export const preprocessKey = (key: string): number[][] => {
  /**
   * @param {string} key - A string of 128, 192, or 256 bits.
   * @returns {number[][]} A 4x4/6/8 matrix representing the key.
  */

  const encoder = new TextEncoder()
  const byteArray = encoder.encode(key)
  
  const validKeyLengths = [128, 192, 256]
  
  if (!validKeyLengths.includes(byteArray.length * 8)) {
    throw new Error('Invalid Key!')
  }

  const N_k = (byteArray.length * 8) / 32

  const matrix: number[][] = [
    Array(N_k).fill(0),
    Array(N_k).fill(0),
    Array(N_k).fill(0),
    Array(N_k).fill(0)
  ]

  // Fill the matrix in column-major order, i.e., top-to-bottom + left-to-right
  // byteArray.length is either 16, 24, or 32.
  for (let i = 0;i < byteArray.length;i++) {
    const row = i % 4
    const col = Math.floor(i / 4)
    matrix[row][col] = byteArray[i]
  }

  return matrix
}