package java_cup;
import java.util.Stack;
/** This class represents an LALR item. Each LALR item consists of
* a production, a "dot" at a position within that production, and
* a set of lookahead symbols (terminal). (The first two of these parts
* are provide by the super class). An item is designed to represent a
* configuration that the parser may be in. For example, an item of the
* form: <pre>
* [A ::= B * C d E , {a,b,c}]
* </pre>
* indicates that the parser is in the middle of parsing the production <pre>
* A ::= B C d E
* </pre>
* that B has already been parsed, and that we will expect to see a lookahead
* of either a, b, or c once the complete RHS of this production has been
* found.<p>
*
* Items may initially be missing some items from their lookahead sets.
* Links are maintained from each item to the set of items that would need
* to be updated if symbols are added to its lookahead set. During
* "lookahead propagation", we add symbols to various lookahead sets and
* propagate these changes across these dependency links as needed.
*
* @see java_cup.lalr_item_set
* @see java_cup.lalr_state
* @version last updated: 11/25/95
* @author Scott Hudson
*/
public class lalr_item extends lr_item_core {
/*-----------------------------------------------------------*/
/*--- Constructor(s) ----------------------------------------*/
/*-----------------------------------------------------------*/
/** Full constructor.
* @param prod the production for the item.
* @param pos the position of the "dot" within the production.
* @param look the set of lookahead symbols.
*/
public lalr_item(production prod, int pos, terminal_set look)
throws internal_error
{
super(prod, pos);
_lookahead = look;
_propagate_items = new Stack();
needs_propagation = true;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Constructor with default position (dot at start).
* @param prod the production for the item.
* @param look the set of lookahead symbols.
*/
public lalr_item(production prod, terminal_set look) throws internal_error
{
this(prod,0,look);
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Constructor with default position and empty lookahead set.
* @param prod the production for the item.
*/
public lalr_item(production prod) throws internal_error
{
this(prod,0,new terminal_set());
}
/*-----------------------------------------------------------*/
/*--- (Access to) Instance Variables ------------------------*/
/*-----------------------------------------------------------*/
/** The lookahead symbols of the item. */
protected terminal_set _lookahead;
/** The lookahead symbols of the item. */
public terminal_set lookahead() {return _lookahead;};
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Links to items that the lookahead needs to be propagated to. */
protected Stack _propagate_items;
/** Links to items that the lookahead needs to be propagated to */
public Stack propagate_items() {return _propagate_items;}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Flag to indicate that this item needs to propagate its lookahead
* (whether it has changed or not).
*/
protected boolean needs_propagation;
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Add a new item to the set of items we propagate to. */
public void add_propagate(lalr_item prop_to)
{
_propagate_items.push(prop_to);
needs_propagation = true;
}
/*-----------------------------------------------------------*/
/*--- General Methods ---------------------------------------*/
/*-----------------------------------------------------------*/
/** Propagate incoming lookaheads through this item to others need to
* be changed.
* @params incoming symbols to potentially be added to lookahead of this item.
*/
public void propagate_lookaheads(terminal_set incoming) throws internal_error
{
boolean change = false;
/* if we don't need to propagate, then bail out now */
if (!needs_propagation && (incoming == null || incoming.empty()))
return;
/* if we have null incoming, treat as an empty set */
if (incoming != null)
{
/* add the incoming to the lookahead of this item */
change = lookahead().add(incoming);
}
/* if we changed or need it anyway, propagate across our links */
if (change || needs_propagation)
{
/* don't need to propagate again */
needs_propagation = false;
/* propagate our lookahead into each item we are linked to */
for (int i = 0; i < propagate_items().size(); i++)
((lalr_item)propagate_items().elementAt(i))
.propagate_lookaheads(lookahead());
}
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Produce the new lalr_item that results from shifting the dot one position
* to the right.
