From 1a856c90999c8f3049adea1b9fcd74256f601420 Mon Sep 17 00:00:00 2001
From: Arkadiusz Kondas <arkadiusz.kondas@gmail.com>
Date: Sat, 22 Jun 2019 22:54:47 +0200
Subject: [PATCH] Fix division by zero in ANOVA for small size dataset (#391)

---
 src/Math/Statistic/ANOVA.php               |  4 +++
 tests/FeatureSelection/SelectKBestTest.php | 42 ++++++++++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/src/Math/Statistic/ANOVA.php b/src/Math/Statistic/ANOVA.php
index d233f84..f89309e 100644
--- a/src/Math/Statistic/ANOVA.php
+++ b/src/Math/Statistic/ANOVA.php
@@ -45,6 +45,10 @@ final class ANOVA
             return $s / $dfbn;
         }, $ssbn);
         $msw = array_map(function ($s) use ($dfwn) {
+            if ($dfwn === 0) {
+                return 1;
+            }
+
             return $s / $dfwn;
         }, $sswn);
 
diff --git a/tests/FeatureSelection/SelectKBestTest.php b/tests/FeatureSelection/SelectKBestTest.php
index ebf119b..5239954 100644
--- a/tests/FeatureSelection/SelectKBestTest.php
+++ b/tests/FeatureSelection/SelectKBestTest.php
@@ -61,6 +61,48 @@ final class SelectKBestTest extends TestCase
         );
     }
 
+    public function testSelectKBestIssue386(): void
+    {
+        $samples = [
+            [
+                0.0006729998475705993,
+                0.0,
+                0.999999773507577,
+                0.0,
+                0.0,
+                6.66666515671718E-7,
+                3.33333257835859E-6,
+                6.66666515671718E-6,
+            ],
+            [
+                0.0006729998475849566,
+                0.0,
+                0.9999997735289103,
+                0.0,
+                0.0,
+                6.666665156859402E-7,
+                3.3333325784297012E-6,
+                1.3333330313718804E-6,
+            ],
+        ];
+
+        $targets = [15.5844, 4.45284];
+
+        $selector = new SelectKBest(2);
+        $selector->fit($samples, $targets);
+
+        self::assertEquals([
+            -2.117582368135751E-22,
+            0.0,
+            0.0,
+            0.0,
+            0.0,
+            1.0097419586828951E-28,
+            0.0,
+            1.4222215779620095E-11,
+        ], $selector->scores());
+    }
+
     public function testThrowExceptionOnEmptyTargets(): void
     {
         $this->expectException(InvalidArgumentException::class);