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lr_parser.java
/*
* Copyright (c) 2003, 2017, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package com.sun.java_cup.internal.runtime;
import java.util.Stack;
/** This class implements a skeleton table driven LR parser. In general,
* LR parsers are a form of bottom up shift-reduce parsers. Shift-reduce
* parsers act by shifting input onto a parse stack until the Symbols
* matching the right hand side of a production appear on the top of the
* stack. Once this occurs, a reduce is performed. This involves removing
* the Symbols corresponding to the right hand side of the production
* (the so called "handle") and replacing them with the non-terminal from
* the left hand side of the production. <p>
*
* To control the decision of whether to shift or reduce at any given point,
* the parser uses a state machine (the "viable prefix recognition machine"
* built by the parser generator). The current state of the machine is placed
* on top of the parse stack (stored as part of a Symbol object representing
* a terminal or non terminal). The parse action table is consulted
* (using the current state and the current lookahead Symbol as indexes) to
* determine whether to shift or to reduce. When the parser shifts, it
* changes to a new state by pushing a new Symbol (containing a new state)
* onto the stack. When the parser reduces, it pops the handle (right hand
* side of a production) off the stack. This leaves the parser in the state
* it was in before any of those Symbols were matched. Next the reduce-goto
* table is consulted (using the new state and current lookahead Symbol as
* indexes) to determine a new state to go to. The parser then shifts to
* this goto state by pushing the left hand side Symbol of the production
* (also containing the new state) onto the stack.<p>
*
* This class actually provides four LR parsers. The methods parse() and
* debug_parse() provide two versions of the main parser (the only difference
* being that debug_parse() emits debugging trace messages as it parses).
* In addition to these main parsers, the error recovery mechanism uses two
* more. One of these is used to simulate "parsing ahead" in the input
* without carrying out actions (to verify that a potential error recovery
* has worked), and the other is used to parse through buffered "parse ahead"
* input in order to execute all actions and re-synchronize the actual parser
* configuration.<p>
*
* This is an abstract class which is normally filled out by a subclass
* generated by the JavaCup parser generator. In addition to supplying
* the actual parse tables, generated code also supplies methods which
* invoke various pieces of user supplied code, provide access to certain
* special Symbols (e.g., EOF and error), etc. Specifically, the following
* abstract methods are normally supplied by generated code:
* <dl compact>
* <dt> short[][] production_table()
* <dd> Provides a reference to the production table (indicating the index of
* the left hand side non terminal and the length of the right hand side
* for each production in the grammar).
* <dt> short[][] action_table()
* <dd> Provides a reference to the parse action table.
* <dt> short[][] reduce_table()
* <dd> Provides a reference to the reduce-goto table.
* <dt> int start_state()
* <dd> Indicates the index of the start state.
* <dt> int start_production()
* <dd> Indicates the index of the starting production.
* <dt> int EOF_sym()
* <dd> Indicates the index of the EOF Symbol.
* <dt> int error_sym()
* <dd> Indicates the index of the error Symbol.
* <dt> Symbol do_action()
* <dd> Executes a piece of user supplied action code. This always comes at
* the point of a reduce in the parse, so this code also allocates and
* fills in the left hand side non terminal Symbol object that is to be
* pushed onto the stack for the reduce.
* <dt> void init_actions()
* <dd> Code to initialize a special object that encapsulates user supplied
* actions (this object is used by do_action() to actually carry out the
* actions).
* </dl>
*
* In addition to these routines that <i>must</i> be supplied by the
* generated subclass there are also a series of routines that <i>may</i>
* be supplied. These include:
* <dl>
* <dt> Symbol scan()
* <dd> Used to get the next input Symbol from the scanner.
* <dt> Scanner getScanner()
* <dd> Used to provide a scanner for the default implementation of
* scan().
* <dt> int error_sync_size()
* <dd> This determines how many Symbols past the point of an error
* must be parsed without error in order to consider a recovery to
* be valid. This defaults to 3. Values less than 2 are not
* recommended.
* <dt> void report_error(String message, Object info)
* <dd> This method is called to report an error. The default implementation
* simply prints a message to System.err and where the error occurred.
* This method is often replaced in order to provide a more sophisticated
* error reporting mechanism.
