src/state.h

   1 /* Type definitions for nondeterministic finite state machine for bison,
   2    Copyright 1984, 1989, 2000 Free Software Foundation, Inc.
   3
   4    This file is part of Bison, the GNU Compiler Compiler.
   5
   6    Bison is free software; you can redistribute it and/or modify
   7    it under the terms of the GNU General Public License as published by
   8    the Free Software Foundation; either version 2, or (at your option)
   9    any later version.
  10
  11    Bison is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14    GNU General Public License for more details.
  15
  16    You should have received a copy of the GNU General Public License
  17    along with Bison; see the file COPYING.  If not, write to
  18    the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  19    Boston, MA 02111-1307, USA.  */
  20
  21
  22 /* These type definitions are used to represent a nondeterministic
  23    finite state machine that parses the specified grammar.  This
  24    information is generated by the function generate_states in the
  25    file LR0.
  26
  27    Each state of the machine is described by a set of items --
  28    particular positions in particular rules -- that are the possible
  29    places where parsing could continue when the machine is in this
  30    state.  These symbols at these items are the allowable inputs that
  31    can follow now.
  32
  33    A core represents one state.  States are numbered in the number
  34    field.  When generate_states is finished, the starting state is
  35    state 0 and nstates is the number of states.  (A transition to a
  36    state whose state number is nstates indicates termination.)  All
  37    the cores are chained together and first_state points to the first
  38    one (state 0).
  39
  40    For each state there is a particular symbol which must have been
  41    the last thing accepted to reach that state.  It is the
  42    accessing_symbol of the core.
  43
  44    Each core contains a vector of nitems items which are the indices
  45    in the ritems vector of the items that are selected in this state.
  46
  47    The link field is used for chaining buckets that hash states by
  48    their itemsets.  This is for recognizing equivalent states and
  49    combining them when the states are generated.
  50
  51    The two types of transitions are shifts (push the lookahead token
  52    and read another) and reductions (combine the last n things on the
  53    stack via a rule, replace them with the symbol that the rule
  54    derives, and leave the lookahead token alone).  When the states are
  55    generated, these transitions are represented in two other lists.
  56
  57    Each shifts structure describes the possible shift transitions out
  58    of one state, the state whose number is in the number field.  The
  59    shifts structures are linked through next and first_shift points to
  60    them.  Each contains a vector of numbers of the states that shift
  61    transitions can go to.  The accessing_symbol fields of those
  62    states' cores say what kind of input leads to them.
  63
  64    A shift to state zero should be ignored.  Conflict resolution
  65    deletes shifts by changing them to zero.
  66
  67    Each reductions structure describes the possible reductions at the
  68    state whose number is in the number field.  The data is a list of
  69    nreds rules, represented by their rule numbers.  first_reduction
  70    points to the list of these structures.
  71
  72    Conflict resolution can decide that certain tokens in certain
  73    states should explicitly be errors (for implementing %nonassoc).
  74    For each state, the tokens that are errors for this reason are
  75    recorded in an errs structure, which has the state number in its
  76    number field.  The rest of the errs structure is full of token
  77    numbers.
  78
  79    There is at least one shift transition present in state zero.  It
  80    leads to a next-to-final state whose accessing_symbol is the
  81    grammar's start symbol.  The next-to-final state has one shift to
  82    the final state, whose accessing_symbol is zero (end of input).
  83    The final state has one shift, which goes to the termination state
  84    (whose number is nstates-1).  The reason for the extra state at the
  85    end is to placate the parser's strategy of making all decisions one
  86    token ahead of its actions.  */
  87
  88 #ifndef STATE_H_
  89 # define STATE_H_
  90
  91 typedef struct core
  92 {
  93   struct core *next;
  94   struct core *link;
  95   short number;
  96   short accessing_symbol;
  97   short nitems;
  98   short items[1];
  99 }
 100 core;
 101
 102
 103
 104 typedef struct shifts
 105 {
 106   struct shifts *next;
 107   short number;
 108   short nshifts;
 109   short shifts[1];
 110 }
 111 shifts;
 112
 113
 114
 115 typedef struct errs
 116 {
 117   short nerrs;
 118   short errs[1];
 119 }
 120 errs;
 121
 122
 123
 124 typedef struct reductions
 125 {
 126   struct reductions *next;
 127   short number;
 128   short nreds;
 129   short rules[1];
 130 }
 131 reductions;
 132
 133 #endif /* !STATE_H_ */