implement RBF kernel function

src/Phpml/Math/Kernel.php

@@ -0,0 +1,17 @@
+<?php
+declare(strict_types = 1);
+
+namespace Phpml\Math;
+
+interface Kernel
+{
+
+    /**
+     * @param float $a
+     * @param float $b
+     *
+     * @return float
+     */
+    public function compute($a, $b);
+
+}

src/Phpml/Math/Kernel/RBF.php

@@ -0,0 +1,39 @@
+<?php
+declare(strict_types = 1);
+
+namespace Phpml\Math\Kernel;
+
+use Phpml\Math\Kernel;
+use Phpml\Math\Product;
+
+class RBF implements Kernel
+{
+    /**
+     * @var float
+     */
+    private $gamma;
+
+    /**
+     * @param float $gamma
+     */
+    public function __construct(float $gamma)
+    {
+        $this->gamma = $gamma;
+    }
+
+    /**
+     * @param float $a
+     * @param float $b
+     *
+     * @return float
+     */
+    public function compute($a, $b)
+    {
+        $score = 2 * Product::scalar($a, $b);
+        $squares = Product::scalar($a, $a) + Product::scalar($b, $b);
+        $result = exp(-$this->gamma * ($squares - $score));
+
+        return $result;
+    }
+
+}

tests/Phpml/Math/Kernel/RBFTest.php

@@ -0,0 +1,26 @@
+<?php
+declare(strict_types = 1);
+
+namespace test\Phpml\Math\Kernel;
+
+use Phpml\Math\Kernel\RBF;
+
+class RBFTest extends \PHPUnit_Framework_TestCase
+{
+
+    public function testComputeRBFKernelFunction()
+    {
+        $rbf = new RBF($gamma = 0.001);
+
+        $this->assertEquals(1, $rbf->compute([1, 2], [1, 2]));
+        $this->assertEquals(0.97336, $rbf->compute([1, 2, 3], [4, 5, 6]), '', $delta = 0.0001);
+        $this->assertEquals(0.00011, $rbf->compute([4, 5], [1, 100]), '', $delta = 0.0001);
+
+        $rbf = new RBF($gamma = 0.2);
+
+        $this->assertEquals(1, $rbf->compute([1, 2], [1, 2]));
+        $this->assertEquals(0.00451, $rbf->compute([1, 2, 3], [4, 5, 6]), '', $delta = 0.0001);
+        $this->assertEquals(0, $rbf->compute([4, 5], [1, 100]));
+    }
+
+}