* <dt> void report_fatal_error(String message, Object info)
* <dd> This method is called when a fatal error that cannot be recovered from
* is encountered. In the default implementation, it calls
* report_error() to emit a message, then throws an exception.
* <dt> void syntax_error(Symbol cur_token)
* <dd> This method is called as soon as syntax error is detected (but
* before recovery is attempted). In the default implementation it
* invokes: report_error("Syntax error", null);
* <dt> void unrecovered_syntax_error(Symbol cur_token)
* <dd> This method is called if syntax error recovery fails. In the default
* implementation it invokes:<br>
* report_fatal_error("Couldn't repair and continue parse", null);
* </dl>
*
* @see com.sun.java_cup.internal.runtime.Symbol
* @see com.sun.java_cup.internal.runtime.Symbol
* @see com.sun.java_cup.internal.runtime.virtual_parse_stack
* @author Frank Flannery
*/
public abstract class lr_parser {
/*-----------------------------------------------------------*/
/*--- Constructor(s) ----------------------------------------*/
/*-----------------------------------------------------------*/
/** Simple constructor. */
public lr_parser()
{
/* nothing to do here */
}
/** Constructor that sets the default scanner. [CSA/davidm] */
public lr_parser(Scanner s) {
this(); /* in case default constructor someday does something */
setScanner(s);
}
/*-----------------------------------------------------------*/
/*--- (Access to) Static (Class) Variables ------------------*/
/*-----------------------------------------------------------*/
/** The default number of Symbols after an error we much match to consider
* it recovered from.
*/
protected final static int _error_sync_size = 3;
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** The number of Symbols after an error we much match to consider it
* recovered from.
*/
protected int error_sync_size() {return _error_sync_size; }
/*-----------------------------------------------------------*/
/*--- (Access to) Instance Variables ------------------------*/
/*-----------------------------------------------------------*/
/** Table of production information (supplied by generated subclass).
* This table contains one entry per production and is indexed by
* the negative-encoded values (reduce actions) in the action_table.
* Each entry has two parts, the index of the non-terminal on the
* left hand side of the production, and the number of Symbols
* on the right hand side.
*/
public abstract short[][] production_table();
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** The action table (supplied by generated subclass). This table is
* indexed by state and terminal number indicating what action is to
* be taken when the parser is in the given state (i.e., the given state
* is on top of the stack) and the given terminal is next on the input.
* States are indexed using the first dimension, however, the entries for
* a given state are compacted and stored in adjacent index, value pairs
* which are searched for rather than accessed directly (see get_action()).
* The actions stored in the table will be either shifts, reduces, or
* errors. Shifts are encoded as positive values (one greater than the
* state shifted to). Reduces are encoded as negative values (one less
* than the production reduced by). Error entries are denoted by zero.
*
* @see com.sun.java_cup.internal.runtime.lr_parser#get_action
*/
public abstract short[][] action_table();
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** The reduce-goto table (supplied by generated subclass). This
* table is indexed by state and non-terminal number and contains
* state numbers. States are indexed using the first dimension, however,
* the entries for a given state are compacted and stored in adjacent
* index, value pairs which are searched for rather than accessed
* directly (see get_reduce()). When a reduce occurs, the handle
* (corresponding to the RHS of the matched production) is popped off
* the stack. The new top of stack indicates a state. This table is
* then indexed by that state and the LHS of the reducing production to
* indicate where to "shift" to.
*
* @see com.sun.java_cup.internal.runtime.lr_parser#get_reduce
*/
public abstract short[][] reduce_table();
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** The index of the start state (supplied by generated subclass). */
public abstract int start_state();
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** The index of the start production (supplied by generated subclass). */
public abstract int start_production();
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** The index of the end of file terminal Symbol (supplied by generated
* subclass).
*/
public abstract int EOF_sym();
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** The index of the special error Symbol (supplied by generated subclass). */
public abstract int error_sym();
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Internal flag to indicate when parser should quit. */
protected boolean _done_parsing = false;
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** This method is called to indicate that the parser should quit. This is
* normally called by an accept action, but can be used to cancel parsing
* early in other circumstances if desired.
