lalr_itempublic class lalr_item extends lr_item_core 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:
[A ::= B * C d E , {a,b,c}]
indicates that the parser is in the middle of parsing the production
A ::= B C d E
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.
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. |
| Fields Summary |
|---|
| protected terminal_set | _lookaheadThe lookahead symbols of the item. | | protected Stack | _propagate_itemsLinks to items that the lookahead needs to be propagated to. | | protected boolean | needs_propagationFlag to indicate that this item needs to propagate its lookahead
(whether it has changed or not). |
| Constructors Summary |
|---|
public lalr_item(production prod, int pos, terminal_set look)Full constructor.
super(prod, pos);
_lookahead = look;
_propagate_items = new Stack();
needs_propagation = true;
| public lalr_item(production prod, terminal_set look)Constructor with default position (dot at start).
this(prod,0,look);
| public lalr_item(production prod)Constructor with default position and empty lookahead set.
this(prod,0,new terminal_set());
|
| Methods Summary |
|---|
| public void | add_propagate(com.sun.java_cup.internal.lalr_item prop_to)Add a new item to the set of items we propagate to.
_propagate_items.push(prop_to);
needs_propagation = true;
| | public terminal_set | calc_lookahead(terminal_set lookahead_after)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.
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;
| | public boolean | equals(com.sun.java_cup.internal.lalr_item other)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).
if (other == null) return false;
return super.equals(other);
| | public boolean | equals(java.lang.Object other)Generic equality comparison.
if (!(other instanceof lalr_item))
return false;
else
return equals((lalr_item)other);
| | public int | hashCode()Return a hash code -- here we only hash the core since we only test core
matching in LALR items.
return super.hashCode();
| | public terminal_set | lookahead()The lookahead symbols of the item.return _lookahead;
| | public boolean | lookahead_visible()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.
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;
| | public java.util.Stack | propagate_items()Links to items that the lookahead needs to be propagated toreturn _propagate_items;
| | public void | propagate_lookaheads(terminal_set incoming)Propagate incoming lookaheads through this item to others need to
be changed.
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());
}
| | public com.sun.java_cup.internal.lalr_item | shift()Produce the new lalr_item that results from shifting the dot one position
to the right.
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;
| | public java.lang.String | toString()Convert to string.
String result = "";
// additional output for debugging:
// result += "(" + obj_hash() + ")";
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+=((lalr_item)(propagate_items().elementAt(i))).obj_hash()+" ";
//
// if (needs_propagation) result += " NP";
return result;
|
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