Re: Read booleans from streams

["Eric G. Miller" <egm2@jps-nospam.net>]

In <mailman.1017011702.18089.comp.lang.ada@ada.eu.org>, Erik Sigra wrote:

> I tried to read bytes from a stream and interpret each bit as a Boolean. But
> it did not work as expected. It reads 1 byte for each bit. How should I do
> instead?
> 
> 
> streamtest.adb
> ==============
> with Ada.Text_IO;           use Ada.Text_IO;
> with Ada.Streams;           use Ada.Streams;
> with Ada.Streams.Stream_IO; use Ada.Streams.Stream_IO;
> 
> procedure Streamtest is
>    type Boolean_Aggregate is array (0 .. 7) of Boolean;
>    for Boolean_Aggregate'Component_Size use 1;
> 
>    Current_Byte : Boolean_Aggregate;
> 
>    The_File : Stream_IO.File_Type;
> 
>    The_Stream : Stream_Access;
> 
> begin
> 
>    Open (The_File, In_File, "data");
> 
>    The_Stream := Stream (The_File);
> 
>    loop
>       Boolean_Aggregate'Read (The_Stream, Current_Byte);
>       for I in Boolean_Aggregate'Range loop
>          Put(Current_Byte (I)'Img & ' ');
>       end loop;
>       New_Line;
>    end loop;
> end Streamtest;
> 
> 
> data
> ====
> 02027203
> 
> 
> user > ./streamtest
> FALSE FALSE FALSE FALSE TRUE FALSE FALSE TRUE
> 
> raised ADA.IO_EXCEPTIONS.END_ERROR : s-stratt.adb:170
> 
> 
> I have checked that the size of Boolean_Aggregate is 8 and not 64.

I think the problem is in the read procedure.  It sees an array of 8 elements,
so wants to read 8 bytes.  I think you need a 'Read wrapper to read a single
byte (type Byte is mod 2**8), and then do an Unchecked_Conversion:

   function To_Boolean_Aggregate is new
	Ada.Unchecked_Conversion(Byte,Boolean_Aggregate);

   procedure Read_Boolean_Aggregate
        (Stream : Stream_Access; Data : in out Boolean_Aggregate) is
        
        Item : Byte;
   begin
        Byte'Read (Stream, Item);
        Data := To_Boolean_Aggregate (Item);
   end Read_Boolean_Aggregate;

...

   while not End_Of_File (File_Type) loop
        Read_Boolean_Aggregate (Stream, Data);

        ...
   end loop;