On 05/29/2012 08:21 PM, Eric Niebler wrote:

On 5/29/2012 1:44 AM, Karsten Ahnert wrote:

I have an arithmetic expression template where multiplication is
commutative. Is there an easy way to order a chain of multiplications
such that terminals with values (like proto::terminal< double >) appear
at the beginning? For example that

arg1 * arg1 * 1.5 * arg1

will be transformed to

1.5 * arg1 * arg1 * arg1

?

I can imagine some complicated algorithms swapping expressions and child
expressions but I wonder if there is a simpler way.

There is no clever built-in Proto algorithm for commutative
transformations like this, I'm afraid. I was going to suggest flattening
to a fusion vector and using fusion sort, but I see there is no fusion
sort! :-( Nevertheless, that seems like a promising direction to me.
Once you have the sorted vector, you should(?) be able to use
fusion::fold to build the correct proto tree from it.

Won't having a way to build it properly from the get go be a better solution ?

This basically require the feature we spoke about earlier so that building a X * Y
node check which from X or Y is a double and put it in the proper place ?

Then when doing X * Expr, check if there is a double at child<0> of expr
and restructure the whole tree at generation time ?