[repo-assist] fix: Seq.meanTruncatedBy trims both ends

src/FSharp.Stats/Seq.fs

@@ -323,18 +323,18 @@ module Seq =
     /// <example>
     /// <code>
     /// let values = [1.0; 2.0; 3.0; 4.0; 5.0]
-    /// let m = Seq.meanTruncatedBy (fun x -> x * 2.0) 0.2 values // returns 7.0
+    /// let m = Seq.meanTruncatedBy (fun x -> x * 2.0) 0.2 values // returns 6.0
     /// </code>
     /// </example>
     let inline meanTruncatedBy  (f : 'T -> ^U) (proportion:float) (data:seq<'T>) : 'U  =
         let zero = LanguagePrimitives.GenericZero< 'U > 
         let n = Seq.length(data)
         if (n > 0) then    
-            let k = int (floor (float n * proportion))
+            let k = int (float n * proportion)
             data
             |> Seq.sort
             |> Seq.skip k
-            |> Seq.take (n - k)
+            |> Seq.take (n - 2 * k)
             |> meanBy f
 
         else

tests/FSharp.Stats.Tests/Seq.fs

@@ -396,6 +396,22 @@ let seqGenTests =
     ]
 
 
+[<Tests>]
+let meanTruncatedByTests =
+    testList "Seq.meanTruncatedBy" [
+        testCase "trims both ends" <| fun () ->
+            let xs = [1.0 .. 10.0]
+            // k = 2: keeps 3..8, doubled mean = 11.0
+            Expect.floatClose Accuracy.high (Seq.meanTruncatedBy (fun x -> x * 2.0) 0.2 xs) 11.0 "Expected both ends trimmed"
+        testCase "matches meanTruncated for identity" <| fun () ->
+            let xs = [9.0; 1.0; 4.0; 100.0; -50.0; 3.0; 7.0; 2.0; 8.0; 5.0]
+            Expect.floatClose Accuracy.high (Seq.meanTruncatedBy id 0.2 xs) (Seq.meanTruncated 0.2 xs) "Expected equal results"
+        testCase "zero proportion equals mean" <| fun () ->
+            let xs = [1.0; 2.0; 3.0; 4.0]
+            Expect.floatClose Accuracy.high (Seq.meanTruncatedBy id 0.0 xs) 2.5 "Expected plain mean"
+    ]
+
+
 [<Tests>]
 let varTests =
     testList "Seq.var" [