implement ConfusionMatrix metric

src/Phpml/Metric/ConfusionMatrix.php

@@ -0,0 +1,71 @@
+<?php
+
+declare (strict_types = 1);
+
+namespace Phpml\Metric;
+
+class ConfusionMatrix
+{
+    /**
+     * @param array $actualLabels
+     * @param array $predictedLabels
+     * @param array $labels
+     *
+     * @return array
+     */
+    public static function compute(array $actualLabels, array $predictedLabels, array $labels = null): array
+    {
+        $labels = $labels ? array_flip($labels) : self::getUniqueLabels($actualLabels);
+        $matrix = self::generateMatrixWithZeros($labels);
+
+        foreach ($actualLabels as $index => $actual) {
+            $predicted = $predictedLabels[$index];
+
+            if (!isset($labels[$actual]) || !isset($labels[$predicted])) {
+                continue;
+            }
+
+            if ($predicted === $actual) {
+                $row = $column = $labels[$actual];
+            } else {
+                $row = $labels[$actual];
+                $column = $labels[$predicted];
+            }
+
+            $matrix[$row][$column] += 1;
+        }
+
+        return $matrix;
+    }
+
+    /**
+     * @param array $labels
+     *
+     * @return array
+     */
+    private static function generateMatrixWithZeros(array $labels): array
+    {
+        $count = count($labels);
+        $matrix = [];
+
+        for ($i = 0; $i < $count; ++$i) {
+            $matrix[$i] = array_fill(0, $count, 0);
+        }
+
+        return $matrix;
+    }
+
+    /**
+     * @param array $labels
+     *
+     * @return array
+     */
+    private static function getUniqueLabels(array $labels): array
+    {
+        $labels = array_values(array_unique($labels));
+        sort($labels);
+        $labels = array_flip($labels);
+
+        return $labels;
+    }
+}

tests/Phpml/Metric/ConfusionMatrixTest.php

@@ -0,0 +1,61 @@
+<?php
+
+declare (strict_types = 1);
+
+namespace tests\Phpml\Metric;
+
+use Phpml\Metric\ConfusionMatrix;
+
+class ConfusionMatrixTest extends \PHPUnit_Framework_TestCase
+{
+    public function testComputeConfusionMatrixOnNumericLabels()
+    {
+        $actualLabels = [2, 0, 2, 2, 0, 1];
+        $predictedLabels = [0, 0, 2, 2, 0, 2];
+
+        $confusionMatrix = [
+            [2, 0, 0],
+            [0, 0, 1],
+            [1, 0, 2],
+        ];
+
+        $this->assertEquals($confusionMatrix, ConfusionMatrix::compute($actualLabels, $predictedLabels));
+    }
+
+    public function testComputeConfusionMatrixOnStringLabels()
+    {
+        $actualLabels = ['cat', 'ant', 'cat', 'cat', 'ant', 'bird'];
+        $predictedLabels = ['ant', 'ant', 'cat', 'cat', 'ant', 'cat'];
+
+        $confusionMatrix = [
+            [2, 0, 0],
+            [0, 0, 1],
+            [1, 0, 2],
+        ];
+
+        $this->assertEquals($confusionMatrix, ConfusionMatrix::compute($actualLabels, $predictedLabels));
+    }
+
+    public function testComputeConfusionMatrixOnLabelsWithSubset()
+    {
+        $actualLabels = ['cat', 'ant', 'cat', 'cat', 'ant', 'bird'];
+        $predictedLabels = ['ant', 'ant', 'cat', 'cat', 'ant', 'cat'];
+        $labels = ['ant', 'bird'];
+
+        $confusionMatrix = [
+            [2, 0],
+            [0, 0],
+        ];
+
+        $this->assertEquals($confusionMatrix, ConfusionMatrix::compute($actualLabels, $predictedLabels, $labels));
+
+        $labels = ['bird', 'ant'];
+
+        $confusionMatrix = [
+            [0, 0],
+            [0, 2],
+        ];
+
+        $this->assertEquals($confusionMatrix, ConfusionMatrix::compute($actualLabels, $predictedLabels, $labels));
+    }
+}