[ET Trac] [Einstein Toolkit] #1020: Reduce WeylScal4 code size

[Einstein Toolkit]

#1020: Reduce WeylScal4 code size
------------------------------------+---------------------------------------
  Reporter:  eschnett               |       Owner:        
      Type:  enhancement            |      Status:  review
  Priority:  major                  |   Milestone:        
 Component:  EinsteinToolkit thorn  |     Version:        
Resolution:                         |    Keywords:        
------------------------------------+---------------------------------------

Comment (by eschnett):

 No, there is no mechanism to give hints to CSE.

 Hmm.
 {{{
 gamma[ua,lb,lc] -> gu[ua,ud] (PD[g[ld,lb],lc] + PD[lb,ld],lc] -
 PD[g[lb,lc],ld])
 CSEHint -> {PD[g[ld,lb],lc] + PD[lb,ld],lc] - PD[g[lb,lc],ld]}
 }}}
 That doesn't look nicer than my solution:
 {{{
 gammal[ld,lb,lc] -> PD[g[ld,lb],lc] + PD[lb,ld],lc] - PD[g[lb,lc],ld],
 gamma[ua,lb,lc] -> gu[ua,ud] gammal[ld,lb,lc]
 }}}

 Or maybe:
 {{{
 gamma[ua,lb,lc] -> gu[ua,ud] Save[PD[g[ld,lb],lc] + PD[lb,ld],lc] -
 PD[g[lb,lc],ld]]
 }}}
 where "Save" indicates that the expression should be saved in a temporary
 variable.

 Or maybe Kranc should do this grouping automatically when it expands
 indices for implicit sums.

 Maybe it would suffice to factor out those terms in a produce that are
 independent of the index that is expanded. That is, a term
 {{{
 A B[ui] C[li]
 }}}
 becomes then
 {{{
 A (B1 C1 + B2 C2 + B3 C3)
 }}}
 instead of repeating A three times. In long expressions, especially if
 multiple indices are expanded in the same product, this could be a big
 winner.

 Alternatively, and much more difficult, CSE could be looking for
 expressions with this pattern:
 {{{
 A X1 + A X2 + A X3
 }}}
 and either transform this to
 {{{
 A (X1 + X2 + X3)
 }}}
 or at least identify A as a common subexpression. Of course, A would be a
 whole expression, and could be "spread out", i.e. wildly intermixed with
 the Xs.

-- 
Ticket URL: <https://trac.einsteintoolkit.org/ticket/1020#comment:5>
Einstein Toolkit <http://einsteintoolkit.org>
The Einstein Toolkit