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digitalmars.D.learn - How can I map bytes to a matrix of structures?

reply teo <teo.ubuntu yahoo.com> writes:
Here is an example of what I am after:

struct DATA
{
  ubyte D1;
  ubyte D2;
  ubyte D3;
  ubyte D4;
}

void main()
{
  ubyte[16] a = [ 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x01, 
0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 ];
  auto b = (cast(DATA*)a.ptr)[0 .. 4];
  auto c = (cast(DATA[]*)b.ptr)[0 .. 2][0 .. 2];
}

I need to have a DATA[2][2]. That code compiles but gives me a 
segmentation fault.
Sep 09 2011
parent reply Timon Gehr <timon.gehr gmx.ch> writes:
On 09/09/2011 05:19 PM, teo wrote:
 Here is an example of what I am after:

 struct DATA
 {
    ubyte D1;
    ubyte D2;
    ubyte D3;
    ubyte D4;
 }

 void main()
 {
    ubyte[16] a = [ 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x01,
 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 ];
    auto b = (cast(DATA*)a.ptr)[0 .. 4];
    auto c = (cast(DATA[]*)b.ptr)[0 .. 2][0 .. 2];
 }

 I need to have a DATA[2][2]. That code compiles but gives me a
 segmentation fault.

If you actually want a dynamic DATA[2][] array of length 2, this works: auto b=(*(cast(DATA[2][2]*)a.ptr))[]; Otherwise: A simple reinterpret cast should do: auto b=*(cast(DATA[2][2]*)a.ptr); but note that this copies the data, because static arrays have value semantics. If you want to have refer the new array to the same location, you can use a union. void main(){ union Myunion{ ubyte[16] a = [ 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 ]; DATA[2][2] b; } Myunion myunion; assert(*(cast(DATA[2][2]*)myunion.a.ptr)==myunion.b); }
Sep 09 2011
next sibling parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Fri, 09 Sep 2011 11:43:04 -0400, Timon Gehr <timon.gehr gmx.ch> wrote:

 On 09/09/2011 05:19 PM, teo wrote:
 Here is an example of what I am after:

 struct DATA
 {
    ubyte D1;
    ubyte D2;
    ubyte D3;
    ubyte D4;
 }

 void main()
 {
    ubyte[16] a = [ 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x01,
 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 ];
    auto b = (cast(DATA*)a.ptr)[0 .. 4];
    auto c = (cast(DATA[]*)b.ptr)[0 .. 2][0 .. 2];
 }

 I need to have a DATA[2][2]. That code compiles but gives me a
 segmentation fault.

If you actually want a dynamic DATA[2][] array of length 2, this works: auto b=(*(cast(DATA[2][2]*)a.ptr))[]; Otherwise: A simple reinterpret cast should do: auto b=*(cast(DATA[2][2]*)a.ptr); but note that this copies the data, because static arrays have value semantics. If you want to have refer the new array to the same location, you can use a union. void main(){ union Myunion{ ubyte[16] a = [ 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 ]; DATA[2][2] b; } Myunion myunion; assert(*(cast(DATA[2][2]*)myunion.a.ptr)==myunion.b); }

You can also use ref, but you have to use a function, as it's impossible to declare a ref local variable except as a function parameter. void main() { ubyte[16] a = ...; void foo(ref DATA[2][2] b) { ... } foo(*(cast(DATA[2][2]*)a.ptr)); } -Steve
Sep 09 2011
prev sibling next sibling parent bearophile <bearophileHUGS lycos.com> writes:
Timon Gehr:

 A simple reinterpret cast should do:
 auto b=*(cast(DATA[2][2]*)a.ptr);

I think this is enough: auto b = cast(Data[2][2])a; Bye, bearophile
Sep 09 2011
prev sibling next sibling parent reply teo <teo.ubuntu yahoo.com> writes:
On Fri, 09 Sep 2011 17:43:04 +0200, Timon Gehr wrote:

 On 09/09/2011 05:19 PM, teo wrote:
 Here is an example of what I am after:

 struct DATA
 {
    ubyte D1;
    ubyte D2;
    ubyte D3;
    ubyte D4;
 }

 void main()
 {
    ubyte[16] a = [ 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08,
    0x01,
 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 ];
    auto b = (cast(DATA*)a.ptr)[0 .. 4];
    auto c = (cast(DATA[]*)b.ptr)[0 .. 2][0 .. 2];
 }

 I need to have a DATA[2][2]. That code compiles but gives me a
 segmentation fault.

