dlstsq Function

function dlstsq(A, b) result(x)
  real(8), intent(in)  :: A(:,:), b(:)
  real(8), allocatable :: x(:)
  integer              :: info, ldb, lwork, m, n, rank
  real(8)              :: rcond
  real(8), allocatable :: work(:), At(:,:), Bt(:,:)
  integer, allocatable :: jpvt(:)
  m = size(A(:,1)) ! = lda
  n = size(A(1,:))
  ldb = size(b)
  allocate(x(n), At(m,n), Bt(ldb,1), jpvt(n), work(1))
  call dgelsy(m, n, 1, At, m, Bt, ldb, jpvt, rcond, rank, work, &
       -1, info)  ! query optimal workspace size
  lwork = int(real(work(1)))
  deallocate(work)
  allocate(work(lwork))  ! allocate with ideal size
  rcond = 0d0
  jpvt(:) = 0
  Bt(:,1) = b(:)  ! only one right-hand side
  At(:,:) = A(:,:)
  call dgelsy(m, n, 1, At, m, Bt, ldb, jpvt, rcond, rank, work, &
       lwork, info)
  if(info /= 0) then
     print *, "dgelsy returned info = ", info
     print *, "the ", -info, "-th argument had an illegal value"
     stop 'dlstsq error: dgelsy'
  endif
  x(:) = Bt(1:n,1)
end function dlstsq