*/
public void done_parsing()
{
_done_parsing = true;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/* Global parse state shared by parse(), error recovery, and
* debugging routines */
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Indication of the index for top of stack (for use by actions). */
protected int tos;
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** The current lookahead Symbol. */
protected Symbol cur_token;
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** The parse stack itself. */
protected Stack<Symbol> stack = new Stack<>();
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Direct reference to the production table. */
protected short[][] production_tab;
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Direct reference to the action table. */
protected short[][] action_tab;
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Direct reference to the reduce-goto table. */
protected short[][] reduce_tab;
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** This is the scanner object used by the default implementation
* of scan() to get Symbols. To avoid name conflicts with existing
* code, this field is private. [CSA/davidm] */
private Scanner _scanner;
/**
* Simple accessor method to set the default scanner.
*/
public void setScanner(Scanner s) { _scanner = s; }
/**
* Simple accessor method to get the default scanner.
*/
public Scanner getScanner() { return _scanner; }
/*-----------------------------------------------------------*/
/*--- General Methods ---------------------------------------*/
/*-----------------------------------------------------------*/
/** Perform a bit of user supplied action code (supplied by generated
* subclass). Actions are indexed by an internal action number assigned
* at parser generation time.
*
* @param act_num the internal index of the action to be performed.
* @param parser the parser object we are acting for.
* @param stack the parse stack of that object.
* @param top the index of the top element of the parse stack.
*/
public abstract Symbol do_action(
int act_num,
lr_parser parser,
Stack<Symbol> stack,
int top)
throws java.lang.Exception;
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** User code for initialization inside the parser. Typically this
* initializes the scanner. This is called before the parser requests
* the first Symbol. Here this is just a placeholder for subclasses that
* might need this and we perform no action. This method is normally
* overridden by the generated code using this contents of the "init with"
* clause as its body.
*/
public void user_init() throws java.lang.Exception { }
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Initialize the action object. This is called before the parser does
* any parse actions. This is filled in by generated code to create
* an object that encapsulates all action code.
*/
protected abstract void init_actions() throws java.lang.Exception;
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Get the next Symbol from the input (supplied by generated subclass).
* Once end of file has been reached, all subsequent calls to scan
* should return an EOF Symbol (which is Symbol number 0). By default
* this method returns getScanner().next_token(); this implementation
* can be overriden by the generated parser using the code declared in
* the "scan with" clause. Do not recycle objects; every call to
* scan() should return a fresh object.
*/
public Symbol scan() throws java.lang.Exception {
return getScanner().next_token();
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Report a fatal error. This method takes a message string and an
* additional object (to be used by specializations implemented in
* subclasses). Here in the base class a very simple implementation
* is provided which reports the error then throws an exception.
*
* @param message an error message.
* @param info an extra object reserved for use by specialized subclasses.
*/
public void report_fatal_error(
String message,
Object info)
throws java.lang.Exception
{
/* stop parsing (not really necessary since we throw an exception, but) */
done_parsing();
/* use the normal error message reporting to put out the message */
report_error(message, info);
/* throw an exception */
throw new Exception("Can't recover from previous error(s)");
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Report a non fatal error (or warning). This method takes a message
* string and an additional object (to be used by specializations
* implemented in subclasses). Here in the base class a very simple
* implementation is provided which simply prints the message to
* System.err.
*
* @param message an error message.
* @param info an extra object reserved for use by specialized subclasses.
*/
public void report_error(String message, Object info)
{
System.err.print(message);
if (info instanceof Symbol)
if (((Symbol)info).left != -1)
System.err.println(" at character " + ((Symbol)info).left +
" of input");
else System.err.println("");
else System.err.println("");
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** This method is called when a syntax error has been detected and recovery
* is about to be invoked. Here in the base class we just emit a
* "Syntax error" error message.
*
* @param cur_token the current lookahead Symbol.
*/
public void syntax_error(Symbol cur_token)
{
report_error("Syntax error", cur_token);
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** This method is called if it is determined that syntax error recovery
* has been unsuccessful. Here in the base class we report a fatal error.
*
* @param cur_token the current lookahead Symbol.
*/
public void unrecovered_syntax_error(Symbol cur_token)
throws java.lang.Exception
{
report_fatal_error("Couldn't repair and continue parse", cur_token);
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Fetch an action from the action table. The table is broken up into
* rows, one per state (rows are indexed directly by state number).
* Within each row, a list of index, value pairs are given (as sequential
* entries in the table), and the list is terminated by a default entry
* (denoted with a Symbol index of -1). To find the proper entry in a row
* we do a linear or binary search (depending on the size of the row).
