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From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: compile time Least Common Multiple,
 was Re: Ada and Internet stuff
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Date: Fri, 31 Aug 2001 17:37:56 GMT
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Xref: archiver1.google.com comp.lang.ada:12611
Date: 2001-08-31T17:37:56+00:00
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>Why are you doing that, though?  What you would generally want to figure
>out with a stream is the minimum number of stream elements that will be
>required to cover your data object's 'size.
  No, I'm not concerned here with converting from my data objects to stream
elements, but with encoding successive 24 bit chunks from a supplied
stream_element_array.  So I need an Unchecked_Conversion from a
subtype of stream_element_array of some length, to an array of 24 bit chunks,
so I need to know how many stream_element's are needed to make that
subtype an even multiple of 24 bits.

  type Twenty_Four_Bits is ...
  for Twenty_Four_Bits'size use 24;

  -- LCM is the Least Common Multiple of 24 and Stream_Element'size

  type Group_Of_Encodees is array(1 .. LCM/24) of Twenty_Four_Bits;
  pragma pack(Group_Of_Encodees);
  for Group_Of_Encodees'size use LCM;

  subtype Set_Of_Stream_Elements
  is Stream_Element_array(1 .. LCM/Stream_Element'size);
  -- assuming System.Storage_Unit = Stream_Element'size,
  -- then Set_Of_Stream_Elements'size = LCM = Group_Of_Encodees'size

  function SE_To_Groups is new Ada.Unchecked_Conversion -- LCM Bits to LCM Bits
    (Source=>Set_Of_Stream_Elements,
     Target=>Group_Of_Encodees);

  function Encode(Source : in Stream_Element_array) return String is
    The_Group : Group_Of_Encodees;
    si : Stream_Element_offset := Source'first;
  begin
    while si <= Source'last loop
      The_Group := SE_To_Groups(Source(si .. si+Group_Of_Encodees'length-1));
      for i in The_Group'range loop
        Encode_And_Emit(The_Group(i));  -- handle a set of 24 Bits
      end loop;
      ...