Add PReLU activation function (#128)

* Implement RELU activation functions

* Add PReLUTest

src/Phpml/NeuralNetwork/ActivationFunction/PReLU.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Phpml\NeuralNetwork\ActivationFunction;
+
+use Phpml\NeuralNetwork\ActivationFunction;
+
+class PReLU implements ActivationFunction
+{
+    /**
+     * @var float
+     */
+    private $beta;
+
+    /**
+     * @param float $beta
+     */
+    public function __construct($beta = 0.01)
+    {
+        $this->beta = $beta;
+    }
+
+    /**
+     * @param float|int $value
+     *
+     * @return float
+     */
+    public function compute($value): float
+    {
+        return $value >= 0 ? $value : $this->beta * $value;
+    }
+}

tests/Phpml/NeuralNetwork/ActivationFunction/PReLUTest.php

@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+namespace tests\Phpml\NeuralNetwork\ActivationFunction;
+
+use Phpml\NeuralNetwork\ActivationFunction\PReLU;
+use PHPUnit\Framework\TestCase;
+
+class PReLUTest extends TestCase
+{
+    /**
+     * @param $beta
+     * @param $expected
+     * @param $value
+     *
+     * @dataProvider preluProvider
+     */
+    public function testPReLUActivationFunction($beta, $expected, $value)
+    {
+        $prelu = new PReLU($beta);
+
+        $this->assertEquals($expected, $prelu->compute($value), '', 0.001);
+    }
+
+    /**
+     * @return array
+     */
+    public function preluProvider()
+    {
+        return [
+            [0.01, 0.367, 0.367],
+            [0.0, 1, 1],
+            [0.3, -0.3, -1],
+            [0.9, 3, 3],
+            [0.02, -0.06, -3],
+        ];
+    }
+}