10.9 Parser Matching Utilities


 
Building Parsers with Java
By Steven  John  Metsker

Table of Contents
Chapter  10.   Matching Mechanics

    Content

Figure 10.1 shows two methods that support matching: bestMatch() and completeMatch() . The bestMatch() method accepts a single assembly, puts it in a vector, and matches against the vector. Then bestMatch() returns one assembly from the output vector of assemblies, choosing the one whose index is advanced furthest. If a grammar matches only part of an input, bestMatch() returns an assembly that shows the progress made. For example, consider matching the grammar

 adjectives = ("steaming"  "hot")*; 

against the string

 "hot hot steaming hot coffee" 

The following code sends bestMatch() to a parser for adjectives , showing that the parser can match up to the word "coffee" :

 package sjm.examples.mechanics;  import sjm.parse.*; import sjm.parse.tokens.*; /**  * Show that <code>Parser.bestMatch()</code> matches a  * parser against an input as far as possible.  */ public class ShowBestMatch { public static void main(String[] args) {     Alternation a = new Alternation();     a.add(new Literal("steaming"));     a.add(new Literal("hot"));     Repetition adjectives = new Repetition(a);     TokenAssembly ta =         new TokenAssembly("hot hot steaming hot coffee");     System.out.println(adjectives.bestMatch(ta)); } } 

This prints the following:

 [hot, hot, steaming, hot]hot/hot/steaming/hot^coffee 

The bestMatch() method performs three tasks that a user of a parser typically needs: placing an assembly in a vector, matching against that vector, and choosing the longest match. Here is the code for this method:

 /**   * Returns an assembly with the greatest possible number of  * elements consumed by matches of this parser.  */ public Assembly bestMatch(Assembly a) {     Vector initialState = new Vector();     initialState.addElement(a);     Vector finalState = matchAndAssemble(initialState);     return best(finalState); } 

Here is the supporting method, best() :

 /**   * Returns the most-matched assembly in a collection.  */ public Assembly best(Vector v) {     Assembly best = null;     Enumeration e = v.elements();     while (e.hasMoreElements()) {         Assembly a = (Assembly) e.nextElement();         if (!a.hasMoreElements()) {             return a;         }         if (best == null) {             best = a;         }else             if (   a.elementsConsumed() >                 best.elementsConsumed()) {                 best = a;             }     }     return best; } 

You usually want to require that a parser consume all of the input text. For example, an arithmetic parser recognizes

 3 * 4 + 5 and more 

as an arithmetic expression followed by unrecognizable text. To ensure that the best match is a complete match, the Parser class provides a completeMatch() method. Here is the code for completeMatch() :

 /**   * Returns either null or a completely matched version of  * the supplied assembly.  */ public Assembly completeMatch(Assembly a) {     Assembly best = bestMatch(a);     if (best != null && !best.hasMoreElements()) {         return best;     }     return null; } 

This method returns either a complete match or null . Here is an example:

 package sjm.examples.mechanics;   import sjm.parse.*; import sjm.parse.tokens.*; import sjm.examples.arithmetic.*; /**  * This class shows that <code>Parser.completeMatch()</code>  * returns a complete match or null.  */ public class ShowCompleteMatch { public static void main(String[] args)     throws ArithmeticException {     Parser p = ArithmeticParser.start();     TokenAssembly ta =         new TokenAssembly("3 * 4 + 5 and more");     System.out.println(p.bestMatch(ta));     System.out.println(p.completeMatch(ta)); } } 

Running this class prints the following:

 [17.0]3.0/*/4.0/+/5.0^and/more null 

This shows that bestMatch() matches as much input as possible, whereas completeMatch() returns null unless the parser can match all of the input.


   
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Building Parsers with Java
Building Parsers With Javaв„ў
ISBN: 0201719622
EAN: 2147483647
Year: 2000
Pages: 169

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