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spongecase.js

/*
 * Cryptographically reversible spongebob-case text transformation
 *
 * Sometimes you want to encode some text in a way that is not completely
 * secret, but is quite hard to read. But what if you need to decode it again?
 *
 * Now you can!
 *
 * to De-sponge some text, simply pass the sponged text back through the sponge 
 * function
 */

function sponge(str) {
  return str
    .split("")
    .map((char, i) => {
      let result = i % 2 == 0 ? toggleCase(char) : char
      if (i % 7 == 0) result = toggleCase(result)
      if (i % 9 == 0) result = toggleCase(result)
      return result
    })
    .join("")
}

const [upp_start, upp_end, low_start, low_end] = "AZaz"
    .split("")
    .map(x => x.charCodeAt(0));
    
function toggleCase(char) {
  const charCode = char.charCodeAt(0);
  let diff = 0;
  if (charCode >= upp_start && charCode <= upp_end) {
    // change uppercase to lowercase
    diff = low_end - upp_end;
  } else if (charCode >= low_start && charCode <= low_end) {
    // change lowercase to uppercase
    diff = upp_end - low_end;
  }
  return String.fromCharCode(charCode + diff);
}