finite automata example (#1364)

algorithms/src/main/java/com/baeldung/automata/FiniteStateMachine.java

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+package com.baeldung.automata;
+
+/**
+ * Finite state machine.
+ */
+public interface FiniteStateMachine {
+
+	/**
+	 * Follow a transition, switch the state of the machine.
+	 * @param c Char.
+	 * @return A new finite state machine with the new state.
+	 */
+	FiniteStateMachine switchState(final CharSequence c);
+
+    /**
+     * Is the current state a final one?
+     * @return true or false.
+     */
+    boolean canStop();
+}

algorithms/src/main/java/com/baeldung/automata/RtFiniteStateMachine.java

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+package com.baeldung.automata;
+
+/**
+ * Default implementation of a finite state machine.
+ * This class is immutable and thread-safe.
+ */
+public final class RtFiniteStateMachine implements FiniteStateMachine {
+
+	/**
+	 * Current state.
+	 */
+	private State current;
+
+	/**
+	 * Ctor.
+	 * @param initial Initial state of this machine.
+	 */
+	public RtFiniteStateMachine(final State initial) {
+		this.current = initial;
+	}
+
+	public FiniteStateMachine switchState(final CharSequence c) {
+		return new RtFiniteStateMachine(this.current.transit(c));
+	}
+
+	public boolean canStop() {
+		return this.current.isFinal();
+	}
+
+}

algorithms/src/main/java/com/baeldung/automata/RtState.java

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+package com.baeldung.automata;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * State in a finite state machine.
+ */
+public final class RtState implements State {
+
+    private List<Transition> transitions;
+    private boolean isFinal;
+
+    public RtState() {
+        this(false);
+    }
+    
+    public RtState(final boolean isFinal) {
+        this.transitions = new ArrayList<>();
+        this.isFinal = isFinal;
+    }
+
+    public State transit(final CharSequence c) {
+        for(final Transition t : this.transitions) {
+            if(t.isPossible(c)) {
+                return t.state();
+            }
+        }
+        throw new IllegalArgumentException("Input not accepted: " + c);
+    }
+
+    public boolean isFinal() {
+        return this.isFinal;
+    }
+
+	@Override
+	public State with(Transition tr) {
+		this.transitions.add(tr);
+		return this;
+	}
+
+}

algorithms/src/main/java/com/baeldung/automata/RtTransition.java

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+package com.baeldung.automata;
+
+
+/**
+ * Transition in finite state machine.
+ */
+public final class RtTransition implements Transition {
+
+	private String rule;
+	private State next;
+	
+	/**
+	 * Ctor.
+	 * @param rule Rule that a character has to meet 
+	 *  in order to get to the next state.  
+	 * @param next Next state.
+	 */
+	public RtTransition (String rule, State next) {
+		this.rule = rule;
+		this.next = next;
+	}
+	
+	public State state() {
+		return this.next;
+	}
+
+	public boolean isPossible(CharSequence c) {
+		return this.rule.equalsIgnoreCase(String.valueOf(c));
+	}
+
+}

algorithms/src/main/java/com/baeldung/automata/State.java

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+package com.baeldung.automata;
+
+/**
+ * State. Part of a finite state machine.
+ */
+public interface State {
+
+	/**
+	 * Add a Transition to this state.
+	 * @param tr Given transition.
+	 * @return Modified State.
+	 */
+	State with(final Transition tr);
+
+    /**
+     * Follow one of the transitions, to get 
+     * to the next state.
+     * @param c Character.
+     * @return State.
+     * @throws IllegalStateException if the char is not accepted.
+     */
+    State transit(final CharSequence c);
+    
+    /**
+     * Can the automaton stop on this state?
+     * @return true or false
+     */
+    boolean isFinal();
+}

algorithms/src/main/java/com/baeldung/automata/Transition.java

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+package com.baeldung.automata;
+
+/**
+ * Transition in a finite State machine.
+ */
+public interface Transition {
+
+	/**
+	 * Is the transition possible with the given character?
+	 * @param c char.
+	 * @return true or false.
+	 */
+	boolean isPossible(final CharSequence c);
+
+	/**
+	 * The state to which this transition leads.
+	 * @return State.
+	 */
+    State state();
+}