package java_cup;
import java.util.Enumeration;
import java.util.Hashtable;
/** This class represents a set of LALR items. For purposes of building
* these sets, items are considered unique only if they have unique cores
* (i.e., ignoring differences in their lookahead sets).<p>
*
* This class provides fairly conventional set oriented operations (union,
* sub/super-set tests, etc.), as well as an LALR "closure" operation (see
* compute_closure()).
*
* @see java_cup.lalr_item
* @see java_cup.lalr_state
* @version last updated: 11/25/95
* @author Scott Hudson
*/
public class lalr_item_set {
/*-----------------------------------------------------------*/
/*--- Constructor(s) ----------------------------------------*/
/*-----------------------------------------------------------*/
/** Constructor for an empty set. */
public lalr_item_set() { }
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Constructor for cloning from another set.
* @param other indicates set we should copy from.
*/
public lalr_item_set(lalr_item_set other)
throws internal_error
{
not_null(other);
_all = (Hashtable)other._all.clone();
}
/*-----------------------------------------------------------*/
/*--- (Access to) Instance Variables ------------------------*/
/*-----------------------------------------------------------*/
/** A hash table to implement the set. We store the items using themselves
* as keys.
*/
protected Hashtable _all = new Hashtable(11);
/** Access to all elements of the set. */
public Enumeration all() {return _all.elements();}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Cached hashcode for this set. */
protected Integer hashcode_cache = null;
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Size of the set */
public int size() {return _all.size();}
/*-----------------------------------------------------------*/
/*--- Set Operation Methods ---------------------------------*/
/*-----------------------------------------------------------*/
/** Does the set contain a particular item?
* @param itm the item in question.
*/
public boolean contains(lalr_item itm) {return _all.containsKey(itm);}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Return the item in the set matching a particular item (or null if not
* found)
* @param itm the item we are looking for.
*/
public lalr_item find(lalr_item itm) {return (lalr_item)_all.get(itm);}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Is this set an (improper) subset of another?
* @param other the other set in question.
*/
public boolean is_subset_of(lalr_item_set other) throws internal_error
{
not_null(other);
/* walk down our set and make sure every element is in the other */
for (Enumeration e = all(); e.hasMoreElements(); )
if (!other.contains((lalr_item)e.nextElement()))
return false;
/* they were all there */
return true;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Is this set an (improper) superset of another?
* @param other the other set in question.
*/
public boolean is_superset_of(lalr_item_set other) throws internal_error
{
not_null(other);
return other.is_subset_of(this);
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Add a singleton item, merging lookahead sets if the item is already
* part of the set. returns the element of the set that was added or
* merged into.
* @param itm the item being added.
*/
public lalr_item add(lalr_item itm) throws internal_error
{
lalr_item other;
not_null(itm);
/* see if an item with a matching core is already there */
other = (lalr_item)_all.get(itm);
/* if so, merge this lookahead into the original and leave it */
if (other != null)
{
other.lookahead().add(itm.lookahead());
return other;
}
/* otherwise we just go in the set */
else
{
/* invalidate cached hashcode */
hashcode_cache = null;
_all.put(itm,itm);
return itm;
}
};
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Remove a single item if it is in the set.
* @param itm the item to remove.
*/
public void remove(lalr_item itm) throws internal_error
{
not_null(itm);
/* invalidate cached hashcode */
hashcode_cache = null;
/* remove it from hash table implementing set */
_all.remove(itm);
};
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Add a complete set, merging lookaheads where items are already in
* the set
* @param other the set to be added.
*/
public void add(lalr_item_set other) throws internal_error
{
not_null(other);
/* walk down the other set and do the adds individually */
for (Enumeration e = other.all(); e.hasMoreElements(); )
add((lalr_item)e.nextElement());
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Remove (set subtract) a complete set.
* @param other the set to remove.