*/
public lalr_item shift() throws internal_error
{
lalr_item result;
/* can't shift if we have dot already at the end */
if (dot_at_end())
throw new internal_error("Attempt to shift past end of an lalr_item");
/* create the new item w/ the dot shifted by one */
result = new lalr_item(the_production(), dot_pos()+1,
new terminal_set(lookahead()));
/* change in our lookahead needs to be propagated to this item */
add_propagate(result);
return result;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Calculate lookahead representing symbols that could appear after the
* symbol that the dot is currently in front of. Note: this routine must
* not be invoked before first sets and nullability has been calculated
* for all non terminals.
*/
public terminal_set calc_lookahead(terminal_set lookahead_after)
throws internal_error
{
terminal_set result;
int pos;
production_part part;
symbol sym;
/* sanity check */
if (dot_at_end())
throw new internal_error(
"Attempt to calculate a lookahead set with a completed item");
/* start with an empty result */
result = new terminal_set();
/* consider all nullable symbols after the one to the right of the dot */
for (pos = dot_pos()+1; pos < the_production().rhs_length(); pos++)
{
part = the_production().rhs(pos);
/* consider what kind of production part it is -- skip actions */
if (!part.is_action())
{
sym = ((symbol_part)part).the_symbol();
/* if its a terminal add it in and we are done */
if (!sym.is_non_term())
{
result.add((terminal)sym);
return result;
}
else
{
/* otherwise add in first set of the non terminal */
result.add(((non_terminal)sym).first_set());
/* if its nullable we continue adding, if not, we are done */
if (!((non_terminal)sym).nullable())
return result;
}
}
}
/* if we get here everything past the dot was nullable
we add in the lookahead for after the production and we are done */
result.add(lookahead_after);
return result;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Determine if everything from the symbol one beyond the dot all the
* way to the end of the right hand side is nullable. This would indicate
* that the lookahead of this item must be included in the lookaheads of
* all items produced as a closure of this item. Note: this routine should
* not be invoked until after first sets and nullability have been
* calculated for all non terminals.
*/
public boolean lookahead_visible() throws internal_error
{
production_part part;
symbol sym;
/* if the dot is at the end, we have a problem, but the cleanest thing
to do is just return true. */
if (dot_at_end()) return true;
/* walk down the rhs and bail if we get a non-nullable symbol */
for (int pos = dot_pos() + 1; pos < the_production().rhs_length(); pos++)
{
part = the_production().rhs(pos);
/* skip actions */
if (!part.is_action())
{
sym = ((symbol_part)part).the_symbol();
/* if its a terminal we fail */
if (!sym.is_non_term()) return false;
/* if its not nullable we fail */
if (!((non_terminal)sym).nullable()) return false;
}
}
/* if we get here its all nullable */
return true;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Equality comparison -- here we only require the cores to be equal since
* we need to do sets of items based only on core equality (ignoring
* lookahead sets).
*/
public boolean equals(lalr_item other)
{
if (other == null) return false;
return super.equals(other);
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Generic equality comparison. */
public boolean equals(Object other)
{
if (!(other instanceof lalr_item))
return false;
else
return equals((lalr_item)other);
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Return a hash code -- here we only hash the core since we only test core
* matching in LALR items.
*/
public int hashCode()
{
return super.hashCode();
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Convert to string. */
public String toString()
{
String result = "";
// additional output for debugging:
// result += "(" + hashCode() + ")";
result += "[";
result += super.toString();
result += ", ";
if (lookahead() != null)
{
result += "{";
for (int t = 0; t < terminal.number(); t++)
if (lookahead().contains(t))
result += terminal.find(t).name() + " ";
result += "}";
}
else
result += "NULL LOOKAHEAD!!";
result += "]";
// additional output for debugging:
// result += " -> ";
// for (int i = 0; i<propagate_items().size(); i++)
// result += propagate_items().elementAt(i).hashCode() + " ";
//
// if (needs_propagation) result += " NP";
return result;
}
/*-----------------------------------------------------------*/
};