Method performDecode

src/Encoder/ADFGXCipher.js:156–196

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

(content)

   * @return {number[]|string|Uint8Array|Chain} Decoded content
   */
  performDecode (content) {
    const variantName = this.getSettingValue('variant')
    const variant = this.constructor.getVariant(variantName)
    const squarePositions = Chain.wrap(variant.squarePositions).getCodePoints()

    // Prepare content by removing non-position characters
    const codePoints = content.getCodePoints()
      .filter(codePoint => squarePositions.indexOf(codePoint) !== -1)
    const length = codePoints.length

    // Derive column map from key
    const key = this.getSettingValue('key')
    const keyLength = key.getLength()
    const columnMap = this.mapElementsToSortedPosition(key.getChars())

    // Calculate the number of columns available in the last row as the
    // plaintext content may not have filled up the entire grid
    const lastRowColumns = (length % keyLength) || keyLength
    const rows = Math.ceil(length / keyLength)

    // Decode code points to Polybius square coordinates
    const polybius = new Array(length)
    let index = 0
    let i, column, row, columnRows

    // Go through columns
    for (i = 0; i < keyLength; i++) {
      column = columnMap[i]

      // The last row of this column may be empty
      columnRows = column >= lastRowColumns ? rows - 1 : rows

      // Transpose each column character
      for (row = 0; row < columnRows; row++) {
        polybius[row * keyLength + column] = codePoints[index++]
      }
    }

    // Decode coordinates using Polybius square
    return this._polybiusSquare.decode(polybius)
  }

  /**
   * Maps the given elements to their corresponding positions in a sorted array.