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--  Heapsort using access to procedure parameters

--  This package provides a heapsort routine that works with access to
--  subprogram parameters, so that it can be used with different types with
--  shared sorting code. See also GNAT.Heap_Sort_G, the generic version,
--  which is a little more efficient but does not allow code sharing.
--  The generic version is also Pure, while the access version can
--  only be Preelaborate.

package GNAT.Heap_Sort_A is
pragma Preelaborate (Heap_Sort_A);

   --  The data to be sorted is assumed to be indexed by integer values from
   --  1 to N, where N is the number of items to be sorted. In addition, the
   --  index value zero is used for a temporary location used during the sort.

   type Move_Procedure is access procedure (From : Natural; To : Natural);
   --  A pointer to a procedure that moves the data item with index From to
   --  the data item with index To. An index value of zero is used for moves
   --  from and to the single temporary location used by the sort.

   type Lt_Function is access function (Op1, Op2 : Natural) return Boolean;
   --  A pointer to a function that compares two items and returns True if
   --  the item with index Op1 is less than the item with index Op2, and False
   --  if the Op1 item is greater than or equal to the Op2 item.

   procedure Sort (N : Natural; Move : Move_Procedure; Lt : Lt_Function);
   --  This procedures sorts items in the range from 1 to N into ascending
   --  order making calls to Lt to do required comparisons, and Move to move
   --  items around. Note that, as described above, both Move and Lt use a
   --  single temporary location with index value zero. This sort is not
   --  stable, i.e. the order of equal elements in the input is not preserved.

end GNAT.Heap_Sort_A;