Category : C Source Code
Archive   : TBISON0.ZIP
Filename : STATE.H

/* state.h Type definitions for nondeterministic finite state machine
for bison, copyright (C) 1984 Bob Corbett and Richard Stallman */


/* These type definitions are used to represent a nondeterministic
finite state machine that parses the specified grammar.
This information is generated by the function generate_states
in the file LR0.

Each state of the machine is described by a set of items --
particular positions in particular rules -- that are the possible
places where parsing could continue when the machine is in this state.
These symbols at these items are the allowable inputs
that can follow now.

A core represents one state. States are numbered in the number field.
When generate_states is finished, the starting state is state 0
and nstates is the number of states. (A transition to a state
whose state number is nstates indicates termination.) All the cores
are chained together and first_state points to the first one (state 0).

For each state there is a particular symbol which must have been the
last thing accepted to reach that state. It is the accessing_symbol
of the core.

Each core contains a vector of nitems items which are the indices
in the ritems vector of the items that are selected in this state.

The link field is used for chaining buckets that hash states by
their itemsets. This is for recognizing equivalent states and
combining them when the states are generated.

The two types of transitions are shifts (push the lookahead token
and read another) and reductions (combine the last n things on the
stack via a rule, replace them with the symbol that the rule derives,
and leave the lookahead token alone). When the states are generated,
these transitions are represented in two other lists.

Each shifts structure describes the possible shift transitions
out of one state, the state whose number is in the number field.
The shifts structures are linked through next and first_shift points
to them. Each contains a vector of numbers of the states that shift
transitions can go to. The accessing_symbol fields of those states'
cores say what kind of input leads to them.

A shift to state zero should be ignored. Conflict resolution
deletes shifts by changing them to zero.

Each reductions structure describes the possible reductions at the
state whose number is in the number field. The data is a list of
nreds rules, represented by their rule numbers. first_reduction
points to the list of these structures.

Conflict resolution can decide that certain tokens in certain
states should explicitly be errors (for implementing %nonassoc).
For each state, the tokens that are errors for this reason
are recorded in an errs structure, which has the state number
in its number field. The rest of the errs structure is full
of token numbers.

There is at least one shift transition present in state zero.
It leads to a next-to-final state whose accessing_symbol is
the grammar's start symbol. The next-to-final state has one shift
to the final state, whose accessing_symbol is zero (end of input).
The final state has one shift, which goes to the termination state
(whose number is nstates, and for which there is no core structure).
The reason for the extra state at the end is to placate the parser's
strategy of making all decisions one token ahead of its actions. */


typedef
struct core
{
struct core *next;
short number;
struct core *link;
short accessing_symbol;
short nitems;
short items[1];
}
core;



typedef
struct shifts
{
struct shifts *next;
short number;
short nshifts;
short shift[1];
}
shifts;



typedef
struct errs
{
short nerrs;
short errors[1];
}
errs;



typedef
struct reductions
{
struct reductions *next;
short number;
short nreds;
short rules[1];
}
reductions;



extern int nstates;
extern core *first_state;
extern shifts *first_shift;
extern reductions *first_reduction;