java_cup

Class lalr_item

public 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.

Version: last updated: 11/25/95

Author: Scott Hudson

See Also: lalr_item_set lalr_state

Field Summary
protected booleanneeds_propagation
Flag to indicate that this item needs to propagate its lookahead (whether it has changed or not).
protected terminal_set_lookahead
The lookahead symbols of the item.
protected Stack_propagate_items
Links to items that the lookahead needs to be propagated to.
Constructor Summary
lalr_item(production prod, int pos, terminal_set look)
Full constructor.
lalr_item(production prod, terminal_set look)
Constructor with default position (dot at start).
lalr_item(production prod)
Constructor with default position and empty lookahead set.
Method Summary
voidadd_propagate(lalr_item prop_to)
Add a new item to the set of items we propagate to.
terminal_setcalc_lookahead(terminal_set lookahead_after)
Calculate lookahead representing symbols that could appear after the symbol that the dot is currently in front of.
booleanequals(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).
booleanequals(Object other)
Generic equality comparison.
inthashCode()
Return a hash code -- here we only hash the core since we only test core matching in LALR items.
terminal_setlookahead()
The lookahead symbols of the item.
booleanlookahead_visible()
Determine if everything from the symbol one beyond the dot all the way to the end of the right hand side is nullable.
Stackpropagate_items()
Links to items that the lookahead needs to be propagated to
voidpropagate_lookaheads(terminal_set incoming)
Propagate incoming lookaheads through this item to others need to be changed.
lalr_itemshift()
Produce the new lalr_item that results from shifting the dot one position to the right.
StringtoString()
Convert to string.

Field Detail

needs_propagation

protected boolean needs_propagation
Flag to indicate that this item needs to propagate its lookahead (whether it has changed or not).

_lookahead

protected terminal_set _lookahead
The lookahead symbols of the item.

_propagate_items

protected Stack _propagate_items
Links to items that the lookahead needs to be propagated to.

Constructor Detail

lalr_item

public lalr_item(production prod, int pos, terminal_set look)
Full constructor.

Parameters: prod the production for the item. pos the position of the "dot" within the production. look the set of lookahead symbols.

lalr_item

public lalr_item(production prod, terminal_set look)
Constructor with default position (dot at start).

Parameters: prod the production for the item. look the set of lookahead symbols.

lalr_item

public lalr_item(production prod)
Constructor with default position and empty lookahead set.

Parameters: prod the production for the item.

Method Detail

add_propagate

public void add_propagate(lalr_item prop_to)
Add a new item to the set of items we propagate to.

calc_lookahead

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.

equals

public boolean equals(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).

equals

public boolean equals(Object other)
Generic equality comparison.

hashCode

public int hashCode()
Return a hash code -- here we only hash the core since we only test core matching in LALR items.

lookahead

public terminal_set lookahead()
The lookahead symbols of the item.

lookahead_visible

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.

propagate_items

public Stack propagate_items()
Links to items that the lookahead needs to be propagated to

propagate_lookaheads

public void propagate_lookaheads(terminal_set incoming)
Propagate incoming lookaheads through this item to others need to be changed.

UNKNOWN: incoming symbols to potentially be added to lookahead of this item.

shift

public lalr_item shift()
Produce the new lalr_item that results from shifting the dot one position to the right.

toString

public String toString()
Convert to string.