116 lines
3.6 KiB
Java
116 lines
3.6 KiB
Java
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import edu.princeton.cs.introcs.StdOut;
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/*************************************************************************
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* Compilation: javac NFA.java
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* Execution: java NFA regexp text
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* Dependencies: Stack.java Bag.java Digraph.java DirectedDFS.java
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*
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* % java NFA "(A*B|AC)D" AAAABD
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* true
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*
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* % java NFA "(A*B|AC)D" AAAAC
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* false
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*
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* % java NFA "(a|(bc)*d)*" abcbcd
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* true
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*
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* % java NFA "(a|(bc)*d)*" abcbcbcdaaaabcbcdaaaddd
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* true
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*
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* Remarks
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* -----------
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* - This version does not suport the + operator or multiway-or.
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*
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* - This version does not handle character classes,
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* metacharacters (either in the text or pattern), capturing
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* capabilities, greedy vs. relucantant modifier, and
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* other features in industrial-strength implementations such
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* as java.util.regexp.
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*
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*************************************************************************/
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public class NFA {
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private Digraph G; // digraph of epsilon transitions
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private String regexp; // regular expression
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private int M; // number of characters in regular expression
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// Create the NFA for the given RE
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public NFA(String regexp) {
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this.regexp = regexp;
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M = regexp.length();
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Stack<Integer> ops = new Stack<Integer>();
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G = new Digraph(M+1);
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for (int i = 0; i < M; i++) {
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int lp = i;
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if (regexp.charAt(i) == '(' || regexp.charAt(i) == '|')
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ops.push(i);
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else if (regexp.charAt(i) == ')') {
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int or = ops.pop();
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// 2-way or operator
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if (regexp.charAt(or) == '|') {
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lp = ops.pop();
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G.addEdge(lp, or+1);
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G.addEdge(or, i);
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}
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else if (regexp.charAt(or) == '(')
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lp = or;
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else assert false;
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}
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// closure operator (uses 1-character lookahead)
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if (i < M-1 && regexp.charAt(i+1) == '*') {
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G.addEdge(lp, i+1);
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G.addEdge(i+1, lp);
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}
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if (regexp.charAt(i) == '(' || regexp.charAt(i) == '*' || regexp.charAt(i) == ')')
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G.addEdge(i, i+1);
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}
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}
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// Does the NFA recognize txt?
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public boolean recognizes(String txt) {
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DirectedDFS dfs = new DirectedDFS(G, 0);
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Bag<Integer> pc = new Bag<Integer>();
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for (int v = 0; v < G.V(); v++)
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if (dfs.marked(v)) pc.add(v);
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// Compute possible NFA states for txt[i+1]
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for (int i = 0; i < txt.length(); i++) {
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Bag<Integer> match = new Bag<Integer>();
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for (int v : pc) {
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if (v == M) continue;
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if ((regexp.charAt(v) == txt.charAt(i)) || regexp.charAt(v) == '.')
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match.add(v+1);
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}
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dfs = new DirectedDFS(G, match);
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pc = new Bag<Integer>();
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for (int v = 0; v < G.V(); v++)
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if (dfs.marked(v)) pc.add(v);
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// optimization if no states reachable
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if (pc.size() == 0) return false;
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}
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// check for accept state
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for (int v : pc)
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if (v == M) return true;
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return false;
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}
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public static void main(String[] args) {
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String regexp = "(" + args[0] + ")";
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String txt = args[1];
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if (txt.indexOf('|') >= 0) {
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throw new IllegalArgumentException("| character in text is not supported");
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}
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NFA nfa = new NFA(regexp);
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StdOut.println(nfa.recognizes(txt));
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}
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}
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