java_cup
Class lalr_item

java.lang.Object
  java_cup.lr_item_core
      java_cup.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.

See Also:

lalr_item_set, lalr_state

Field Summary

protected terminal_set	_lookahead The lookahead symbols of the item.
protected java.util.Stack	_propagate_items Links to items that the lookahead needs to be propagated to.
protected boolean	needs_propagation Flag to indicate that this item needs to propagate its lookahead (whether it has changed or not).

Fields inherited from class java_cup.lr_item_core

_core_hash_cache, _dot_pos, _symbol_after_dot, _the_production

Constructor Summary

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

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).

Method Summary

void	add_propagate(lalr_item prop_to) Add a new item to the set of items we propagate to.
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.
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).
boolean	equals(java.lang.Object other) Generic equality comparison.
int	hashCode() Return a hash code -- here we only hash the core since we only test core matching in LALR items.
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.
terminal_set	lookahead() The lookahead symbols of the item.
java.util.Stack	propagate_items() Links to items that the lookahead needs to be propagated to
void	propagate_lookaheads(terminal_set incoming) Propagate incoming lookaheads through this item to others need to be changed.
lalr_item	shift() Produce the new lalr_item that results from shifting the dot one position to the right.
java.lang.String	toString() Convert to string.

Methods inherited from class java_cup.lr_item_core

core_equals, core_hashCode, dot_at_end, dot_before_nt, dot_pos, equals, obj_hash, shift_core, symbol_after_dot, the_production, to_simple_string

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Field Detail

_lookahead

protected terminal_set _lookahead

The lookahead symbols of the item.

_propagate_items

protected java.util.Stack _propagate_items

Links to items that the lookahead needs to be propagated to.

needs_propagation

protected boolean needs_propagation

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

Constructor Detail

lalr_item

public lalr_item(production prod,
                 int pos,
                 terminal_set look)
          throws internal_error

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)
          throws internal_error

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)
          throws internal_error

Constructor with default position and empty lookahead set.

Parameters:

prod - the production for the item.

Method Detail

lookahead

public terminal_set lookahead()

The lookahead symbols of the item.

propagate_items

public java.util.Stack propagate_items()

Links to items that the lookahead needs to be propagated to

add_propagate

public void add_propagate(lalr_item prop_to)

Add a new item to the set of items we propagate to.

propagate_lookaheads

public void propagate_lookaheads(terminal_set incoming)
                          throws internal_error

Propagate incoming lookaheads through this item to others need to be changed.

Throws:

internal_error

shift

public lalr_item shift()
                throws internal_error

Produce the new lalr_item that results from shifting the dot one position to the right.

Throws:

internal_error

calc_lookahead

public terminal_set calc_lookahead(terminal_set lookahead_after)
                            throws internal_error

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.

Throws:

internal_error

lookahead_visible

public boolean lookahead_visible()
                          throws internal_error

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.

Throws:

internal_error

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(java.lang.Object other)

Generic equality comparison.

Overrides:

equals in class lr_item_core

hashCode

public int hashCode()

Return a hash code -- here we only hash the core since we only test core matching in LALR items.

Overrides:

hashCode in class lr_item_core

toString

public java.lang.String toString()

Convert to string.

Overrides:

toString in class lr_item_core