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digitalmars.D.learn - data mapping, more elegant solution?

reply mandel <oh no.es> writes:
Hi,

I want to map a uint to a position of an ubyte array.
Atm. I use this:

ubyte[8] array;
*cast(uint*) &array.ptr[0] = 42;

But is there a nicer solution to do achive this?

Smth. like  "array[0..4] = 42;", but this tries to assign 42 to every 
byte (resulting in [0x2a, 0x2a, 0x2a, 0x2a, 0x00, 0x00, 0x00, 0x00]).
Dec 13 2007
next sibling parent reply Regan Heath <regan netmail.co.nz> writes:
mandel wrote:

 Hi,
 
 I want to map a uint to a position of an ubyte array.
 Atm. I use this:
 
 ubyte[8] array;
 *cast(uint*) &array.ptr[0] = 42;
 
 But is there a nicer solution to do achive this?
 
 Smth. like  "array[0..4] = 42;", but this tries to assign 42 to every 
 byte (resulting in [0x2a, 0x2a, 0x2a, 0x2a, 0x00, 0x00, 0x00, 0x00]).


Use a union?

union thing
{
   ubyte[8] ub;
   uint     ui;
}

void main()
{
   thing a;
   a.ui = 42;
}

Regan
Dec 13 2007
parent reply mandel <oh no.es> writes:
On Thu, 13 Dec 2007 13:35:35 +0000, Regan Heath wrote:

 Use a union?
 
 union thing
 {
    ubyte[8] ub;
    uint     ui;
 }
 
 void main()
 {
    thing a;
    a.ui = 42;
 }
 


This way I can't insert data at arbitrary places, e.g. array[4..8].
I also would have to cast thing to ubyte[8] when I pass it to functions.
It's also hackish. :P
Dec 13 2007
next sibling parent reply Regan Heath <regan netmail.co.nz> writes:
mandel wrote:

 On Thu, 13 Dec 2007 13:35:35 +0000, Regan Heath wrote:

 Use a union?

 union thing
 {
    ubyte[8] ub;
    uint     ui;
 }

 void main()
 {
    thing a;
    a.ui = 42;
 }


 This way I can't insert data at arbitrary places, e.g. array[4..8].
 I also would have to cast thing to ubyte[8] when I pass it to functions.
 It's also hackish. :P


If you want to insert a 'short' into it then add an array of shorts too:

union thing
{
    ubyte[8]  ub;
    ushort[2] us;
    uint      ui;
}

thing a;

a.us[0] = 1; //ub[0..4]
a.us[1] = 2; //ub[4..8]

It's less hackish, and cleaner/clearer than using casts.

Regan
Dec 13 2007
parent Regan Heath <regan netmail.co.nz> writes:
Regan Heath wrote:

 mandel wrote:

 On Thu, 13 Dec 2007 13:35:35 +0000, Regan Heath wrote:

 Use a union?

 union thing
 {
    ubyte[8] ub;
    uint     ui;
 }

 void main()
 {
    thing a;
    a.ui = 42;
 }


 This way I can't insert data at arbitrary places, e.g. array[4..8].
 I also would have to cast thing to ubyte[8] when I pass it to functions.
 It's also hackish. :P


 
 If you want to insert a 'short' into it then add an array of shorts too:
 
 union thing
 {
    ubyte[8]  ub;
    ushort[2] us;
    uint      ui;
 }
 
 thing a;
 
 a.us[0] = 1; //ub[0..4]
 a.us[1] = 2; //ub[4..8]
 
 It's less hackish, and cleaner/clearer than using casts.


I must have been half asleep when I posted that.  I got the sizes 
confused.  You have 64 bits of data, so your complete union would look 
something like:

union thing
{
   ubyte[8]	ub;
   ushort[4]	us;
   uint[2]	ui;
   ulong		ul;
}

I got the assignments correct though, a.us[0] assigns to a.ub[0..4] and 
so on.  You may need to deal with big/little endian issues, not sure.

If you really hate the union and would rather use a cast I suggest:

ubyte[8] array;

(cast(uint*)&array)[0] = 1;  //array[0..4]
(cast(uint*)&array)[1] = 2;  //array[4..8]

I think that works, I don't have a compiler here to test it, it might 
need to be:

(cast(uint*)&array)[0..2][0] = 1;  //array[0..4]
(cast(uint*)&array)[0..2][1] = 2;  //array[4..8]

The above creates a temporary dynamic array of type uint with length == 
2, which is less efficient than the union .. and more typing too.

Regan
Dec 13 2007
prev sibling parent Mike <vertex gmx.at> writes:
On Thu, 13 Dec 2007 14:43:51 +0100, mandel <oh no.es> wrote:


 On Thu, 13 Dec 2007 13:35:35 +0000, Regan Heath wrote:

 Use a union?

 union thing
 {
    ubyte[8] ub;
    uint     ui;
 }

 void main()
 {
    thing a;
    a.ui =3D 42;
 }


 This way I can't insert data at arbitrary places, e.g. array[4..8].
 I also would have to cast thing to ubyte[8] when I pass it to function=


s.

 It's also hackish. :P


Then pack the hackish things away into something:

struct thing
{
     union
     {
         ubyte[8] ub;
         uint ui;
     }

     void opAssign(uint value)
     {
         ui =3D value;
     }

     uint opCall()
     {
         return value;
     }

     void opIndexAssign(byte value, uint idx)
     {
         ub[idx] =3D value;
     }

     byte opIndex(uint idx)
     {
         return ub[idx];
     }
}

Better? You can pass that to functions. For arbitrary places you're eith=
er  =

stuck with the casting or you maybe can do something with opSlice.

-Mike

-- =

Using Opera's revolutionary e-mail client: http://www.opera.com/mail/
Dec 13 2007
prev sibling parent 0ffh <frank frankhirsch.youknow.what.todo.net> writes:
mandel wrote:

 Hi,
 
 I want to map a uint to a position of an ubyte array.
 Atm. I use this:
 
 ubyte[8] array;
 *cast(uint*) &array.ptr[0] = 42;
 
 But is there a nicer solution to do achive this?
 
 Smth. like  "array[0..4] = 42;", but this tries to assign 42 to every 
 byte (resulting in [0x2a, 0x2a, 0x2a, 0x2a, 0x00, 0x00, 0x00, 0x00]).


Well, if "array[0..4]=42;" does what you say, I think it does the right
thing. Your solution is the right one, altough I'd use more braces.
It's hard to be more elegant when crossing type boundaries. Some other
languages I could mention would not even allow the canonical C solution
(which is what you are doing)...

regards, frank
Dec 13 2007