[2017-12-08] Solution for both puzzles

Makefile

@@ -22,6 +22,8 @@ day7:
 	php day7/main.php
 
 day8:
+	php day8/main.php
+
 day9:
 day10:
 day11:

day8/main.php

@@ -0,0 +1,70 @@
+<?php
+// https://adventofcode.com/2017/day/8
+
+function puzzle(string $input, bool $returnTopValue = false) : int {
+    $lines = explode(PHP_EOL, $input);
+    $variables = [];
+    $topValue = 0;
+
+    $shouldRun = function (string $comparison, int $subject, int $compareTo) : bool {
+        if ($comparison === '>') {
+            return $subject > $compareTo;
+        } elseif ($comparison === '<') {
+            return $subject < $compareTo;
+        } elseif ($comparison === '==') {
+            return $subject === $compareTo;
+        } elseif ($comparison === '>=') {
+            return $subject >= $compareTo;
+        } elseif ($comparison === '<=') {
+            return $subject <= $compareTo;
+        } elseif ($comparison === '!=') {
+            return $subject !== $compareTo;
+        }
+
+        echo "Missing comparison code: ".$comparison.PHP_EOL;
+        exit;
+    };
+
+    $runCommandFunction = function (string $type, int $argument, int $base) : int {
+        if ($type === 'inc') {
+            return $base + $argument;
+        } elseif ($type === 'dec') {
+            return $base - $argument;
+        }
+
+        echo "Missing command: ".$type.PHP_EOL;
+        exit;
+    };
+
+    foreach ($lines as $line) {
+        $parts = explode(' ', $line);
+
+        // Extract parts
+        $subject = $parts[0];
+        $command = $parts[1];
+        $commandArgument = $parts[2];
+        $ifSubject = $parts[4];
+        $ifComparison = $parts[5];
+        $ifCompareTo = $parts[6];
+
+        // Default values to zero if they aren't set
+        $variables[$subject] = $variables[$subject] ?? 0;
+        $variables[$ifSubject] = $variables[$ifSubject] ?? 0;
+
+        if ($shouldRun($ifComparison, $variables[$ifSubject], $ifCompareTo)) {
+            $variables[$subject] = $runCommandFunction($command, $commandArgument, $variables[$subject]);
+
+            if ($variables[$subject] > $topValue) {
+                $topValue = $variables[$subject];
+            }
+        }
+    }
+
+    if ($returnTopValue) {
+        return $topValue;
+    }
+
+    rsort($variables);
+
+    return array_shift($variables);
+}