Day 2 Part 2

2024/02/puzzle.php

@@ -1,30 +1,59 @@
 <?php
 
 $input = fopen('input.txt', 'r+');
-$safe = 0 ;
+$safe = 0;
 $unsafe = 0;
-while (!feof($input)) {
-    $readings = fgetcsv($input, separator: ' ');
+$line = 0;
 
-    for($i = 0; $i < count($readings); $i++) {
-        if (array_key_exists($i+1, $readings) && abs($readings[$i]  - $readings[$i+1]) > 3) {
-            echo "Difference is too great. {$readings[$i]} and " . $readings[$i+1] . PHP_EOL;
-            continue 2;
-        }
+// This worked for Part 1, so assuming it checks out.
+function checkReading(array $readings): ?int
+{
+    for ($i = 0; $i < count($readings); $i++) {
+        if (array_key_exists($i + 1, $readings) && abs($readings[$i] - $readings[$i + 1]) > 3) {
+            return $i;
 
-        if (array_key_exists($i+1, $readings) && array_key_exists($i-1, $readings) && (($readings[$i-1] <=> $readings[$i]) !== ($readings[$i] <=> $readings[$i+1]))) {
-            echo 'Trend change. ' . join(', ', [$readings[$i-1], $readings[$i], $readings[$i+1]]) . PHP_EOL;
-            continue 2;
         }
 
-        if (array_key_exists($i+1, $readings) && $readings[$i] == $readings[$i+1]) {
-            echo "No change! {$readings[$i]} {$readings[$i+1]}" . PHP_EOL;
+        if (array_key_exists($i + 1, $readings) && array_key_exists($i - 1,
+                $readings) && (($readings[$i - 1] <=> $readings[$i]) !== ($readings[$i] <=> $readings[$i + 1]))) {
+            return $i;
         }
 
+        if (array_key_exists($i + 1, $readings) && $readings[$i] == $readings[$i + 1]) {
+            return $i;
+        }
     }
+    return null;
+}
+
+function duplicateValues(array $readings): array
+{
+    return array_keys(array_filter(array_count_values($readings), fn (int $count) => $count > 1));
+}
 
-    ++$safe;
+while (!feof($input)) {
+    ++$line;
+    $readings = fgetcsv($input, separator: ' ');
+
+    // Does the simple check work?
+    if (checkReading($readings) === null) {
+        // Great, easy!
+        ++$safe;
+        continue;
+    }
 
+    for ($i = 0; $i < count($readings); $i++) {
+        $test = $readings;
+        array_splice($test, $i, 1);
+        if (checkReading($test) === null) {
+            // Great, easy!
+            ++$safe;
+            continue 2; // Safe version found, next reading!
+        }
+    }
 }
 
-echo $safe . PHP_EOL;
+if ($safe <= 488 || $safe >= 538) {
+    echo 'WRONG'.PHP_EOL;
+}
+echo $safe.PHP_EOL;