[Boost-users] large enum and native array indexing

Subject: [Boost-users] large enum and native array indexing
From: Hicham Mouline (hicham_at_[hidden])
Date: 2009-01-14 13:18:52

hello,
we have a 3rd party lib which provides an enum OP with some hundred members,
but each member is defined with numbers that are not contiguous.

enum OP { op1=56, op2=6, op3=5 ..... };

We have a performance critical function f that looks like this:

status f( double& result, OP , int thing );

typical usage calls f( result0, op0 ) then f(result1, op1) .... f(resultn, opn) 
on the same thing.
these calls, if merged, could have some speedup as there is some repetitive work 
done for say op3 and op7.

I then thought of doing

void f( std::map< Op, pair<status, result> >& output, const std::set<OP>&, int 
thing ) to collapse the above calls to 1.
Inside this f, we set results in output with the [] operator.

then I compared this to

void f( double results[], status statues[], const std::set<OP>&, int thing ), 
thanks to the native array contiguous access, it was 4 times faster.

therefore i wrote a numerical pseudo-metafunction "Position" to take the enum OP 
as an input and return a size_t, reflecting the index for the array.

A call would be like:

std::set<OP> opset;   opset.set( op1 ); opset.set( op2 ); ... opset.set( op7 );
double results[7]; status statuses[7];
f( results, statuses, opset, thing );

// then to read output
results[  Position<OP5>::value ];

Now, the question is about defining the metafunction Position.

template< OP op > struct Position {};
template<> struct Position<OP5> { const size_t value=4; };
....

The 3rd party enum OP in the header file I can't change.

Is there a way to generate my metafunction automatically?
Any facility in mpl or fusion instead of defining my own metafunction?

regards,