*
* @param state the state index of the action being accessed.
* @param sym the Symbol index of the action being accessed.
*/
protected final short get_action(int state, int sym)
{
short tag;
int first, last, probe;
short[] row = action_tab[state];
/* linear search if we are < 10 entries */
if (row.length < 20)
for (probe = 0; probe < row.length; probe++)
{
/* is this entry labeled with our Symbol or the default? */
tag = row[probe++];
if (tag == sym || tag == -1)
{
/* return the next entry */
return row[probe];
}
}
/* otherwise binary search */
else
{
first = 0;
last = (row.length-1)/2 - 1; /* leave out trailing default entry */
while (first <= last)
{
probe = (first+last)/2;
if (sym == row[probe*2])
return row[probe*2+1];
else if (sym > row[probe*2])
first = probe+1;
else
last = probe-1;
}
/* not found, use the default at the end */
return row[row.length-1];
}
/* shouldn't happened, but if we run off the end we return the
default (error == 0) */
return 0;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Fetch a state from the reduce-goto table. The table is broken up into
* rows, one per state (rows are indexed directly by state number).
* Within each row, a list of index, value pairs are given (as sequential
* entries in the table), and the list is terminated by a default entry
* (denoted with a Symbol index of -1). To find the proper entry in a row
* we do a linear search.
*
* @param state the state index of the entry being accessed.
* @param sym the Symbol index of the entry being accessed.
*/
protected final short get_reduce(int state, int sym)
{
short tag;
short[] row = reduce_tab[state];
/* if we have a null row we go with the default */
if (row == null)
return -1;
for (int probe = 0; probe < row.length; probe++)
{
/* is this entry labeled with our Symbol or the default? */
tag = row[probe++];
if (tag == sym || tag == -1)
{
/* return the next entry */
return row[probe];
}
}
/* if we run off the end we return the default (error == -1) */
return -1;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** This method provides the main parsing routine. It returns only when
* done_parsing() has been called (typically because the parser has
* accepted, or a fatal error has been reported). See the header
* documentation for the class regarding how shift/reduce parsers operate
* and how the various tables are used.
*/
public Symbol parse() throws java.lang.Exception
{
/* the current action code */
int act;
/* the Symbol/stack element returned by a reduce */
Symbol lhs_sym = null;
/* information about production being reduced with */
short handle_size, lhs_sym_num;
/* set up direct reference to tables to drive the parser */
production_tab = production_table();
action_tab = action_table();
reduce_tab = reduce_table();
/* initialize the action encapsulation object */
init_actions();
/* do user initialization */
user_init();
/* get the first token */
cur_token = scan();
/* push dummy Symbol with start state to get us underway */
stack.removeAllElements();
stack.push(new Symbol(0, start_state()));
tos = 0;
/* continue until we are told to stop */
for (_done_parsing = false; !_done_parsing; )
{
/* Check current token for freshness. */
if (cur_token.used_by_parser)
throw new Error("Symbol recycling detected (fix your scanner).");
/* current state is always on the top of the stack */
/* look up action out of the current state with the current input */
act = get_action((stack.peek()).parse_state, cur_token.sym);
/* decode the action -- > 0 encodes shift */
if (act > 0)
{
/* shift to the encoded state by pushing it on the stack */
cur_token.parse_state = act-1;
cur_token.used_by_parser = true;
stack.push(cur_token);
tos++;
/* advance to the next Symbol */
cur_token = scan();
}
/* if its less than zero, then it encodes a reduce action */
else if (act < 0)
{
/* perform the action for the reduce */
lhs_sym = do_action((-act)-1, this, stack, tos);
/* look up information about the production */
lhs_sym_num = production_tab[(-act)-1][0];
handle_size = production_tab[(-act)-1][1];
/* pop the handle off the stack */
for (int i = 0; i < handle_size; i++)
{
stack.pop();
tos--;
}
/* look up the state to go to from the one popped back to */
act = get_reduce((stack.peek()).parse_state, lhs_sym_num);
/* shift to that state */
lhs_sym.parse_state = act;
lhs_sym.used_by_parser = true;
stack.push(lhs_sym);
tos++;
}
/* finally if the entry is zero, we have an error */
else if (act == 0)
{
/* call user syntax error reporting routine */
syntax_error(cur_token);
/* try to error recover */
if (!error_recovery(false))
{
/* if that fails give up with a fatal syntax error */
unrecovered_syntax_error(cur_token);
/* just in case that wasn't fatal enough, end parse */
done_parsing();
} else {
lhs_sym = stack.peek();
}
}
}
return lhs_sym;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Write a debugging message to System.err for the debugging version
* of the parser.
