Change array type: 8bit type to 6bit type - bit

Question

I have two types and two arrays of that types in file.ads
type Ebit is mod 2**8;
type Sbit is mod 2**6;
type Data_Type is array (Positive range <>) of Ebit;
type Changed_Data_Type is array (Positive range <>) of Sbit;
and function:
function ChangeDataType (D : in Data_Type) return Changed_Data_Type
with
Pre => D'Length rem 3 = 0 and D'Last < Positive'Last / 4,
Post => ChangeDataType'Result'Length = 4 * (D'Length / 3)
Ok i can understand all of this.
For example we have arrays of:
65, 66, 65, 65, 66, 65 in 8bit values function should give to us 16, 20, 9, 1, 16, 20, 9, 1 in 6bit values.
I dont know how i can build a 6bit table from 8 bit table.
My idea of sollutions is for example taking bit by bit from type:
fill all bites in 6bit type to 0 (propably default)
if first bit (2**1) is 1 set bit (2**1) in 6bit type to 1;
and do some iterations
But i dont know how to do this, always is a problem with types. Is this good idea or i can do this with easier way? I spend last nigt to try write this but without success.
Edit:
I wrote some code, its working but i have problem with array initialization.
function ChangeDataType (D: in Data_Type) return Changed_Data_Type
is
length: Natural := (4*(D'Length / 3));
ER: Changed_Data_type(length);
Temp: Ebit;
Temp1: Ebit;
Temp2: Ebit;
Actual: Ebit;
n: Natural;
k: Natural;
begin
n := 0;
k := 0;
Temp := 2#00000000#;
Temp1 := 2#00000000#;
Temp2 := 2#00000000#;
Array_loop:
for k in D'Range loop
case n is
when 0 =>
Actual := D(k);
Temp1 := Actual / 2**2;
ER(k) := Sbit(Temp1);
Temp := Actual * ( 2**4);
n := 2;
when 2 =>
Actual := D(k);
Temp1 := Actual / 2**4;
Temp2 := Temp1 or Temp;
ER(k) := Sbit(Temp2);
Temp := Actual * ( 2**2);
n := 4;
when 4 =>
Actual := D(k);
Temp1 := Actual / 2**6;
Temp2 := Temp1 or Temp;
ER(k) := Sbit(Temp2);
n := 6;
when 6 =>
Temp1 := Actual * ( 2**2);
Temp2 := Actual / 2**2;
ER(k) := Sbit(Temp2);
n := 0;
when others =>
n := 0;
end case;
end loop Array_Loop;
return ER;
end;

Answer 1

IF I understand what you're asking... it's that you want to re-pack the same 8-bit data into 6-bit values such that the "leftover" bits of the first EBit become the first bits (highest or lowest?) of the second Sbit.
One way you can do this - at least for fixed size arrays, e.g. your 6 words * 8 bits, 8 words * 6 bits example, is by specifying the exact layout in memory for each array type, using packing, and representation aspects (or pragmas, before Ada-2012) which are nicely described here.
I haven't tested the following, but it may serve as a starting point.
type Ebit is mod 2**8;
type Sbit is mod 2**6;
for Ebit'Size use 8;
for Sbit'Size use 6;
type Data_Type is array (1 .. 6) of Ebit
with Alignment => 0; -- this should pack tightly
type Changed_Data_Type is array (1 .. 8) of Sbit
with Alignment => 0;
Then you can instantiate the generic Unchecked_Conversion function with the two array types, and use that function to convert from one array to the other.
with Ada.Unchecked_Conversion;
function Change_Type is new Ada.Unchecked_Conversion(Data_Type, Changed_Data_Type);
declare
Packed_Bytes : Changed_Data_Type := Change_Type(Original_Bytes);
begin ...
In terms of code generated, it's not slow, because Unchecked_Conversion doesn't do anything, except tell the compile-time type checking to look the other way.
I view Unchecked_Conversion like the "I meant to do that" look my cat gives me after falling off the windowledge. Again...
Alternatively, if you wish to avoid copying, you can declare Original_Bytes as aliased, and use a similar trick with access types and Unchecked_Access to overlay both arrays on the same memory (like a Union in C). I think this is what DarkestKhan calls "array overlays" in a comment below. See also section 3 of this rather dated page which describes the technique further. It notes the overlaid variable must not only be declared aliased but also volatile so that accesses to one view aren't optimised into registers, but reflect any changes made via the other view. Another approach to overlays is in the Ada Wikibook here.
Now this may be vulnerable to endian-ness considerations, i.e. it may work on some platforms but not others. The second reference above gives an example of a record with exact bit-alignment of its members : we can at least take the Bit_Order aspect, as in with Alignment => 0, Bit_Order => Low_Order_First; for the arrays above...
-- code stolen from "Rationale" ... see link above p.11
type RR is record
Code: Opcode;
R1: Register;
R2: Register;
end record
with Alignment => 2, Bit_Order => High_Order_First;
for RR use record
Code at 0 range 0 .. 7;
R1 at 1 range 0 .. 3;
R2 at 1 range 4 .. 7;
end record;
One thing that's not clear to me is if there's a formulaic way to specify the exact layout of each element in an array, as is done in a record here - or even if there's a potential need to. If necessary, one workaround would be to replace the arrays above with records. But I'd love to see a better answer if there is one.