Package java_cup
Class lalr_item
java.lang.Object
java_cup.lr_item_core
java_cup.lalr_item
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 Ethat 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.
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Field Summary
FieldsModifier and TypeFieldDescriptionprotected terminal_set
The lookahead symbols of the item.protected Stack
Links to items that the lookahead needs to be propagated to.protected boolean
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
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Constructor Summary
ConstructorsConstructorDescriptionlalr_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
Modifier and TypeMethodDescriptionvoid
add_propagate
(lalr_item prop_to) Add a new item to the set of items we propagate to.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
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
Generic equality comparison.int
hashCode()
Return a hash code -- here we only hash the core since we only test core matching in LALR items.The lookahead symbols of the item.boolean
Determine if everything from the symbol one beyond the dot all the way to the end of the right hand side is nullable.Links to items that the lookahead needs to be propagated tovoid
propagate_lookaheads
(terminal_set incoming) Propagate incoming lookaheads through this item to others need to be changed.shift()
Produce the new lalr_item that results from shifting the dot one position to the right.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
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Field Details
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_lookahead
The lookahead symbols of the item. -
_propagate_items
Links to items that the lookahead needs to be propagated to. -
needs_propagation
protected boolean needs_propagationFlag to indicate that this item needs to propagate its lookahead (whether it has changed or not).
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Constructor Details
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lalr_item
Full constructor.- Parameters:
prod
- the production for the item.pos
- the position of the "dot" within the production.look
- the set of lookahead symbols.- Throws:
internal_error
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lalr_item
Constructor with default position (dot at start).- Parameters:
prod
- the production for the item.look
- the set of lookahead symbols.- Throws:
internal_error
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lalr_item
Constructor with default position and empty lookahead set.- Parameters:
prod
- the production for the item.- Throws:
internal_error
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Method Details
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lookahead
The lookahead symbols of the item. -
propagate_items
Links to items that the lookahead needs to be propagated to -
add_propagate
Add a new item to the set of items we propagate to. -
propagate_lookaheads
Propagate incoming lookaheads through this item to others need to be changed.- Throws:
internal_error
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shift
Produce the new lalr_item that results from shifting the dot one position to the right.- Throws:
internal_error
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calc_lookahead
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
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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.- Throws:
internal_error
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equals
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
Generic equality comparison.- Overrides:
equals
in classlr_item_core
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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 classlr_item_core
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toString
Convert to string.- Overrides:
toString
in classlr_item_core
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