# Boost Users :

Subject: Re: [Boost-users] Needs advices on design ( mpl or processor )
From: Larry Evans (cppljevans_at_[hidden])
Date: 2012-01-10 23:22:58

On 01/10/12 15:35, Larry Evans wrote:
> On 01/10/12 14:42, Larry Evans wrote:
>> 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.
>>
>> Is that about right?
>>
>> -regards,
>> Larry
> Also, something similar is done for multidimensional array classes.
> for example, given an m-dimensional array, a, the array index
> expression:
>
> a[i0][i1]...[im],
>
> accesses some element in internal data array:
>
> Type data[(n0+1)*(n1+1)*...*(nm+1)];
>
> where ni, for i=1...m, is the max index for the i-th dimension.
>
> The offset value into data for the is given by:
>
> i0*s0+i1*s1+...+im*sm
>
> where s0,...sm are the strides of the array, and the strides are just
> the partial products of the ni's. IOW, for fortran storage order:
>
> s0=1;
> s1=s0*(n0+1);
> s2=s1*(n1+1);
> ...
>
> I think indices_at_offset here:
>

>
> which returns the array indices corresponding to some offset into
> the array, is doing something similar to what your get<Tag>() is doing.
> However, get<Tag> just returns 1 index where Tag corresponds to some
> dimension in the array and indices_at_offset resturns all the
> indices.
>
> Sound right?
>
> -regards,
> Larry

Based on what I just learned from your explanations and code, there's
no need for either the CRTP or the if_recur. Neither is used in
the attached, which just implements the compressed tuple and produces
output:

> :eos_t::stride=24
> compressed_enums_impl
> < pair< T1, E1>
> : compressed_enums_impl
> < pair< T2, E2>
> : compressed_enums_impl
> < pair< T3, E3>
> : compressed_enums_impl
> <
> >::stride=1
> >::stride=4
> >::stride=12
>>::stride=24
> :get<T1>=0
> :get<T2>=0
> :get<T3>=0
> putting:
> put<T1>(1)
> put<T2>(2)
> put<T3>(3)
> :get<T1>=1
> :get<T2>=2
> :get<T3>=3
> putting:
> put<T1>(0)
> put<T2>(1)
> put<T3>(2)
> :get<T1>=0
> :get<T2>=1
> :get<T3>=2

Looking at your code made me realize there was no need vor the CRTP
to allow access to storage, it could just be passed as an argument
to functions higher up in the inheritance hierarchy. Thanks.

-regards,
Larry