Method performDecode

src/Encoder/TrifidCipher.js:126–171

* Performs decode on given content. * @protected * @param {Chain} content * @return {number[]|string|Uint8Array|Chain} Decoded content

(content)

   * @return {number[]|string|Uint8Array|Chain} Decoded content
   */
  async performDecode (content) {
    const { key, groupSize } = this.getSettingValues()

    // Derive mixed-alphabet from key
    const alphabet = Chain.wrap(key).extend(baseAlphabet).getCodePoints()

    // Map Unicode code points to their respective alphabet positions
    const positions =
      content.toLowerCase().getCodePoints()
        .map(codePoint => alphabet.indexOf(codePoint))
        .filter(codePoint => codePoint !== -1)

    // Fill in table horizontally
    // Refer to the `performEncode` method for more detailed explanations
    const length = positions.length
    const table = new Array(length * 3)
    let i, j, coordinates

    for (i = 0; i < length; i++) {
      coordinates = this.cartesianCoordinatesFromIndex(positions[i])
      for (j = 0; j < 3; j++) {
        table[i * 3 + j] = coordinates[j]
      }
    }

    // Read trigrams vertically for each character
    const result = new Array(length)
    let group, index, size

    for (i = 0; i < length; i++) {
      // Gather group facts
      group = Math.floor(i / groupSize)
      index = i - group * groupSize
      size = Math.min(groupSize, length - group * groupSize)

      // Read one character from vertical trigram
      for (j = 0; j < 3; j++) {
        coordinates[j] = table[index + group * groupSize * 3 + size * j]
      }

      index = this.indexFromCartesianCoordinates(coordinates)
      result[i] = alphabet[index]
    }

    return result
  }

  /**
   * Translates a one-dimensional index to three-dimensional cartesian