[phylo] performance boost when there's lots of periods

src/Gargantext/Core/Viz/Phylo/PhyloMaker.hs

@@ -385,7 +385,14 @@ toSeriesOfClusteringFis phylo phyloDocs = fromList $ parMap rpar (func (corpusPa
 toSeriesOfClusteringMaxClique :: Phylo -> Map (Date, Date) [Document] -> (Double, MaxCliqueFilter) -> Map (Date,Date) [Clustering]
 toSeriesOfClusteringMaxClique phylo phyloDocs (thr, filterType) = fromList mcl
  where
-   mcl  = parMap rpar (\(prd,docs) ->
+   -- This looks innocent but is a big performance
+   -- improvement. Suppose there are few docs but many periods (like
+   -- when one computes phylo with second or minute
+   -- resolution). Without pattern matching on empty docs (many
+   -- periods will have no docs associated), one will fire
+   -- `getMaxCliques` with all the Accelerate machinery.
+   mapFunc (prd, []) = (prd, [])
+   mapFunc (prd, docs) = 
                let cooc = map round
                         $ foldl sumCooc empty
                         $ map (\d -> listToMatrix $ ngramsToIdx (text d) (getRoots phylo)) docs
@@ -395,8 +402,8 @@ toSeriesOfClusteringMaxClique phylo phyloDocs (thr, filterType) = fromList mcl
                                             , _clustering_period = prd
                                             , _clustering_visWeighting = Nothing
                                             , _clustering_visFiltering = [] })
-                         $ getMaxCliques filterType Conditional thr cooc))
-          $ toList phyloDocs
+                         $ getMaxCliques filterType Conditional thr cooc)
+   mcl  = parMap rpar mapFunc $ toList phyloDocs
 --------------------------------------
 
         -- dev viz graph maxClique getMaxClique