*
* @param mess the text of the debugging message.
*/
public void debug_message(String mess)
{
System.err.println(mess);
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Dump the parse stack for debugging purposes. */
public void dump_stack()
{
if (stack == null)
{
debug_message("# Stack dump requested, but stack is null");
return;
}
debug_message("============ Parse Stack Dump ============");
/* dump the stack */
for (int i=0; i<stack.size(); i++)
{
debug_message("Symbol: " + (stack.get(i)).sym +
" State: " + (stack.get(i)).parse_state);
}
debug_message("==========================================");
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Do debug output for a reduce.
*
* @param prod_num the production we are reducing with.
* @param nt_num the index of the LHS non terminal.
* @param rhs_size the size of the RHS.
*/
public void debug_reduce(int prod_num, int nt_num, int rhs_size)
{
debug_message("# Reduce with prod #" + prod_num + " [NT=" + nt_num +
", " + "SZ=" + rhs_size + "]");
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Do debug output for shift.
*
* @param shift_tkn the Symbol being shifted onto the stack.
*/
public void debug_shift(Symbol shift_tkn)
{
debug_message("# Shift under term #" + shift_tkn.sym +
" to state #" + shift_tkn.parse_state);
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Do debug output for stack state. [CSA]
*/
public void debug_stack() {
StringBuilder sb=new StringBuilder("## STACK:");
for (int i=0; i<stack.size(); i++) {
Symbol s = stack.get(i);
sb.append(" <state "+s.parse_state+", sym "+s.sym+">");
if ((i%3)==2 || (i==(stack.size()-1))) {
debug_message(sb.toString());
sb = new StringBuilder(" ");
}
}
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Perform a parse with debugging output. This does exactly the
* same things as parse(), except that it calls debug_shift() and
* debug_reduce() when shift and reduce moves are taken by the parser
* and produces various other debugging messages.
*/
public Symbol debug_parse()
throws java.lang.Exception
{
/* the current action code */
int act;
/* the Symbol/stack element returned by a reduce */
Symbol lhs_sym = null;
/* information about production being reduced with */
short handle_size, lhs_sym_num;
/* set up direct reference to tables to drive the parser */
production_tab = production_table();
action_tab = action_table();
reduce_tab = reduce_table();
debug_message("# Initializing parser");
/* initialize the action encapsulation object */
init_actions();
/* do user initialization */
user_init();
/* the current Symbol */
cur_token = scan();
debug_message("# Current Symbol is #" + cur_token.sym);
/* push dummy Symbol with start state to get us underway */
stack.removeAllElements();
stack.push(new Symbol(0, start_state()));
tos = 0;
/* continue until we are told to stop */
for (_done_parsing = false; !_done_parsing; )
{
/* Check current token for freshness. */
if (cur_token.used_by_parser)
throw new Error("Symbol recycling detected (fix your scanner).");
/* current state is always on the top of the stack */
//debug_stack();
/* look up action out of the current state with the current input */
act = get_action((stack.peek()).parse_state, cur_token.sym);
/* decode the action -- > 0 encodes shift */
if (act > 0)
{
/* shift to the encoded state by pushing it on the stack */
cur_token.parse_state = act-1;
cur_token.used_by_parser = true;
debug_shift(cur_token);
stack.push(cur_token);
tos++;
/* advance to the next Symbol */
cur_token = scan();
debug_message("# Current token is " + cur_token);
}
/* if its less than zero, then it encodes a reduce action */
else if (act < 0)
{
/* perform the action for the reduce */
lhs_sym = do_action((-act)-1, this, stack, tos);
/* look up information about the production */
lhs_sym_num = production_tab[(-act)-1][0];
handle_size = production_tab[(-act)-1][1];
debug_reduce((-act)-1, lhs_sym_num, handle_size);
/* pop the handle off the stack */
for (int i = 0; i < handle_size; i++)
{
stack.pop();
tos--;
}
/* look up the state to go to from the one popped back to */
act = get_reduce((stack.peek()).parse_state, lhs_sym_num);
