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/**
 * Makes parallel HTTP requests to the given array of URLs, with a constraint on the number
 * of simultaneous requests allowed. Once all requests are completed, calls the provided
 * callback function with an array of text responses in the same order as the input URLs.
 *
 * @param {string[]} urls - An array of URLs to make HTTP requests to.
 * @param {number} limit - The maximum number of simultaneous HTTP requests allowed.
 * @param {function(string[]): void} callback - The callback function to be called with an
 * array of text responses once all requests are completed.
 * 
 * 10
 * -> 10
 * 
 * <-2
 * 
 * -> 8
 */


function parallelHttpRequests(urls, limit, callback) {
  const results = Array(urls.length);
  let countResults = 0;
  let lastCalledUrlIdx = 0;

  limit = Math.min(urls.length, limit);

  const memForRepeated = {};

  const uniqsUrls = new Set();

  urls.forEach(url => {
    if (uniqsUrls.has(url)) {
      return memForRepeated[url] = null;
    }
    uniqsUrls.add(url);
  })

  const fetchUrl = (urlIdx) => {
    const onSuccess = (res) => {
      results[urlIdx] = res;
      countResults++;
      
      if (countResults === urls.length) {
        return callback(results);
      }

      if (urls.length - 1 > lastCalledUrlIdx) {
        fetchUrl(++lastCalledUrlIdx)
      }
    }

    const url = urls[urlIdx];
    
    if (typeof memForRepeated[url] === 'string') {
      return onSuccess(memForRepeated[url]);
    }

    fetch(url).then(onSuccess);
  }

  
  for (; lastCalledUrlIdx < limit; lastCalledUrlIdx++) {
    fetchUrl(lastCalledUrlIdx)
  }
}
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