WARRANTY; without even the implied warranty of
11// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12// GNU General Public License for more details.
13//
14// You should have received a copy of the GNU General Public License
15// along with this program. If not, see <http://www.gnu.org/licenses/>.
16// ===
17///
18/// \file gstatemachine.h
19///
20
21#ifndef G_STATE_MACHINE_H
22#define G_STATE_MACHINE_H
23
24#include "gdef.h"
25#include "gexception.h"
26#include <map>
27
28namespace G
29{
30
31G_EXCEPTION_CLASS( StateMachine_Error , "invalid state machine transition" ) ;
32
33//| \class StateMachine
34/// A finite state machine class template.
35///
36/// The finite state machine has a persistant 'state'. When an 'event' is
37/// apply()d to the state machine, it undergoes a state 'transition'
38/// and then calls the associated 'action' method.
39///
40/// Any action method can return a boolean predicate value which is used to
41/// select between two transitions -- the 'normal' transition if the predicate
42/// is true, and an 'alternative' transition if false.
43///
44/// Transition states can be implemented by having the relevant action
45/// method call apply() on the state-machine. The state machine's state
46/// is always changed before any action method is called -- although only
47/// using the 'normal' transition, not the 'alternative' -- so this sort
48/// of reentrancy is valid, as long as the action method going into
49/// the transition state returns a predicate value of 'true'.
50///
51/// Default transitions for a given state are not supported directly. But
52/// note that protocol errors do not invalidate the state machine and
53/// do not result in a change of state. This means that client code can
54/// achieve the effect of default transitions by handling protocol errors
55/// for that state in a special manner.
56///
57/// Special states 'same' and 'any' can be defined to simplify the
58/// definition of state transitions. A transition with a 'source'
59/// state of 'any' will match any state. This is typically used
60/// for error events or timeouts. A transition with a 'destination'
61/// state of 'same' will not result in a state change. This is
62/// sometimes used when handling predicates -- the predicate can
63/// be used to control whether the state changes, or stays the
64/// same. The 'any' state is also used as a return value from
65/// apply() to signal a protocol error.
66///
67/// If the 'any' state is numerically the largest then it can be used
68/// to identify a default transition for the given event; transitions
69/// identified by an exact match with the current state will be
70/// chosen in preference to the 'any' transition.
71///
72/// The 'end' state is special in that predicates are ignored for
73/// transitions which have 'end' as their 'normal' destintation
74/// state. This is because of a special implementation feature
75/// which allows the state machine object to be deleted within the
76/// action method which causes a transition to the 'end' state.
77/// (This feature also means that transitions with an 'alternative'
78/// state of 'end' are not valid.)
79///
80/// Usage:
81/// \code
82/// class Protocol
83/// {
84/// struct ProtocolError {} ;
85/// enum class State { s_Same , sFoo , sBar , sEnd , s_Any } ;
86/// enum class Event { eFoo , eBar , eError } ;
87/// typedef StateMachine<Protocol,State,Event> Fsm ;
88/// Fsm m_fsm ;
89/// void doFoo( const std::string & , bool & ) {}
90/// void doBar( const std::string & , bool & ) { delete this ; }
91/// Event decode( const std::string & ) const ;
92/// public:
93/// Protocol() : m_fsm(State::sFoo,State::sBar,State::s_Same,State::s_Any)
94/// {
95/// m_fsm(Event::eFoo,State::sFoo,sBar,&Protocol::doFoo) ;
96/// m_fsm(Event::eBar,State::sBar,sEnd,&Protocol::doBar) ;
97/// }
98/// void apply( const std::string & event_string )
99/// {
100/// State s = m_fsm.apply( *this , decode(event_string) , event_string ) ;
101/// if( s == State::sEnd ) return ; // this already deleted by doBar()
102/// if( s == State::sAny ) throw ProtocolError() ;
103/// }
104/// } ;
105/// \endcode
106///
107template <typename T, typename State, typename Event, typename Arg>
109{
110public:
111 using Action = void (T::*)(Arg, bool &) ;
112 using Error = StateMachine_Error ;
113
114 StateMachine( State s_start , State s_end , State s_same , State s_any ) ;
115 ///< Constructor.
116
117 void operator()( Event event , State from , State to , Action action ) ;
118 ///< Adds a transition. Special semantics apply if 'from' is
119 ///< 's_any', or if 'to' is 's_same'.
120
121 void operator()( Event event , State from , State to , Action action , State alt ) ;
122 ///< An overload which adds a transition with predicate support.
123 ///< The 'alt' state is taken as an alternative 'to' state
124 ///< if the action's predicate is returned as false.