debug_message("# Reduce rule: top state " +
(stack.peek()).parse_state +
", lhs sym " + lhs_sym_num + " -> state " + act);
/* shift to that state */
lhs_sym.parse_state = act;
lhs_sym.used_by_parser = true;
stack.push(lhs_sym);
tos++;
debug_message("# Goto state #" + act);
}
/* finally if the entry is zero, we have an error */
else if (act == 0)
{
/* call user syntax error reporting routine */
syntax_error(cur_token);
/* try to error recover */
if (!error_recovery(true))
{
/* if that fails give up with a fatal syntax error */
unrecovered_syntax_error(cur_token);
/* just in case that wasn't fatal enough, end parse */
done_parsing();
} else {
lhs_sym = stack.peek();
}
}
}
return lhs_sym;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/* Error recovery code */
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Attempt to recover from a syntax error. This returns false if recovery
* fails, true if it succeeds. Recovery happens in 4 steps. First we
* pop the parse stack down to a point at which we have a shift out
* of the top-most state on the error Symbol. This represents the
* initial error recovery configuration. If no such configuration is
* found, then we fail. Next a small number of "lookahead" or "parse
* ahead" Symbols are read into a buffer. The size of this buffer is
* determined by error_sync_size() and determines how many Symbols beyond
* the error must be matched to consider the recovery a success. Next,
* we begin to discard Symbols in attempt to get past the point of error
* to a point where we can continue parsing. After each Symbol, we attempt
* to "parse ahead" though the buffered lookahead Symbols. The "parse ahead"
* process simulates that actual parse, but does not modify the real
* parser's configuration, nor execute any actions. If we can parse all
* the stored Symbols without error, then the recovery is considered a
* success. Once a successful recovery point is determined, we do an
* actual parse over the stored input -- modifying the real parse
* configuration and executing all actions. Finally, we return the the
* normal parser to continue with the overall parse.
*
* @param debug should we produce debugging messages as we parse.
*/
protected boolean error_recovery(boolean debug)
throws java.lang.Exception
{
if (debug) debug_message("# Attempting error recovery");
/* first pop the stack back into a state that can shift on error and
do that shift (if that fails, we fail) */
if (!find_recovery_config(debug))
{
if (debug) debug_message("# Error recovery fails");
return false;
}
/* read ahead to create lookahead we can parse multiple times */
read_lookahead();
/* repeatedly try to parse forward until we make it the required dist */
for (;;)
{
/* try to parse forward, if it makes it, bail out of loop */
if (debug) debug_message("# Trying to parse ahead");
if (try_parse_ahead(debug))
{
break;
}
/* if we are now at EOF, we have failed */
if (lookahead[0].sym == EOF_sym())
{
if (debug) debug_message("# Error recovery fails at EOF");
return false;
}
/* otherwise, we consume another Symbol and try again */
if (debug)
debug_message("# Consuming Symbol #" + cur_err_token().sym);
restart_lookahead();
}
/* we have consumed to a point where we can parse forward */
if (debug) debug_message("# Parse-ahead ok, going back to normal parse");
/* do the real parse (including actions) across the lookahead */
parse_lookahead(debug);
/* we have success */
return true;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Determine if we can shift under the special error Symbol out of the
* state currently on the top of the (real) parse stack.
*/
protected boolean shift_under_error()
{
/* is there a shift under error Symbol */
return get_action((stack.peek()).parse_state, error_sym()) > 0;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Put the (real) parse stack into error recovery configuration by
* popping the stack down to a state that can shift on the special
* error Symbol, then doing the shift. If no suitable state exists on
* the stack we return false
*
* @param debug should we produce debugging messages as we parse.
