# linflat.py - FlatSystem subclass for linear systems
# RMM, 10 November 2012
#
# This file defines a FlatSystem class for a linear system.
#
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import numpy as np
import control
from .flatsys import FlatSystem
from ..statesp import StateSpace


class LinearFlatSystem(FlatSystem, StateSpace):
    """Base class for a linear, differentially flat system.

    This class is used to create a differentially flat system representation
    from a linear system.

    Parameters
    ----------
    linsys : StateSpace
        LTI StateSpace system to be converted
    inputs : int, list of str or None, optional
        Description of the system inputs.  This can be given as an integer
        count or as a list of strings that name the individual signals.
        If an integer count is specified, the names of the signal will be
        of the form `s[i]` (where `s` is one of `u`, `y`, or `x`).  If
        this parameter is not given or given as `None`, the relevant
        quantity will be determined when possible based on other
        information provided to functions using the system.
    outputs : int, list of str or None, optional
        Description of the system outputs.  Same format as `inputs`.
    states : int, list of str, or None, optional
        Description of the system states.  Same format as `inputs`.
    dt : None, True or float, optional
        System timebase.  None (default) indicates continuous
        time, True indicates discrete time with undefined sampling
        time, positive number is discrete time with specified
        sampling time.
    params : dict, optional
        Parameter values for the systems.  Passed to the evaluation
        functions for the system as default values, overriding internal
        defaults.
    name : string, optional
        System name (used for specifying signals)

    """

    def __init__(self, linsys, **kwargs):
        """Define a flat system from a SISO LTI system.

        Given a reachable, single-input/single-output, linear time-invariant
        system, create a differentially flat system representation.

        """
        # Make sure we can handle the system
        if (not control.isctime(linsys)):
            raise control.ControlNotImplemented(
                "requires continuous time, linear control system")
        elif (not control.issiso(linsys)):
            raise control.ControlNotImplemented(
                "only single input, single output systems are supported")

        # Initialize the object as a StateSpace system
        StateSpace.__init__(self, linsys, **kwargs)

        # Find the transformation to chain of integrators form
        # Note: store all array as ndarray, not matrix
        zsys, Tr = control.reachable_form(linsys)
        Tr = np.array(Tr[::-1, ::])     # flip rows

        # Extract the information that we need
        self.F = np.array(zsys.A[0, ::-1])      # input function coeffs
        self.T = Tr                             # state space transformation
        self.Tinv = np.linalg.inv(Tr)           # compute inverse once

        # Compute the flat output variable z = C x
        Cfz = np.zeros(np.shape(linsys.C)); Cfz[0, 0] = 1
        self.Cf = Cfz @ Tr

    # Compute the flat flag from the state (and input)
    def forward(self, x, u, params):
        """Compute the flat flag given the states and input.

        See :func:`control.flatsys.FlatSystem.forward` for more info.

        """
        x = np.reshape(x, (-1, 1))
        u = np.reshape(u, (1, -1))
        zflag = [np.zeros(self.nstates + 1)]
        zflag[0][0] = (self.Cf @ x).item()
        H = self.Cf                     # initial state transformation
        for i in range(1, self.nstates + 1):
            zflag[0][i] = (H @ (self.A @ x + self.B @ u)).item()
            H = H @ self.A       # derivative for next iteration
        return zflag

    # Compute state and input from flat flag
    def reverse(self, zflag, params):
        """Compute the states and input given the flat flag.

        See :func:`control.flatsys.FlatSystem.reverse` for more info.

        """
        z = zflag[0][0:-1]
        x = self.Tinv @ z
        u = zflag[0][-1] - self.F @ z
        return np.reshape(x, self.nstates), np.reshape(u, self.ninputs)

    # Update function
    def _rhs(self, t, x, u):
        # Use StateSpace._rhs instead of default (MRO) NonlinearIOSystem
        return StateSpace._rhs(self, t, x, u)

    # output function
    def _out(self, t, x, u):
        # Use StateSpace._out instead of default (MRO) NonlinearIOSystem
        return StateSpace._out(self, t, x, u)
