<?php

$reports = array_filter(explode("\n", file_get_contents(__DIR__.'/input.txt')), static fn($line) =>!empty($line));

$number_of_safe_reports = 0;
function is_report_safe(string $report)
{
    $levels = explode(' ', $report);
    $current_number = null;
    $current_way = null;
    foreach ($levels as $level) {
        if ($current_number === null) {
            $current_number = $level;
        } else {
            if ($current_number === $level) {
                return false;
            }
            if ($current_number > $level) {
                if ($current_number - $level > 3) {
                    return false;
                }
                if ($current_way === null) {
                    $current_way = 'd';
                } elseif ($current_way !== 'd') {
                    return false;
                }
            }
            if ($current_number < $level) {
                if ($level - $current_number > 3) {
                    return false;
                }
                if ($current_way === null) {
                    $current_way = 'a';
                } elseif ($current_way !== 'a') {
                    return false;
                }
            }
            $current_number = $level;
        }
    }

    return true;
}

foreach ($reports as $report) {
    if (is_report_safe($report)) {
        $number_of_safe_reports++;
    }
}

echo sprintf('Number of safe reports : %u', $number_of_safe_reports);
exit;