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Filename : reed-solomon-encoder.js
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const Polynomial = require('./polynomial')

function ReedSolomonEncoder (degree) {
  this.genPoly = undefined
  this.degree = degree

  if (this.degree) this.initialize(this.degree)
}

/**
 * Initialize the encoder.
 * The input param should correspond to the number of error correction codewords.
 *
 * @param  {Number} degree
 */
ReedSolomonEncoder.prototype.initialize = function initialize (degree) {
  // create an irreducible generator polynomial
  this.degree = degree
  this.genPoly = Polynomial.generateECPolynomial(this.degree)
}

/**
 * Encodes a chunk of data
 *
 * @param  {Uint8Array} data Buffer containing input data
 * @return {Uint8Array}      Buffer containing encoded data
 */
ReedSolomonEncoder.prototype.encode = function encode (data) {
  if (!this.genPoly) {
    throw new Error('Encoder not initialized')
  }

  // Calculate EC for this data block
  // extends data size to data+genPoly size
  const paddedData = new Uint8Array(data.length + this.degree)
  paddedData.set(data)

  // The error correction codewords are the remainder after dividing the data codewords
  // by a generator polynomial
  const remainder = Polynomial.mod(paddedData, this.genPoly)

  // return EC data blocks (last n byte, where n is the degree of genPoly)
  // If coefficients number in remainder are less than genPoly degree,
  // pad with 0s to the left to reach the needed number of coefficients
  const start = this.degree - remainder.length
  if (start > 0) {
    const buff = new Uint8Array(this.degree)
    buff.set(remainder, start)

    return buff
  }

  return remainder
}

module.exports = ReedSolomonEncoder