fix apply on js and native

vecxt/js/src/package.scala

@@ -113,7 +113,7 @@ object extensions:
 
   extension (vec: NArray[Double])
 
-    inline def idxBoolean(index: js.Array[Boolean])(using inline boundsCheck: BoundsCheck.BoundsCheck) =
+    inline def apply(index: js.Array[Boolean])(using inline boundsCheck: BoundsCheck.BoundsCheck) =
       dimCheck(vec, index)
       val trues = index.countTrue
       val newVec = Float64Array(trues)
@@ -125,7 +125,7 @@ object extensions:
           j = 1 + j
       end for
       newVec
-    end idxBoolean
+    end apply
 
     def increments: Float64Array =
       val out = Float64Array(vec.length)

vecxt/jvm/src/package.scala

@@ -271,7 +271,28 @@ object extensions:
     def variance: Double =
       // https://www.cuemath.com/sample-variance-formula/
       val μ = vec.mean
-      vec.map(i => (i - μ) * (i - μ)).sum / (vec.length - 1)
+      // vec.map(i => (i - μ) * (i - μ)).sum / (vec.length - 1)
+      val sp = Matrix.doubleSpecies
+      val l = sp.length()
+      var tmp = DoubleVector.zero(sp)
+
+      var i = 0
+      while i < sp.loopBound(vec.length) do
+        val v = DoubleVector.fromArray(sp, vec, i)
+        val diff = v.sub(μ)
+        tmp = tmp.add(diff.mul(diff))
+        i += l
+      end while
+
+      var sumSqDiff = tmp.reduceLanes(VectorOperators.ADD)
+
+      while i < vec.length do
+        sumSqDiff = sumSqDiff + (vec(i) - μ) * (vec(i) - μ)
+        i += 1
+      end while
+
+      sumSqDiff / (vec.length - 1)
+
     end variance
 
     inline def stdDev: Double =

vecxt/native/src/package.scala

@@ -87,7 +87,7 @@ object extensions:
 
   extension (vec: Array[Double])
 
-    def idxBoolean(index: Array[Boolean]) =
+    def apply(index: Array[Boolean]) =
       val trues = index.countTrue
       val newVec = new Array[Double](trues)
       var j = 0
@@ -98,7 +98,7 @@ object extensions:
           j += 1
       end for
       newVec
-    end idxBoolean
+    end apply
 
     def increments: Array[Double] =
       val out = new Array[Double](vec.length)

vecxt/test/src/arrayExtensions.test.scala

@@ -211,6 +211,14 @@ class ArrayExtensionSuite extends munit.FunSuite:
     assertEqualsDouble(afterIndex.last, 3.0, 0.0001)
   }
 
+  test("variance".only) {
+    // https://www.storyofmathematics.com/sample-variance/#:~:text=7.%20Divide%20the%20number%20you%20get%20in%20step%206%20by example 3
+    val ages = NArray[Double](26.0, 48.0, 67.0, 39.0, 25.0, 25.0, 36.0, 44.0, 44.0, 47.0, 53.0, 52.0, 52.0, 51.0, 52.0,
+      40.0, 77.0, 44.0, 40.0, 45.0, 48.0, 49.0, 19.0, 54.0, 82.0)
+    val variance = ages.variance
+    assertEqualsDouble(variance, 216.82, 0.01)
+  }
+
   test("tvar") {
     import vecxt.rpt.tVar
     val v1 = NArray.tabulate(100)(_.toDouble)