14th day both parts

14/part2.php

@@ -0,0 +1,39 @@
+<?php
+$lines = file('input', FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
+
+$mapings         = [];
+$before          = [];
+$initialMappings = [];
+$template        = array_shift($lines);
+for ($i = 0; $i < strlen($template) - 1; $i++) {
+    $pair          = substr($template, $i, 2);
+    $before[$pair] = 1;
+}
+foreach ($lines as $item) {
+    $line                      = explode(' -> ', $item);
+    $mapings[$line[0]]         = $line[1];
+    $initialMappings[$line[0]] = 0;
+}
+
+for ($step = 1; $step <= 40; $step++) {
+    $after = $initialMappings;
+    foreach ($before as $pair => $count) {
+        $left  = $pair[0] . $mapings[$pair];
+        $right = $mapings[$pair] . $pair[1];
+        $after[$left]  += $count;
+        $after[$right] += $count;
+    }
+    $before = $after;
+}
+$letters = [];
+foreach ($before as $pair => $count) {
+    $left = $pair[0];
+    if (!isset($letters[$left])) {
+        $letters[$left] = 0;
+    }
+    $letters[$left] += $count;
+}
+$letters[substr($template, -1)] += 1;
+sort($letters);
+
+echo array_pop($letters) - array_shift($letters); //2516901104210