*/
protected boolean find_recovery_config(boolean debug)
{
Symbol error_token;
int act;
if (debug) debug_message("# Finding recovery state on stack");
/* Remember the right-position of the top symbol on the stack */
int right_pos = (stack.peek()).right;
int left_pos = (stack.peek()).left;
/* pop down until we can shift under error Symbol */
while (!shift_under_error())
{
/* pop the stack */
if (debug)
debug_message("# Pop stack by one, state was # " +
(stack.peek()).parse_state);
left_pos = stack.pop().left;
tos--;
/* if we have hit bottom, we fail */
if (stack.empty())
{
if (debug) debug_message("# No recovery state found on stack");
return false;
}
}
/* state on top of the stack can shift under error, find the shift */
act = get_action((stack.peek()).parse_state, error_sym());
if (debug)
{
debug_message("# Recover state found (#" +
(stack.peek()).parse_state + ")");
debug_message("# Shifting on error to state #" + (act-1));
}
/* build and shift a special error Symbol */
error_token = new Symbol(error_sym(), left_pos, right_pos);
error_token.parse_state = act-1;
error_token.used_by_parser = true;
stack.push(error_token);
tos++;
return true;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Lookahead Symbols used for attempting error recovery "parse aheads". */
protected Symbol lookahead[];
/** Position in lookahead input buffer used for "parse ahead". */
protected int lookahead_pos;
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Read from input to establish our buffer of "parse ahead" lookahead
* Symbols.
*/
protected void read_lookahead() throws java.lang.Exception
{
/* create the lookahead array */
lookahead = new Symbol[error_sync_size()];
/* fill in the array */
for (int i = 0; i < error_sync_size(); i++)
{
lookahead[i] = cur_token;
cur_token = scan();
}
/* start at the beginning */
lookahead_pos = 0;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Return the current lookahead in our error "parse ahead" buffer. */
protected Symbol cur_err_token() { return lookahead[lookahead_pos]; }
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Advance to next "parse ahead" input Symbol. Return true if we have
* input to advance to, false otherwise.
*/
protected boolean advance_lookahead()
{
/* advance the input location */
lookahead_pos++;
/* return true if we didn't go off the end */
return lookahead_pos < error_sync_size();
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Reset the parse ahead input to one Symbol past where we started error
* recovery (this consumes one new Symbol from the real input).
*/
protected void restart_lookahead() throws java.lang.Exception
{
/* move all the existing input over */
for (int i = 1; i < error_sync_size(); i++)
lookahead[i-1] = lookahead[i];
/* read a new Symbol into the last spot */
cur_token = scan();
lookahead[error_sync_size()-1] = cur_token;
/* reset our internal position marker */
lookahead_pos = 0;
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Do a simulated parse forward (a "parse ahead") from the current
* stack configuration using stored lookahead input and a virtual parse
* stack. Return true if we make it all the way through the stored
* lookahead input without error. This basically simulates the action of
* parse() using only our saved "parse ahead" input, and not executing any
* actions.
*
* @param debug should we produce debugging messages as we parse.
*/
protected boolean try_parse_ahead(boolean debug)
throws java.lang.Exception
{
int act;
short lhs, rhs_size;
/* create a virtual stack from the real parse stack */
virtual_parse_stack vstack = new virtual_parse_stack(stack);
/* parse until we fail or get past the lookahead input */
for (;;)
{
/* look up the action from the current state (on top of stack) */
act = get_action(vstack.top(), cur_err_token().sym);
/* if its an error, we fail */
if (act == 0) return false;
/* > 0 encodes a shift */
if (act > 0)
{
/* push the new state on the stack */
vstack.push(act-1);
if (debug) debug_message("# Parse-ahead shifts Symbol #" +
cur_err_token().sym + " into state #" + (act-1));
/* advance simulated input, if we run off the end, we are done */
if (!advance_lookahead()) return true;
}
/* < 0 encodes a reduce */
else
{
/* if this is a reduce with the start production we are done */
if ((-act)-1 == start_production())
{
if (debug) debug_message("# Parse-ahead accepts");
return true;
}
/* get the lhs Symbol and the rhs size */
lhs = production_tab[(-act)-1][0];
rhs_size = production_tab[(-act)-1][1];
/* pop handle off the stack */
for (int i = 0; i < rhs_size; i++)
vstack.pop();
if (debug)
debug_message("# Parse-ahead reduces: handle size = " +
rhs_size + " lhs = #" + lhs + " from state #" + vstack.top());
/* look up goto and push it onto the stack */
vstack.push(get_reduce(vstack.top(), lhs));
if (debug)
debug_message("# Goto state #" + vstack.top());
}
}
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
/** Parse forward using stored lookahead Symbols. In this case we have
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