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-- $Source: /nosc/work/parser/RCS/ParseStk.spc,v $
-- $Revision: 4.0 $ -- $Date: 85/02/19 11:33:03 $ -- $Author: carol $

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

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

package Parsestack is
    --| Elements awaiting parsing

    --| Overview
    --|
    --| The ParseStack 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

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

    package Pd renames Parserdeclarations;

    ------------------------------------------------------------------
    -- Declarations Global to Package ParseStack
    ------------------------------------------------------------------

    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 Pd.Parsestackelement);  --| element to add

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

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

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

    function Pop --| Removes top element in stack
                return Pd.Parsestackelement;

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

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

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

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

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

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

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

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

    --| Effects
    --|
    --| Pops and discards top N 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 Parsestack;

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