| java.lang.Object
 java_cup.lalr_state
| lalr_state | | public class lalr_state (Code) | | This class represents a state in the LALR viable prefix recognition machine.
A state consists of an LALR item set and a set of transitions to other
states under terminal and non-terminal symbols. Each state represents
a potential configuration of the parser. If the item set of a state
includes an item such as:
[A ::= B * C d E , {a,b,c}]
this indicates that when the parser is in this state it is currently
looking for an A of the given form, has already seen the B, and would
expect to see an a, b, or c after this sequence is complete. Note that
the parser is normally looking for several things at once (represented
by several items). In our example above, the state would also include
items such as:
[C ::= * X e Z, {d}]
[X ::= * f, {e}]
to indicate that it was currently looking for a C followed by a d (which
would be reduced into a C, matching the first symbol in our production
above), and the terminal f followed by e.
At runtime, the parser uses a viable prefix recognition machine made up
of these states to parse. The parser has two operations, shift and reduce.
In a shift, it consumes one Symbol and makes a transition to a new state.
This corresponds to "moving the dot past" a terminal in one or more items
in the state (these new shifted items will then be found in the state at
the end of the transition). For a reduce operation, the parser is
signifying that it is recognizing the RHS of some production. To do this
it first "backs up" by popping a stack of previously saved states. It
pops off the same number of states as are found in the RHS of the
production. This leaves the machine in the same state is was in when the
parser first attempted to find the RHS. From this state it makes a
transition based on the non-terminal on the LHS of the production. This
corresponds to placing the parse in a configuration equivalent to having
replaced all the symbols from the the input corresponding to the RHS with
the symbol on the LHS.
See Also: java_cup.lalr_item
See Also: java_cup.lalr_item_set
See Also: java_cup.lalr_transition
version:
last updated: 7/3/96
author:
Frank Flannery |
| Method Summary | |
| public void | add_transition(symbol on_sym, lalr_state to_st)
Add a transition out of this state to another. | | public static Enumeration | all()
Collection of all states. | | public static lalr_state | build_machine(production start_prod)
Build an LALR viable prefix recognition machine given a start
production. | | public void | build_table_entries(parse_action_table act_table, parse_reduce_table reduce_table)
Fill in the parse table entries for this state. | | public static void | clear()
| | protected static void | dump_state(lalr_state st)
Helper routine for debugging -- produces a dump of the given state
onto System.out. | | public boolean | equals(lalr_state other)
Equality comparison. | | public boolean | equals(Object other)
Generic equality comparison. | | public static lalr_state | find_state(lalr_item_set itms)
Find and return state with a given a kernel item set (or null if not
found). | | protected boolean | fix_with_precedence(production p, int term_index, parse_action_row table_row, parse_action act)
Procedure that attempts to fix a shift/reduce error by using
precedences. | | public int | hashCode()
Produce a hash code. | | public int | index()
| | protected parse_action | insert_action(parse_action a1, parse_action a2, int act_type)
| | protected parse_action | insert_reduce(parse_action a1, parse_action a2)
| | protected parse_action | insert_shift(parse_action a1, parse_action a2)
| | public lalr_item_set | items()
The item set for this state. | | public static int | number()
Indicate total number of states there are. | | protected static void | propagate_all_lookaheads()
Propagate lookahead sets through the constructed viable prefix
recognizer. | | protected void | propagate_lookaheads()
Propagate lookahead sets out of this state. | | protected void | report_conflicts(terminal_set conflict_set)
Produce warning messages for all conflicts found in this state. | | protected void | report_reduce_reduce(lalr_item itm1, lalr_item itm2)
Produce a warning message for one reduce/reduce conflict. | | protected void | report_shift_reduce(lalr_item red_itm, int conflict_sym)
Produce a warning message for one shift/reduce conflict. | | public String | toString()
Convert to a string. | | public lalr_transition | transitions()
List of transitions out of this state. |
| _all_kernels | | protected static Hashtable _all_kernels(Code) | | Hash table to find states by their kernels (i.e, the original,
unclosed, set of items -- which uniquely define the state). This table
stores state objects using (a copy of) their kernel item sets as keys.
|
| _index | | protected int _index(Code) | | Index of this state in the parse tables
|
| next_index | | protected static int next_index(Code) | | Static counter for assigning unique state indexes.
|
| lalr_state | | public lalr_state(lalr_item_set itms) throws internal_error(Code) | | Constructor for building a state from a set of items.
Parameters:
itms - the set of items that makes up this state. |
| add_transition | | public void add_transition(symbol on_sym, lalr_state to_st) throws internal_error(Code) | | Add a transition out of this state to another.
Parameters:
on_sym - the symbol the transition is under.
Parameters:
to_st - the state the transition goes to. |
| build_machine | | public static lalr_state build_machine(production start_prod) throws internal_error(Code) | | Build an LALR viable prefix recognition machine given a start
production. This method operates by first building a start state
from the start production (based on a single item with the dot at
the beginning and EOF as expected lookahead). Then for each state
it attempts to extend the machine by creating transitions out of
the state to new or existing states. When considering extension
from a state we make a transition on each symbol that appears before
the dot in some item. For example, if we have the items:
[A ::= a b * X c, {d,e}]
[B ::= a b * X d, {a,b}]
in some state, then we would be making a transition under X to a new
state. This new state would be formed by a "kernel" of items
corresponding to moving the dot past the X. In this case:
[A ::= a b X * c, {d,e}]
[B ::= a b X * Y, {a,b}]
The full state would then be formed by "closing" this kernel set of
items so that it included items that represented productions of things
the parser was now looking for. In this case we would items
corresponding to productions of Y, since various forms of Y are expected
next when in this state (see lalr_item_set.compute_closure() for details
on closure).
