index.js

// This Library uses STuF
// https://github.com/stuffyerface/STuF


class STuF {
  /**
   * Decodes a string in Standardized Truncated url Format.
   * @param {String} string - The String to Decode.
   */
  static decode = (string) => {
    if (!string.startsWith('l$')) {
      throw new Error('String does not appear to be in STuF');
    }
    let prefix = string[2];
    let suffix = string[3];
    let dotIndices = string.slice(4, string.indexOf('|')).split('').map(Number);
    let urlBody = string.slice(string.indexOf('|') + 1);

    let first9 = urlBody.slice(0, 9 - dotIndices.length);
    let then = urlBody.slice(9 - dotIndices.length).replace(/\^/g, '.');

    let url = first9 + then;
    url = charInc(url, -1)

    // Restore the dots in the first part of the URL
    dotIndices.forEach((index) => {
      url = url.slice(0, index) + '.' + url.slice(index);
    });

    // Add the prefix back
    if (prefix === 'h') {
      url = 'http://' + url;
    } else if (prefix === 'H') {
      url = 'https://' + url;
    }

    // Add the suffix back
    if (suffix === '1') {
      url += '.png';
    } else if (suffix === '2') {
      url += '.jpg';
    } else if (suffix === '3') {
      url += '.jpeg';
    } else if (suffix === '4') {
      url += '.gif';
    }

    return url;
  };

  /**
   * Encodes a string in Standardized Truncated url Format.
   * @param {String} url - The URL to Encode.
   */
  static encode = (url) => {
    let encoded = "l$"
    if (url.startsWith('http://')) {
      encoded += 'h';
      url = url.slice(7); // Remove the 'http://' part
    } else if (url.startsWith('https://')) {
      encoded += 'H';
      url = url.slice(8); // Remove the 'https://' part
    }

    if (url.endsWith('.png')) {
      encoded += '1';
      url = url.slice(0, -4); // Remove the '.png' part
    } else if (url.endsWith('.jpg')) {
      encoded += '2';
      url = url.slice(0, -4); // Remove the '.jpg' part
    } else if (url.endsWith('.jpeg')) {
      encoded += '3';
      url = url.slice(0, -5); // Remove the '.jpeg' part
    } else if (url.endsWith('.gif')) {
      encoded += '4';
      url = url.slice(0, -4); // Remove the '.gif' part
    } else {
      encoded += '0';
    }

    let dotIndices = [];
    for (let i = 0; (i < url.length) && (i <= 8); i++) {
      if (url[i] === '.') {
        dotIndices.push(i);
        if (dotIndices.length === 9) break; // Stop after 9 dots
      }
    }

    let first9 = url.substring(0, 9)
    let then = url.substring(9).replace(/\./g, '^');
    first9 = first9.replace(/\./g, '');
    let shifted = charInc(first9 + then, 1)

    encoded += dotIndices.map(index => index.toString()).join('') + '|';
    encoded += shifted


    return encoded;
  }
}

function charInc(str, int) {
  const charSet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
  let incrementedStr = '';
  for (let i = 0; i < str.length; i++) {
    let char = str[i];
    let index = charSet.indexOf(char);

    if (index == -1) {
      incrementedStr += char;
    } else {
      let offset = index + int
      while(offset >= charSet.length){
        offset -= charSet.length
      }
      while(offset < 0){
        offset += charSet.length
      }
      let nextChar = charSet[offset];
      incrementedStr += nextChar;
    }
  }
  return incrementedStr;
}

module.exports = {
  STuF
}