change intersect order to reduce time (#36339)

base/abstractset.jl

@@ -146,6 +146,14 @@ Set{Float64} with 1 element:
 ```
 """
 function intersect(s::AbstractSet, itr, itrs...)
+    # heuristics to try to `intersect` with the shortest Set on the left
+    if length(s)>50 && haslength(itr) && all(haslength, itrs)
+        min_length, min_idx = findmin(length, itrs)
+        if length(itr) > min_length
+            new_itrs = setindex(itrs, itr, min_idx)
+            return intersect(s, itrs[min_idx], new_itrs...)
+        end
+    end
     T = promote_eltype(s, itr, itrs...)
     if T == promote_eltype(s, itr)
         out = intersect(s, itr)
@@ -156,7 +164,13 @@ function intersect(s::AbstractSet, itr, itrs...)
     return intersect!(out, itrs...)
 end
 intersect(s) = union(s)
-intersect(s::AbstractSet, itr) = mapfilter(in(s), push!, itr, emptymutable(s, promote_eltype(s, itr)))
+function intersect(s::AbstractSet, itr)
+    if haslength(itr) && hasfastin(itr) && length(s) < length(itr)
+        return mapfilter(in(itr), push!, s, emptymutable(s, promote_eltype(s, itr)))
+    else
+        return mapfilter(in(s), push!, itr, emptymutable(s, promote_eltype(s, itr)))
+    end
+end
 
 const ∩ = intersect
 

test/sets.jl

@@ -242,6 +242,9 @@ end
         s = S([1,2]) ∩ S([3,4])
         @test s == S()
         s = intersect(S([5,6,7,8]), S([7,8,9]))
+        slong = S(collect(3:63))
+        # test #36339 length/order short-cut
+        @test intersect(S([5,6,7,8]), slong) == intersect(slong, S([5,6,7,8]))
         @test s == S([7,8])
         @test intersect(S([2,3,1]), S([4,2,3]), S([5,4,3,2])) == S([2,3])
         let s1 = S([1,2,3])