125
126 State apply( T & t , Event event , Arg arg ) ;
127 ///< Applies an event. Calls the appropriate action method
128 ///< on object "t" and changes state. The state change
129 ///< takes into account the predicate returned by the
130 ///< action method.
131 ///<
132 ///< If the event is valid then the new state is returned.
133 ///< If the event results in a protocol error the StateMachine's
134 ///< state is unchanged, no action method is called, and
135 ///< this method returns 's_any' (see ctor).
136 ///<
137 ///< As a special implementation feature the StateMachine
138 ///< object may be deleted during the last action method
139 ///< callback (ie. the one which takes the state to the
140 ///< 's_end' state).
141
142 State state() const ;
143 ///< Returns the current state.
144
145 State reset( State new_state ) ;
146 ///< Sets the current state. Returns the old state.
147
148private:
149 struct Transition /// A private structure used by G::StateMachine<>.
150 {
151 State from ;
152 State to ;
153 State alt ; // alternate "to" state if predicate false
154 Action action ;
155 Transition(State s1,State s2,Action a,State s3) :
156 from(s1) , to(s2) , alt(s3) , action(a) {}
157 } ;
158 using Map = std::multimap<Event,Transition> ;
159 using Map_value_type = typename Map::value_type ;
160 Map m_map ;
161 State m_state ;
162 State m_end ;
163 State m_same ;
164 State m_any ;
165} ;
166
167template <typename T, typename State, typename Event, typename Arg>
168StateMachine<T,State,Event,Arg>::StateMachine( State s_start , State s_end , State s_same , State s_any ) :
169 m_state(s_start) ,
170 m_end(s_end) ,
171 m_same(s_same) ,
172 m_any(s_any)
173{
174}
175
176template <typename T, typename State, typename Event, typename Arg>
177void StateMachine<T,State,Event,Arg>::operator()( Event event , State from , State to , Action action )
178{
179 operator()( event , from , to , action , to ) ;
180}
181
182template <typename T, typename State, typename Event, typename Arg>
183void StateMachine<T,State,Event,Arg>::operator()( Event event , State from , State to , Action action , State alt )
184{
185 if( to == m_any || alt == m_any )
186 throw Error( "\"to any\" is invalid" ) ;
187
188 if( from == m_same )
189 throw Error( "\"from same\" is invalid" ) ;
190
191 if( to == m_end && alt != to )
192 throw Error( "predicates on end-state transitions are invalid" ) ;
193
194 if( alt == m_end && to != m_end )
195 throw Error( "false predicates cannot take you to the end state" ) ;
196
197 m_map.insert( Map_value_type( event , Transition(from,to,action,alt) ) ) ;
198}
199
200template <typename T, typename State, typename Event, typename Arg>
202{
203 State old_state = m_state ;
204 m_state = new_state ;
205 return old_state ;
206}
207
208template <typename T, typename State, typename Event, typename Arg>
210{
211 return m_state ;
212}
213
214template <typename T, typename State, typename Event, typename Arg>
215State StateMachine<T,State,Event,Arg>::apply( T & t , Event event , Arg arg )
216{
217 State state = m_state ;
218 auto p = m_map.find( event ) ; // look up in the multimap keyed on event + current-state
219 for( ; p != m_map.end() && (*p).first == event ; ++p )
220 {
221 if( (*p).second.from == m_any || (*p).second.from == m_state )
222 {
223 State old_state = m_state ; // change state
224 if( (*p).second.to != m_same )
225 state = m_state = (*p).second.to ;
226
227 State end = m_end ; // (avoid using members after the action method call)
228
229 bool predicate = true ;
230 (t.*((*p).second.action))( arg , predicate ) ; // perform action
231
232 if( state != end && !predicate ) // respond to predicate
233 {
234 State alt_state = (*p).second.alt ;
235 state = m_state = alt_state == m_same ? old_state : alt_state ;
236 }
237 return state ;
238 }
239 }
240 return m_any ;
241}
242
243}
244
245#endif
246
A class which holds a represention of the argc/argv command line array, and supports simple command-l...
Definition: garg.h:44
A finite state machine class template.
void operator()(Event event, State from, State to, Action action)
Adds a transition.
State reset(State new_state)
Sets the current state. Returns the old state.
StateMachine(State s_start, State s_end, State s_same, State s_any)
Constructor.
void operator()(Event event, State from, State to, Action action, State alt)
An overload which adds a transition with predicate support.
State apply(T &t, Event event, Arg arg)
Applies an event.
State state() const
Returns the current state.
Low-level classes.
Definition: galign.h:28