WorkList


1   package polyglot.util;
2   
3   import java.util.Collection  ;
4   import java.util.Collections  ;
5   import java.util.HashMap  ;
6   import java.util.Iterator  ;
7   import java.util.LinkedList  ;
8   import java.util.ListIterator  ;
9   import java.util.Map  ;
10  
11  /**
12   * This class represents a set of calculations to be performed, some
13   * of which have already been completed.  It optionally stores the results of
14   * those calculations.
15   *
16   * Requires: every 'calculation' object has a working hashcode and
17   * toString method.
18   **/
19  public class WorkList {
20  
21    /**
22     * Creates a new, empty worklist.
23     **/
24    public WorkList() {
25      pending = new LinkedList  ();
26      results = new HashMap  ();
27      size = 0;
28    }
29  
30    /**
31     * Adds the calculation represented by <o> to the worklist, if it has not
32     * already been calculated.
33     **/
34    public void addWork(Object   o) {
35      if (! results.containsKey(o)) {
36        results.put(o, NOT_CALCULATED);
37        pending.addLast(o);
38        size++;
39      }
40    }
41  
42    /**
43     * Adds every member of the collection <c> to the worklist, if it
44     * has not already been calculted.
45     **/
46    public void addWork(Collection   c) {
47      for (Iterator   i = c.iterator(); i.hasNext(); )
48        addWork(i.next());
49    }
50  
51    /**
52     * Returns true iff there is no more work to do.
53     **/
54    public boolean finished() {
55      return size == 0;
56    }
57  
58    /**
59     * Returns the first element with no known result.  Throws
60     * NoSuchElementException if no such element exists.
61     **/
62    public Object   getWork() {
63      if (size>0) 
64        return pending.getFirst();
65      else
66        throw new java.util.NoSuchElementException  ("WorkList.getWork");
67    }
68  
69    /**
70     * Announces that we have finished the calculation represented by <work>,
71     * getting the result <result>.  Removes <work> from the pending list,
72     * and sets its result to <result>
73     **/
74    public void finishWork(Object   work, Object   result) {
75      if (results.get(work) == NOT_CALCULATED) {
76        for (ListIterator   i = pending.listIterator(); i.hasNext(); ) {
77      if (i.next().equals(work))
78        i.remove();
79        }
80      }
81      results.put(work, result);
82    }
83  
84    /**
85     * Announces that we have finished the calculation represented by <work>.
86     **/
87    public void finishWork(Object   work) {
88      finishWork(work, null);
89    }
90  
91    /**
92     * Returns true iff <work> has been completed.
93     **/
94    public boolean isFinished(Object   work) {
95      return results.containsKey(work) && results.get(work) != NOT_CALCULATED;
96    }
97  
98    /**
99     * Returns an immutable view of a map from calculation objects to
100    * their results.  Non-computed values map to NOT_CALCULATED
101    **/
102   public Map   getMap() {
103     return Collections.unmodifiableMap(results);
104   }
105 
106   // The list of objects to be calculated on.  The oldest element is first.
107   // RI: Every member of <pending> is a key in <results>.
108   LinkedList   pending;
109   // A map from all objects to their results.  Any object with no result
110   // maps to NOT_CALCULATED.
111   HashMap   results;
112   // The number of elements in pending.
113   int size;
114 
115   public static final Object   NOT_CALCULATED = new Object  ();
116 }
117
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