Add ThresholdedReLU activation function (#129)

src/Phpml/NeuralNetwork/ActivationFunction/ThresholdedReLU.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Phpml\NeuralNetwork\ActivationFunction;
+
+use Phpml\NeuralNetwork\ActivationFunction;
+
+class ThresholdedReLU implements ActivationFunction
+{
+    /**
+     * @var float
+     */
+    private $theta;
+
+    /**
+     * @param float $theta
+     */
+    public function __construct($theta = 1.0)
+    {
+        $this->theta = $theta;
+    }
+
+    /**
+     * @param float|int $value
+     *
+     * @return float
+     */
+    public function compute($value): float
+    {
+        return $value > $this->theta ? $value : 0.0;
+    }
+}

tests/Phpml/NeuralNetwork/ActivationFunction/ThresholdedReLUTest.php

@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace tests\Phpml\NeuralNetwork\ActivationFunction;
+
+use Phpml\NeuralNetwork\ActivationFunction\ThresholdedReLU;
+use PHPUnit\Framework\TestCase;
+
+class ThresholdedReLUTest extends TestCase
+{
+    /**
+     * @param $theta
+     * @param $expected
+     * @param $value
+     *
+     * @dataProvider thresholdProvider
+     */
+    public function testThresholdedReLUActivationFunction($theta, $expected, $value)
+    {
+        $thresholdedReLU = new ThresholdedReLU($theta);
+
+        $this->assertEquals($expected, $thresholdedReLU->compute($value));
+    }
+
+    /**
+     * @return array
+     */
+    public function thresholdProvider()
+    {
+        return [
+            [1.0, 0, 1.0],
+            [0.5, 3.75, 3.75],
+            [0.0, 0.5, 0.5],
+            [0.9, 0, 0.1]
+        ];
+    }
+}