Iterate over smaller set when computing intersection (#15497)

Closes #15496

1 files changed, 9 insertions, 2 deletions

diff --git a/lib/pure/collections/sets.nim b/lib/pure/collections/sets.nim
index 67e407576..98dedc5ff 100644
--- a/lib/pure/collections/sets.nim
+++ b/lib/pure/collections/sets.nim
@@ -422,8 +422,15 @@ proc intersection*[A](s1, s2: HashSet[A]): HashSet[A] =
     assert c == toHashSet(["b"])
 
   result = initHashSet[A](max(min(s1.data.len, s2.data.len), 2))
-  for item in s1:
-    if item in s2: incl(result, item)
+  
+  # iterate over the elements of the smaller set
+  if s1.data.len < s2.data.len:
+    for item in s1:
+      if item in s2: incl(result, item)
+  else:
+    for item in s2:
+      if item in s1: incl(result, item)
+  
 
 proc difference*[A](s1, s2: HashSet[A]): HashSet[A] =
   ## Returns the difference of the sets `s1` and `s2`.