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-- $Source: /nosc/work/parser/RCS/StateStk.spc,v $
-- $Revision: 4.0 $ -- $Date: 85/02/19 11:43:44 $ -- $Author: carol $

----------------------------------------------------------------------

with Parserdeclarations;  -- declarations for the Parser
use Parserdeclarations;

package Statestack is
    --| Elements awaiting parsing

    --| Overview
    --|
    --| The StateStack used by the parser.
    --|
    --| This data structure has the following sets of operations:
    --|
    --| 1) A set that add and delete elements.
    --| This set can raise the exceptions Underflow and Overflow.
    --| The set includes:
    --|
    --|     Pop
    --|     Push
    --|     Reduce
    --|
    --| 2) A function that returns the number of elements in the
    --| data structure.
    --| This set raises no exceptions.
    --| The set includes:
    --|
    --|     Length
    --|
    --| 3) A copy operations, to return the top of the stack.
    --| The exception, UnderFlow,
    --| is utilized to indicate the end of a sequential examination.
    --| The set includes:
    --|
    --|     CopyTop
    --|     InitCopy
    --|     CopyNext

    --| Notes
    --|
    --|     Under some implementations the exception
    --| ParserDeclarations.MemoryOverflow could be raised.
    --|

    ------------------------------------------------------------------
    -- Declarations Global to Package StateStack
    ------------------------------------------------------------------

    Overflow : exception;
    --| raised if no more space in stack.
    Underflow : exception;
    --| raised if no more elements in stack.

    ------------------------------------------------------------------

    procedure Push ( --| Adds new top element to stack
                    Element : in Statestackelement);  --| element to add

    --|
    --| Raises
    --|
    --| OverFlow - no more space in stack.

    --| Effects
    --|
    --| This subprogram adds an element to the top of the stack.
    --|

    ------------------------------------------------------------------

    function Pop return Statestackelement;  --| Removes top element in stack

    --| Raises
    --|
    --| UnderFlow - no more elements in stack.

    --| Effects
    --|
    --| This subprogram pops the element at the top of the stack.
    --|

    ------------------------------------------------------------------

    function Copytop return Statestackelement;
    --| Copy top element in stack

    --| Raises
    --|
    --| UnderFlow - no more elements in stack.
    --|

    --| Effects
    --|
    --| Returns the top of the stack.

    ------------------------------------------------------------------

    function Copynext return Statestackelement;
    --| Copy element after previous one copied

    --| Raises
    --|
    --| UnderFlow - no more elements in stack.

    --| Effects
    --|
    --| This subprogram is used in conjunction with
    --| CopyTop or Init Copy to sequentially examine the stack.
    --|

    ------------------------------------------------------------------

    function Length return Stateparsestacksindex;
    --| Returns the number of elements in the stack

    --| Effects
    --|
    --| This subprogram returns the number of elements in the stack.
    --|

    ------------------------------------------------------------------

    procedure Initcopy;  --| Initialize sequential examination of
    --| the data structure

    --| Effects
    --|
    --| Initializes the copy function,
    --| so that subsequent calls to CopyNext will sequentially examine
    --| the elements in the data structure.
    --|

    ------------------------------------------------------------------

    function Copythisone ( --| returns element given by parm 'which_one'
                          Which_One : in Stateparsestacksrange)
                         return Statestackelement;

    --| Overview
    --|
    --| Returns the state stack element indicated by the parameter
    --| 'which_one'.  This operation is needed by LocalStateStack
    --| because, in essence, the state stack is being copied in two
    --| nested loops and the Next_To_Copy counter can therefore only
    --| be used for one of the series of copies.

    ------------------------------------------------------------------

    procedure Reduce ( --| Pops and discards top n elements on
                      --| the stack.
                      Topn : in Stateparsestacksindex);
    --| Number of elements to pop.

    --| Raises:
    --|
    --| Underflow - no more elements in stack.

    --| Effects
    --|
    --| Pops and discards TopN elements on the stack.
    --| If TopN is greater than the number of elements in the stack,
    --| Underflow is raised.
    --| This subprogram is used by the parser to reduce the stack during
    --| a reduce action.
    --| This stack reduction could be done with a for
    --| loop and the Pop subprogram at a considerable cost in execution
    --| time.
    --|

    ------------------------------------------------------------------

end Statestack;

----------------------------------------------------------------------