dionsaputra

// lhs (left-hand-side), rhs (right-hand-side)
fun solve(lhs: Matrix, rhs: Matrix): Matrix? {
  return inverse(lhs)?.let { it x rhs }
}

dionsaputra

// matrix cross-product
infix fun x(other: Matrix): Matrix {
  require(cols == other.rows)
  val res = Matrix(rows, other.cols, Array(rows * other.cols) { 0.0 })
  for (i in 0 until rows) {
    for (j in 0 until other.cols) {
      for (k in 0 until cols) res[i,j] += (this[i,k] * other[k,j])
    }
  }
  return res

dionsaputra

fun inverse(matrix: Matrix): Matrix? {
  require(matrix.rows == matrix.cols)

  // check matrix invertible
  if (determinant(matrix).equalsDelta(0.0)) return null

  // copy matrix
  val inverse = Matrix.diagonal(matrix.rows, 1.0, 0.0)
  val temp = Matrix(
    matrix.rows, 

dionsaputra

fun determinant(matrix: Matrix): Double {
  require(matrix.rows == matrix.cols)

  if (matrix.rows == 1) return matrix[0,0]
  if (matrix.rows == 2) return (
    matrix[0,0] * matrix[1,1] - 
    matrix[0,1] * matrix[1,0]
  )

  var det = 0.0

dionsaputra

data class Matrix(val rows: Int, val cols: Int) {
  
  lateinit var elements: DoubleArray
  
  private fun index(r: Int, c: Int) = r * cols + c
  
  operator fun get(r: Int, c: Int) = elements[index(r,c)]
  
  operator fun set(r: Int, c: Int, element: Double) {
    elements[index(r,c)] = element