If you actually want a dynamic DATA[2][] array of length 2, this works: auto b=(*(cast(DATA[2][2]*)a.ptr))[]; Otherwise: A simple reinterpret cast should do: auto b=*(cast(DATA[2][2]*)a.ptr); but note that this copies the data, because static arrays have value semantics. If you want to have refer the new array to the same location, you can use a union. void main(){ union Myunion{ ubyte[16] a = [ 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08,

 			0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 ];
 	DATA[2][2] b;
      }
      Myunion myunion;
      assert(*(cast(DATA[2][2]*)myunion.a.ptr)==myunion.b);
 }

Thank you Timon for the good explanation. Just one more question (I suspect the answer will be no, but let me ask): is it possible to directly cast to ubyte[][]?
Sep 09 2011
parent Timon Gehr <timon.gehr gmx.ch> writes:
On 09/09/2011 10:25 PM, teo wrote:
 On Fri, 09 Sep 2011 17:43:04 +0200, Timon Gehr wrote:

 On 09/09/2011 05:19 PM, teo wrote:
 Here is an example of what I am after:

 struct DATA
 {
     ubyte D1;
     ubyte D2;
     ubyte D3;
     ubyte D4;
 }

 void main()
 {
     ubyte[16] a = [ 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08,
     0x01,
 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 ];
     auto b = (cast(DATA*)a.ptr)[0 .. 4];
     auto c = (cast(DATA[]*)b.ptr)[0 .. 2][0 .. 2];
 }

 I need to have a DATA[2][2]. That code compiles but gives me a
 segmentation fault.

If you actually want a dynamic DATA[2][] array of length 2, this works: auto b=(*(cast(DATA[2][2]*)a.ptr))[]; Otherwise: A simple reinterpret cast should do: auto b=*(cast(DATA[2][2]*)a.ptr); but note that this copies the data, because static arrays have value semantics. If you want to have refer the new array to the same location, you can use a union. void main(){ union Myunion{ ubyte[16] a = [ 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08,

 			0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 ];
 	DATA[2][2] b;
       }
       Myunion myunion;
       assert(*(cast(DATA[2][2]*)myunion.a.ptr)==myunion.b);
 }

Thank you Timon for the good explanation.

You are welcome. As bearophile suggests, those are nicer though: auto b=(cast(DATA[2][2]a); // static array auto b=(cast(DATA[2][2]a)[];// dynamic array
 Just one more question (I suspect the answer will be no, but let me ask):
 is it possible to directly cast to ubyte[][]?

Not directly, because you have to build some structure in memory. an ubyte[][] is an array of dynamic arrays. Each of those dynamic arrays is a 2 field struct consisting of a ptr and a length field. That data you have to construct manually. in case you wanted to turn the ubyte[16] a array to a ubyte[][] b array, with b.length and b[i].length equal to 4, this would probably do the job: ubyte[16] a=[ 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 ]; auto b=new ubyte[][](4);// create a ubyte[][] array that can hold 4 ubyte[] values foreach(i,ref x;b) x=a[i*4 .. (i+1)*4]; // compute the values by slicing the original array
Sep 09 2011
prev sibling parent teo <teo.ubuntu yahoo.com> writes:
On Fri, 09 Sep 2011 13:10:41 -0400, bearophile wrote:

 Timon Gehr:
 
 A simple reinterpret cast should do:
 auto b=*(cast(DATA[2][2]*)a.ptr);

I think this is enough: auto b = cast(Data[2][2])a; Bye, bearophile

That works. Thanks.
Sep 09 2011