🎉 Hello world

day_4/part_1.php

@@ -0,0 +1,59 @@
+<?php
+
+[$min, $max] = explode('-', file_get_contents('input.txt'));
+
+/**
+ * @param int|string $big_number
+ *
+ * @return bool
+ */
+function has_at_least_two_adjacent_number($big_number)
+{
+    $big_number = (string) $big_number;
+    $current_number = null;
+    for ($i = 0, $l = strlen($big_number); $i < $l; $i++) {
+        if ($current_number === $big_number[$i]) {
+            return true;
+        }
+        $current_number = $big_number[$i];
+    }
+
+    return false;
+}
+
+/**
+ * @param int|string $big_number
+ * @return bool
+ */
+function never_decrease($big_number)
+{
+    $big_number = (string) $big_number;
+    $current_number = null;
+    for ($i = 0, $l = strlen($big_number); $i < $l; $i++) {
+        if ($current_number > $big_number[$i]) {
+            return false;
+        }
+        $current_number = $big_number[$i];
+    }
+
+    return true;
+}
+
+/**
+ * @param int|string $number
+ *
+ * @return bool
+ */
+function is_valid($number)
+{
+    return has_at_least_two_adjacent_number($number) && never_decrease($number);
+}
+
+$valids = [];
+for ($i = $min; $i <= $max; $i++) {
+    if (is_valid($i)) {
+        $valids[] = $i;
+    }
+}
+
+echo count($valids);