# Boost Users :

Subject: Re: [Boost-users] Needs advices on design ( mpl or processor )
From: Larry Evans (cppljevans_at_[hidden])
Date: 2012-01-10 15:42:41

On 01/10/12 14:08, Allan Nielsen wrote:
[snip]
>> the largest enumeration. However, in the case of StructModel_r, I
>> don't understand:
>>
>> static const size_t size = head::size * _tail::size;
>>
>> around line 200. I would think that there would just be additions
>> since you want to store one value after another. That's why, in my
>> previous post, in template sum_bits, there's:
>>
>> , integral_c<unsigned,Bits+enum_bits<Enum>::size>
>>
>> Could you explain why multiplication instead of addition is used to
>> calculate the size in StructModel_r?
>
> This is because the offset is not really an offset, I just could not
> come up with a better name. Here is the explanation:
>
> Assume we need to store 5 different enumerated types which each
> represents 3 different values ( like the A, B C).
>
> These 5 enums can all together represent 3^5 = 243 different values (
> AAAAA, AAAAB, AAAAC, AAABA, AAABB ...).
>
> As 243 is less than 256 ( all the different values in a char), it is
> possible to store the values in a char. But in a binary number system
> 2 bits are required to store one tri-state value. If we use 3 * 5 bits
> we end up with 15 bits ( more than one char) which is not the most
> efficient encoding.
>
> Therefore we encode this in a base_3 number system which is defined
> by: ... d_2 * n^2 + d_1 * n^1 + d_0 * n^0, where d is the digits we
> want to encode, and n is 3 because all the enums are tri-states.
>
> Example: Encode BCACB -> 12021
>
> 1*3^4 + 2*3^3 + 0*3^2 + 2*3^1 + 1*3^0 = 142
>
> To extract enum number 4 the reverse operation must be done: (142 / 3^3) % 3 = 2
>
> If different enums are with different sizes are used then n in the
> equation above will be different for each digit. Then to set digit
> number 4 one has to multiply n_0 * n_1 * n_2 * n_3 and then use this
> as base. Therefore the multiplications.

Ah! Now I see. This reminds me of apl decode and encode:

the radix vector mentioned there represents the sizes (=max value+1)
of the enums to be stored. For example, let:

enum E_0{e0_0,e0_1,e0_2,...,e0_n0};
enum E_1{e1_0,e1_1,...,e1_n1};
...
enum E_m{em_0,em_1,...,em_nm};

then the radix vector, rv, would be:

unsigned rv[m+1]={n0+1,n1+2,...,nm+1};

and to encode the enum values:

struct Ev
{ E_0 e0;
E_1 e1;
...
E_m em;
};
Ev ev={e0_i0, e1_i1, ..., em_im};

decode would be used:

unsigned dv=decode(rv,ev)

where decode is the c++ equivalent of the decode described on encode.htm.