Re: First time Ada has let me down

[nospam.andrew.logue@cdcgy.com (Shifty)]

Ken.Garlington@computer.org (Ken Garlington) wrote in 
<sJAJ5.8835$NP.676523@news.flash.net>:

>I'm curious about the "typecasting a 4 bit value into an 8 bit integer"
>statement. Why would you need to do this?

Apparently I don't, per comments from Ted and others.  I just assumed
that it wouldn't be considered an integer type since it is smaller than
the system.storage_unit size.

>I would have thought the record representation clause approach would be much
>preferable than a bunch of
>#DEFINEs to describe the header layout. Granted, the fact that the headers
>are variable length complicates
>things a bit, but just looking at an IPv4 header, I would expect something
>like the source code below. In fact,
>if you use this approach (records with variant), you might be able to avoid
>some of the offset calculations altogether.
>You certainly don't need to do any low-level bit twiddling (which is why
>this wasn't as big of a deal in Ada83 as
>you might think...)
>
>package IPv4 is
>
>type Byte is range 0 .. 16#FF#;
>for Byte'Size use 8;
>
>type Version_Type is range 4 .. 4;

>subtype Header_Length_Type is Integer range 5 .. 15;
>subtype Total_Length_Type is Integer range 0 .. 65_535;
>
>-- more types for the rest of the header
>
>type Option_Type is array (1 .. 4) of Byte;
>pragma Pack (Option_Type);
>type Option_Type_Array is array (Integer range <>) of Option_Type;
>pragma Pack (Option_Type_Array);
>
>type Header_Type (IHL : Header_Length_Type) is record
>   Version : Version_Type := 4;
>   Type_of_Service : Byte := 0;
>   Total_Length : Total_Length_Type;
>
>   -- the rest of the mandatory fields
>
>   case IHL is
>      when 5 =>
>         null;
>      when 6 .. Header_Length_Type'Last =>
>         Option : Option_Type_Array(6 .. IHL);
>   end case;
>end record;
>
>-- record rep spec goes here
>
>end IPv4;

Yep, this looks like a good solution, but it just seemed like too much
code to simply calculate the IP header length.  In the end I came up with
something like this:

  function Calculate_Ip_Header_Length  
             (Byte_At : in System.Address) return Integer is

    Version_Length_Descriptor : Octet_Type;
    for Version_Length_Descriptor use at Byte_At;

    Temp : Integer := Integer (Version_Length_Descriptor);

    --  7 6 5 4 3 2 1 0 
    -- +-+-+-+-+-+-+-+-+
    -- | | | | | | | | |
    -- +-+-+-+-+-+-+-+-+
    -- | Ver   | Hlen  |

    -- Define a mask to mask out the high-order bits.
    --                                   ||||
    Version_Mask : constant Integer := 2#00001111#;

    -- Mask it.
    --
    Length : Integer := V_Bits.Bit_And (Temp, Version_Mask);

  begin -- Calculate_Ip_Header_Length

    -- The Hlen field gives the datagram hdr len measured in 32-bit words. 
    --
    return Length * (32 / System.Storage_Unit); 

  end Calculate_Ip_Header_Length;


I don't want to start a war over this, since there is _always_ a better
way of doing things, but this solution seems simple and elegant, especially
when I don't give hoot about the rest of the header structure.  You just
pass the start address of the IP datagram to this function and it quickly
returns the byte-length of the header.  (assuming, of course, that Hlen
contains the right value!)

Thankfully Ada95 has native language support for bitwise integer operators.


Thank you for your comments guys.


Andrew.