*/
public void remove(lalr_item_set other) throws internal_error
{
not_null(other);
/* walk down the other set and do the removes individually */
for (Enumeration e = other.all(); e.hasMoreElements(); )
remove((lalr_item)e.nextElement());
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Remove and return one item from the set (done in hash order). */
public lalr_item get_one() throws internal_error
{
Enumeration the_set;
lalr_item result;
the_set = all();
if (the_set.hasMoreElements())
{
result = (lalr_item)the_set.nextElement();
remove(result);
return result;
}
else
return null;
}
/*-----------------------------------------------------------*/
/*--- General Methods ---------------------------------------*/
/*-----------------------------------------------------------*/
/** Helper function for null test. Throws an interal_error exception if its
* parameter is null.
* @param obj the object we are testing.
*/
protected void not_null(Object obj) throws internal_error
{
if (obj == null)
throw new internal_error("Null object used in set operation");
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Compute the closure of the set using the LALR closure rules. Basically
* for every item of the form: <pre>
* [L ::= a *N alpha, l]
* </pre>
* (where N is a a non terminal and alpha is a string of symbols) make
* sure there are also items of the form: <pre>
* [N ::= *beta, first(alpha l)]
* </pre>
* corresponding to each production of N. Items with identical cores but
* differing lookahead sets are merged by creating a new item with the same
* core and the union of the lookahead sets (the LA in LALR stands for
* "lookahead merged" and this is where the merger is). This routine
* assumes that nullability and first sets have been computed for all
* productions before it is called.
*/
public void compute_closure()
throws internal_error
{
lalr_item_set consider;
lalr_item itm, new_itm, add_itm;
non_terminal nt;
terminal_set new_lookaheads;
Enumeration p;
production prod;
boolean need_prop;
/* invalidate cached hashcode */
hashcode_cache = null;
/* each current element needs to be considered */
consider = new lalr_item_set(this);
/* repeat this until there is nothing else to consider */
while (consider.size() > 0)
{
/* get one item to consider */
itm = consider.get_one();
/* do we have a dot before a non terminal */
nt = itm.dot_before_nt();
if (nt != null)
{
/* create the lookahead set based on first after dot */
new_lookaheads = itm.calc_lookahead(itm.lookahead());
/* are we going to need to propagate our lookahead to new item */
need_prop = itm.lookahead_visible();
/* create items for each production of that non term */
for (p = nt.productions(); p.hasMoreElements(); )
{
prod = (production)p.nextElement();
/* create new item with dot at start and that lookahead */
new_itm = new lalr_item(prod,new_lookaheads);
/* add/merge item into the set */
add_itm = add(new_itm);
/* if propagation is needed link to that item */
if (need_prop)
itm.add_propagate(add_itm);
/* was this was a new item*/
if (add_itm == new_itm)
{
/* that may need further closure, consider it also */
consider.add(new_itm);
}
}
}
}
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Equality comparison. */
public boolean equals(lalr_item_set other)
{
if (other == null || other.size() != size()) return false;
/* once we know they are the same size, then improper subset does test */
try {
return is_subset_of(other);
} catch (internal_error e) {
/* can't throw error from here (because superclass doesn't) so crash */
e.crash();
return false;
}
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Generic equality comparison. */
public boolean equals(Object other)
{
if (!(other instanceof lalr_item_set))
return false;
else
return equals((lalr_item_set)other);
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Return hash code. */
public int hashCode()
{
int result = 0;
Enumeration e;
int cnt;
/* only compute a new one if we don't have it cached */
if (hashcode_cache == null)
{
/* hash together codes from at most first 5 elements */
for (e = all(), cnt=0 ; e.hasMoreElements() && cnt<5; cnt++)
result ^= ((lalr_item)e.nextElement()).hashCode();
hashcode_cache = new Integer(result);
}
return hashcode_cache.intValue();
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Convert to string. */
public String toString()
{
StringBuffer result = new StringBuffer();
result.append("{\n");
for (Enumeration e=all(); e.hasMoreElements(); )
{
result.append(" " + (lalr_item)e.nextElement() + "\n");
}
result.append("}");
return result.toString();
}
/*-----------------------------------------------------------*/
};