$output_array[1],
'sector_id' => $output_array[2],
'checksum' => $output_array[3]
);
}
return false;
}
function generateChecksum($room_name) {
// calculate char occurrences
$char_count_array = count_chars(str_replace('-', '', $room_name), 1);
// sort by occurrence count
arsort($char_count_array);
// group chars into occurrence count
$dedupe = array();
foreach($char_count_array as $char => $occurrence) {
$dedupe[$occurrence][] = $char;
}
// cross my fingers and hope this actually works
$checksum = '';
foreach($dedupe as $occurrence => $chars) {
sort($chars);
for($i = 0; $i < count($chars); $i++) {
$checksum .= chr($chars[$i]);
}
}
// cut off the excess and return
return substr($checksum, 0, 5);
}
function decryptRoomName($name, $sector_id) {
$key = $sector_id % 26; // this should work right?
$a_start = ord('a');
$output = '';
for($i = 0; $i < strlen($name); $i++) {
$a = ord(substr($name, $i, 1));
if($a == ord('-'))
$output .= ' ';
else
$output .= chr($a_start + ((($a - $a_start) + $key) % 26));
}
return $output;
}
$input = file('day4_input.txt');
//$input = array("aaaaa-bbb-z-y-x-123[abxyz]", "a-b-c-d-e-f-g-h-987[abcde]", "not-a-real-room-404[oarel]");
// Part 1
$valid_room_sum = 0;
$valid_rooms = array();
foreach($input as $line) {
$room_info = extractRoomInfo($line);
$generated_checksum = generateChecksum($room_info['room_name']);
if($generated_checksum == $room_info['checksum']) {
$valid_rooms[] = $room_info;
$valid_room_sum += (int)$room_info['sector_id'];
}
}
echo "valid room sum: $valid_room_sum
\n";
// Part 2
foreach($valid_rooms as $room) {
$decrypted_name = decryptRoomName($room['room_name'], (int)$room['sector_id']);
echo "room name: $decrypted_name, sector: {$room['sector_id']}
\n";
}