<?php

class UrlManipulator
{
    private $paramsArray = ['param1', 'param2', 'param3', 'param4'];
    
    public function manipulateUrl($url)
    {
        if (!filter_var($url, FILTER_VALIDATE_URL)) {
            return ['error' => 'Invalid URL', 'input' => $url];
        }

        $parsedUrl = parse_url($url);
        $existingParams = [];
        if (isset($parsedUrl['query'])) {
            parse_str($parsedUrl['query'], $existingParams);
        }


        if (empty($this->paramsArray)) {
            return ['error' => 'Params array is full', 'input' => $url];
        }

        $newParam = $this->getNewParam($existingParams);

        if (!$newParam) {
            return ['error' => 'Unable to find a unique parameter to add', 'input' => $url];
        }

        $randomValue = $this->generateRandomValue();

        $modifiedUrl = $this->addParamToUrl($url, $newParam, $randomValue);

        return [
            'before' => $url,
            'after' => $modifiedUrl,
        ];
    }

    private function addParamToUrl($url, $param, $value)
    {
        $separator = (parse_url($url, PHP_URL_QUERY) == null) ? '?' : '&';

        if (strpos($url, $param . '=') !== false) {
            $modifiedUrl = preg_replace('/(' . $param . '=)[^&]+/', '$1' . urlencode($value), $url);
        } else {
            $modifiedUrl = $url . $separator . urlencode($param) . '=' . urlencode($value);
        }

        return $modifiedUrl;
    }

    private function generateRandomValue()
    {
        return uniqid();
    }

    private function getNewParam($existingParams){
      while (!empty($this->paramsArray)) {
          $newParam = array_pop($this->paramsArray);

          if (!array_key_exists($newParam, $existingParams)) {
              return $newParam;
          }
      }

      return null; 
    }

}

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    $urlManipulator = new UrlManipulator();
    $url = $_POST['url'];
    $result = $urlManipulator->manipulateUrl($url);

    header('Content-Type: application/json');
    echo json_encode($result);
}