implement minkowski distance metric function

src/Phpml/Metric/Distance/Minkowski.php

@@ -0,0 +1,49 @@
+<?php
+
+declare (strict_types = 1);
+
+namespace Phpml\Metric\Distance;
+
+use Phpml\Exception\InvalidArgumentException;
+use Phpml\Metric\Distance;
+
+class Minkowski implements Distance
+{
+    /**
+     * @var float
+     */
+    private $lambda;
+
+    /**
+     * @param float $lambda
+     */
+    public function __construct(float $lambda = 3)
+    {
+        $this->lambda = $lambda;
+    }
+
+
+    /**
+     * @param array $a
+     * @param array $b
+     *
+     * @return float
+     *
+     * @throws InvalidArgumentException
+     */
+    public function distance(array $a, array $b): float
+    {
+        if (count($a) !== count($b)) {
+            throw InvalidArgumentException::sizeNotMatch();
+        }
+
+        $distance = 0;
+        $count = count($a);
+
+        for ($i = 0; $i < $count; ++$i) {
+            $distance += pow(abs($a[$i] - $b[$i]), $this->lambda);
+        }
+
+        return pow($distance, 1 / $this->lambda);
+    }
+}

tests/Phpml/Metric/Distance/MinkowskiTest.php

@@ -0,0 +1,77 @@
+<?php
+
+declare (strict_types = 1);
+
+namespace tests\Phpml\Metric;
+
+use Phpml\Metric\Distance\Minkowski;
+
+class MinkowskiTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * @var Minkowski
+     */
+    private $distanceMetric;
+
+    public function setUp()
+    {
+        $this->distanceMetric = new Minkowski();
+    }
+
+    /**
+     * @expectedException \Phpml\Exception\InvalidArgumentException
+     */
+    public function testThrowExceptionOnInvalidArguments()
+    {
+        $a = [0, 1, 2];
+        $b = [0, 2];
+
+        $this->distanceMetric->distance($a, $b);
+    }
+
+    public function testCalculateDistanceForOneDimension()
+    {
+        $a = [4];
+        $b = [2];
+
+        $expectedDistance = 2;
+        $actualDistance = $this->distanceMetric->distance($a, $b);
+
+        $this->assertEquals($expectedDistance, $actualDistance);
+    }
+
+    public function testCalculateDistanceForTwoDimensions()
+    {
+        $a = [4, 6];
+        $b = [2, 5];
+
+        $expectedDistance = 2.080;
+        $actualDistance = $this->distanceMetric->distance($a, $b);
+
+        $this->assertEquals($expectedDistance, $actualDistance, '', $delta=0.001);
+    }
+
+    public function testCalculateDistanceForThreeDimensions()
+    {
+        $a = [6, 10, 3];
+        $b = [2, 5, 5];
+
+        $expectedDistance = 5.819;
+        $actualDistance = $this->distanceMetric->distance($a, $b);
+
+        $this->assertEquals($expectedDistance, $actualDistance, '', $delta=0.001);
+    }
+
+    public function testCalculateDistanceForThreeDimensionsWithDifferentLambda()
+    {
+        $distanceMetric = new Minkowski($lambda = 5);
+
+        $a = [6, 10, 3];
+        $b = [2, 5, 5];
+
+        $expectedDistance = 5.300;
+        $actualDistance = $distanceMetric->distance($a, $b);
+
+        $this->assertEquals($expectedDistance, $actualDistance, '', $delta=0.001);
+    }
+}