Improve performance symmetry of Set.intersect

docs/release-notes/.FSharp.Core/10.0.300.md

@@ -1,5 +1,6 @@
 ### Fixed
 
+* Optimize Set.intersect performance symmetry and preserve identity from the first set argument. ([PR #19292](https://github.com/dotnet/fsharp/pull/19291)) (Fixes #19292)
 * Fix anonymous record field ordering in LINQ expression conversion to produce consistent expression trees regardless of field declaration order. ([Issue #11131](https://github.com/dotnet/fsharp/issues/11131), [Issue #15648](https://github.com/dotnet/fsharp/issues/15648))
 * Fix array indexing in LINQ expressions to generate proper array index expressions instead of GetArray method calls, enabling LINQ providers like Azure Cosmos DB to translate array access. ([Issue #16918](https://github.com/dotnet/fsharp/issues/16918))
 * Fix tuple join conditions and groupBy operations to properly compare tuple keys using structural equality. AnonymousObject types now implement Equals and GetHashCode, enabling inline tuple joins like `join b on ((a.Id1, a.Id2) = (b.Id1, b.Id2))` to work correctly. ([Issue #7885](https://github.com/dotnet/fsharp/issues/7885), [Issue #47](https://github.com/dotnet/fsharp/issues/47))

src/FSharp.Core/set.fs

@@ -267,6 +267,21 @@ module internal SetTree =
                 elif c = 0 then true
                 else mem comparer k tn.Right
 
+    let rec tryGet (comparer: IComparer<'T>) k (t: SetTree<'T>) =
+        if isEmpty t then
+            None
+        else
+            let c = comparer.Compare(k, t.Key)
+
+            if t.Height = 1 then
+                if c = 0 then Some t.Key else None
+            else
+                let tn = asNode t
+
+                if c < 0 then tryGet comparer k tn.Left
+                elif c = 0 then Some tn.Key
+                else tryGet comparer k tn.Right
+
     let rec iter f (t: SetTree<'T>) =
         if isEmpty t then
             ()
@@ -391,6 +406,24 @@ module internal SetTree =
 
                 balance comparer (union comparer t2n.Left lo) t2n.Key (union comparer t2n.Right hi)
 
+    let rec intersectionAuxFromSmall comparer a (t: SetTree<'T>) acc =
+        if isEmpty t then
+            acc
+        else if t.Height = 1 then
+            match tryGet comparer t.Key a with
+            | Some v -> add comparer v acc
+            | None -> acc
+        else
+            let tn = asNode t
+            let acc = intersectionAuxFromSmall comparer a tn.Right acc
+
+            let acc =
+                match tryGet comparer tn.Key a with
+                | Some v -> add comparer v acc
+                | None -> acc
+
+            intersectionAuxFromSmall comparer a tn.Left acc
+
     let rec intersectionAux comparer b (t: SetTree<'T>) acc =
         if isEmpty t then
             acc
@@ -412,7 +445,13 @@ module internal SetTree =
             intersectionAux comparer b tn.Left acc
 
     let intersection comparer a b =
-        intersectionAux comparer b a empty
+        let n1 = count a
+        let n2 = count b
+
+        if n1 <= n2 then
+            intersectionAux comparer b a empty
+        else
+            intersectionAuxFromSmall comparer a b empty
 
     let partition1 comparer f k (acc1, acc2) =
         if f k then