implement standard deviation of population function

src/Phpml/Exception/InvalidArgumentException.php

@@ -23,4 +23,22 @@ class InvalidArgumentException extends \Exception
     {
         return new self(sprintf('%s must be between 0.0 and 1.0', $name));
     }
+
+    /**
+     * @return InvalidArgumentException
+     */
+    public static function arrayCantBeEmpty()
+    {
+        return new self('The array has zero elements');
+    }
+
+    /**
+     * @param int $minimumSize
+     *
+     * @return InvalidArgumentException
+     */
+    public static function arraySizeToSmall($minimumSize = 2)
+    {
+        return new self(sprintf('The array must have at least %s elements', $minimumSize));
+    }
 }

src/Phpml/Math/Statistic/StandardDeviation.php

@@ -0,0 +1,45 @@
+<?php
+declare(strict_types = 1);
+
+namespace Phpml\Math\Statistic;
+
+use Phpml\Exception\InvalidArgumentException;
+
+class StandardDeviation
+{
+
+    /**
+     * @param array|float[] $a
+     * @param bool $sample
+     *
+     * @return float
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function population(array $a, $sample = true)
+    {
+        if(empty($a)) {
+            throw InvalidArgumentException::arrayCantBeEmpty();
+        }
+        
+        $n = count($a);
+
+        if ($sample && $n === 1) {
+            throw InvalidArgumentException::arraySizeToSmall(2);
+        }
+        
+        $mean = array_sum($a) / $n;
+        $carry = 0.0;
+        foreach ($a as $val) {
+            $d = $val - $mean;
+            $carry += $d * $d;
+        };
+
+        if($sample) {
+            --$n;
+        }
+
+        return sqrt($carry / $n);
+    }
+
+}

src/Phpml/Regression/LeastSquares.php

@@ -16,6 +16,16 @@ class LeastSquares implements Regression
      */
     private $targets;
 
+    /**
+     * @var float
+     */
+    private $slope;
+
+    /**
+     * @var
+     */
+    private $intercept;
+
     /**
      * @param array $features
      * @param array $targets

tests/Phpml/Math/Statistic/StandardDeviationTest.php

@@ -0,0 +1,43 @@
+<?php
+
+declare (strict_types = 1);
+
+namespace test\Phpml\Math\StandardDeviation;
+
+use Phpml\Math\Statistic\StandardDeviation;
+
+class StandardDeviationTest extends \PHPUnit_Framework_TestCase
+{
+    public function testStandardDeviationOfPopulationSample()
+    {
+        //https://pl.wikipedia.org/wiki/Odchylenie_standardowe
+        $delta = 0.001;
+        $population = [5, 6, 8, 9];
+        $this->assertEquals(1.825, StandardDeviation::population($population), '', $delta);
+
+        //http://www.stat.wmich.edu/s216/book/node126.html
+        $delta = 0.5;
+        $population = [7100, 15500, 4400, 4400, 5900, 4600, 8800, 2000, 2750, 2550,  960, 1025];
+        $this->assertEquals(4079, StandardDeviation::population($population), '', $delta);
+
+        $population = [9300,  10565,  15000,  15000,  17764,  57000,  65940,  73676,  77006,  93739, 146088, 153260];
+        $this->assertEquals(50989, StandardDeviation::population($population), '', $delta);
+    }
+
+    /**
+     * @expectedException \Phpml\Exception\InvalidArgumentException
+     */
+    public function testThrowExceptionOnEmptyArrayIfNotSample()
+    {
+        StandardDeviation::population([], false);
+    }
+
+    /**
+     * @expectedException \Phpml\Exception\InvalidArgumentException
+     */
+    public function testThrowExceptionOnToSmallArray()
+    {
+        StandardDeviation::population([1]);
+    }
+
+}