The process of building the viable prefix recognizer terminates when no
new states can be added. However, in order to build a smaller number of
states (i.e., corresponding to LALR rather than canonical LR) the state
building process does not maintain full loookaheads in all items.
Consequently, after the machine is built, we go back and propagate
lookaheads through the constructed machine using a call to
propagate_all_lookaheads(). This makes use of propagation links
constructed during the closure and transition process.
Parameters:
start_prod - the start production of the grammar
See Also: java_cup.lalr_item_set.compute_closure
See Also: java_cup.lalr_state.propagate_all_lookaheads |
| build_table_entries | | public void build_table_entries(parse_action_table act_table, parse_reduce_table reduce_table) throws internal_error(Code) | | Fill in the parse table entries for this state. There are two
parse tables that encode the viable prefix recognition machine, an
action table and a reduce-goto table. The rows in each table
correspond to states of the machine. The columns of the action table
are indexed by terminal symbols and correspond to either transitions
out of the state (shift entries) or reductions from the state to some
previous state saved on the stack (reduce entries). All entries in the
action table that are not shifts or reduces, represent errors. The
reduce-goto table is indexed by non terminals and represents transitions
out of a state on that non-terminal.
Conflicts occur if more than one action needs to go in one entry of the
action table (this cannot happen with the reduce-goto table). Conflicts
are resolved by always shifting for shift/reduce conflicts and choosing
the lowest numbered production (hence the one that appeared first in
the specification) in reduce/reduce conflicts. All conflicts are
reported and if more conflicts are detected than were declared by the
user, code generation is aborted.
Parameters:
act_table - the action table to put entries in.
Parameters:
reduce_table - the reduce-goto table to put entries in. |
| clear | | public static void clear()(Code) | | |
| dump_state | | protected static void dump_state(lalr_state st) throws internal_error(Code) | | Helper routine for debugging -- produces a dump of the given state
onto System.out.
|
| equals | | public boolean equals(Object other)(Code) | | Generic equality comparison.
|
| find_state | | public static lalr_state find_state(lalr_item_set itms)(Code) | | Find and return state with a given a kernel item set (or null if not
found). The kernel item set is the subset of items that were used to
originally create the state. These items are formed by "shifting the
dot" within items of other states that have a transition to this one.
The remaining elements of this state's item set are added during closure.
Parameters:
itms - the kernel set of the state we are looking for. |
| fix_with_precedence | | protected boolean fix_with_precedence(production p, int term_index, parse_action_row table_row, parse_action act) throws internal_error(Code) | | Procedure that attempts to fix a shift/reduce error by using
precedences. --frankf 6/26/96
if a production (also called rule) or the lookahead terminal
has a precedence, then the table can be fixed. if the rule
has greater precedence than the terminal, a reduce by that rule
in inserted in the table. If the terminal has a higher precedence,
it is shifted. if they have equal precedence, then the associativity
of the precedence is used to determine what to put in the table:
if the precedence is left associative, the action is to reduce.
if the precedence is right associative, the action is to shift.
if the precedence is non associative, then it is a syntax error.
Parameters:
p - the production
Parameters:
term_index - the index of the lokahead terminal
Parameters:
parse_action_row - a row of the action table
Parameters:
act - the rule in conflict with the table entry |
| hashCode | | public int hashCode()(Code) | | Produce a hash code.
|
| index | | public int index()(Code) | | Index of this state in the parse tables
|
| number | | public static int number()(Code) | | Indicate total number of states there are.
|
| propagate_all_lookaheads | | protected static void propagate_all_lookaheads() throws internal_error(Code) | | Propagate lookahead sets through the constructed viable prefix
recognizer. When the machine is constructed, each item that results
in the creation of another such that its lookahead is included in the
other's will have a propagate link set up for it. This allows additions
to the lookahead of one item to be included in other items that it
was used to directly or indirectly create.
|
| propagate_lookaheads | | protected void propagate_lookaheads() throws internal_error(Code) | | Propagate lookahead sets out of this state. This recursively
propagates to all items that have propagation links from some item
in this state.
|
| report_conflicts | | protected void report_conflicts(terminal_set conflict_set) throws internal_error(Code) | | Produce warning messages for all conflicts found in this state.
|
| report_reduce_reduce | | protected void report_reduce_reduce(lalr_item itm1, lalr_item itm2) throws internal_error(Code) | | Produce a warning message for one reduce/reduce conflict.
Parameters:
itm1 - first item in conflict.
Parameters:
itm2 - second item in conflict. |
| report_shift_reduce | | protected void report_shift_reduce(lalr_item red_itm, int conflict_sym) throws internal_error(Code) | | Produce a warning message for one shift/reduce conflict.
Parameters:
red_itm - the item with the reduce.
Parameters:
conflict_sym - the index of the symbol conflict occurs under. |
|
|