# ifstack.icn:  Object to manage conditional expansion for template.icn
#--

$ifndef __IFSTACK_ICN__
$define __IFSTACK_ICN__

$define  IF_STACK_REVISION  "$Revision: 1.3 $"
$define  IF_STACK_DATE      "$Date: 1995/11/16 18:52:45 $"

#================================================================
# Class IfStack:  Represents the state of all currently open
# conditional expansion tokens in a template, that is, all
# "%ifnext;" and similar conditionals for which we have not yet
# seen the matching "%endif;".
#----------------------------------------------------------------
# ifStack  :=  If_Stack_New ( )
#   [ returns an empty ifStack object ]
#--
# If_Stack_Start ( ifStack, cond )
#   [ pushes `cond' onto the ifStack ]
#--
# cond  :=  If_Stack_End ( ifStack )
#   [ if the ifStack is empty -> fail
#   | else ->
#       ifStack  :=  ifStack minus its top condition
#       returns the (former) top condition
#   ]
#--
# If_Stack_Else ( ifStack )
#   [ if the ifStack is empty -> fail
#   | else ->
#       ifStack  :=  ifStack with its top condition's .skipping
#                    switch inverted
#       return ifStack with its top condition's .skipping switch inverted
#   ]
#--
# If_Stack_Is_Skipping ( ifStack )
#   [ if the stack of conditions in ifStack implies that we are currently
#     skipping tokens ->
#       return &null
#   | else -> fail
#   ]
#----------------------------------------------------------------
  record ifStackTag (
     L )        # Pushdown stack of condition objects
#================================================================
# INVARIANT:
#   .L is a list of zero or more `condition' objects
#----------------------------------------------------------------


# - - -   I f _ S t a c k _ N e w   - - -

procedure If_Stack_New ( )
  local ifStack

  ifStack    :=  ifStackTag ( );
  ifStack.L  :=  [];

  return ifStack;
end # --- If_Stack_New ---


# - - -   I f _ S t a c k _ S t a r t   - - -

procedure If_Stack_Start ( ifStack, condition )
  push ( ifStack.L, condition );
end


# - - -   I f _ S t a c k _ E n d   - - -

procedure If_Stack_End ( ifStack )
  if  ( * ifStack.L ) = 0  then
    fail;

  return pop ( ifStack.L );
end


# - - -   I f _ S t a c k _ E l s e   - - -

procedure If_Stack_Else ( ifStack )
  local top

  if  ( * ifStack.L ) = 0  then
    fail;

  top  :=  ifStack.L[1];
  Condition_Else ( top );
  return ifStack;
end


# - - -   I f _ S t a c k _ I s _ S k i p p i n g   - - -

procedure If_Stack_Is_Skipping ( ifStack )
  local cond

  if  ( * ifStack.L ) = 0  then
    fail;       # No conditionals in effect -> hence, not skipping

  every  cond := ! ifStack.L  do
  { #-- Check all active conditions
    if  Condition_Is_Skipping ( cond )  then
      return &null;     # At least one conditional is skipping
  } #-- Check all active conditions

  fail;         # None of the conditionals are skipping
end

$endif