"""Byrd-Omojokun Trust-Region SQP method."""

from scipy.sparse import eye_array as speye
from .projections import projections
from .qp_subproblem import modified_dogleg, projected_cg, box_intersections
import numpy as np
from numpy.linalg import norm

__all__ = ['equality_constrained_sqp']


def default_scaling(x):
    n, = np.shape(x)
    return speye(n)


def equality_constrained_sqp(fun_and_constr, grad_and_jac, lagr_hess,
                             x0, fun0, grad0, constr0,
                             jac0, stop_criteria,
                             state,
                             initial_penalty,
                             initial_trust_radius,
                             factorization_method,
                             trust_lb=None,
                             trust_ub=None,
                             scaling=default_scaling):
    """Solve nonlinear equality-constrained problem using trust-region SQP.

    Solve optimization problem:

        minimize fun(x)
        subject to: constr(x) = 0

    using Byrd-Omojokun Trust-Region SQP method described in [1]_. Several
    implementation details are based on [2]_ and [3]_, p. 549.

    References
    ----------
    .. [1] Lalee, Marucha, Jorge Nocedal, and Todd Plantenga. "On the
           implementation of an algorithm for large-scale equality
           constrained optimization." SIAM Journal on
           Optimization 8.3 (1998): 682-706.
    .. [2] Byrd, Richard H., Mary E. Hribar, and Jorge Nocedal.
           "An interior point algorithm for large-scale nonlinear
           programming." SIAM Journal on Optimization 9.4 (1999): 877-900.
    .. [3] Nocedal, Jorge, and Stephen J. Wright. "Numerical optimization"
           Second Edition (2006).
    """
    PENALTY_FACTOR = 0.3  # Rho from formula (3.51), reference [2]_, p.891.
    LARGE_REDUCTION_RATIO = 0.9
    INTERMEDIARY_REDUCTION_RATIO = 0.3
    SUFFICIENT_REDUCTION_RATIO = 1e-8  # Eta from reference [2]_, p.892.
    TRUST_ENLARGEMENT_FACTOR_L = 7.0
    TRUST_ENLARGEMENT_FACTOR_S = 2.0
    MAX_TRUST_REDUCTION = 0.5
    MIN_TRUST_REDUCTION = 0.1
    SOC_THRESHOLD = 0.1
    TR_FACTOR = 0.8  # Zeta from formula (3.21), reference [2]_, p.885.
    BOX_FACTOR = 0.5

    n, = np.shape(x0)  # Number of parameters

    # Set default lower and upper bounds.
    if trust_lb is None:
        trust_lb = np.full(n, -np.inf)
    if trust_ub is None:
        trust_ub = np.full(n, np.inf)

    # Initial values
    x = np.copy(x0)
    trust_radius = initial_trust_radius
    penalty = initial_penalty
    # Compute Values
    f = fun0
    c = grad0
    b = constr0
    A = jac0
    S = scaling(x)
    # Get projections
    try:
        Z, LS, Y = projections(A, factorization_method)
    except ValueError as e:
        if str(e) == "expected square matrix":
            # can be the case if there are more equality
            # constraints than independent variables
            raise ValueError(
                "The 'expected square matrix' error can occur if there are"
                " more equality constraints than independent variables."
                " Consider how your constraints are set up, or use"
                " factorization_method='SVDFactorization'."
            ) from e
        else:
            raise e

    # Compute least-square lagrange multipliers
    v = -LS.dot(c)
    # Compute Hessian
    H = lagr_hess(x, v)

    # Update state parameters
    optimality = norm(c + A.T.dot(v), np.inf)
    constr_violation = norm(b, np.inf) if len(b) > 0 else 0
    cg_info = {'niter': 0, 'stop_cond': 0,
               'hits_boundary': False}

    last_iteration_failed = False
    while not stop_criteria(state, x, last_iteration_failed,
                            optimality, constr_violation,
                            trust_radius, penalty, cg_info):
        # Normal Step - `dn`
        # minimize 1/2*||A dn + b||^2
        # subject to:
        # ||dn|| <= TR_FACTOR * trust_radius
        # BOX_FACTOR * lb <= dn <= BOX_FACTOR * ub.
        dn = modified_dogleg(A, Y, b,
                             TR_FACTOR*trust_radius,
                             BOX_FACTOR*trust_lb,
                             BOX_FACTOR*trust_ub)

        # Tangential Step - `dt`
        # Solve the QP problem:
        # minimize 1/2 dt.T H dt + dt.T (H dn + c)
        # subject to:
        # A dt = 0
        # ||dt|| <= sqrt(trust_radius**2 - ||dn||**2)
        # lb - dn <= dt <= ub - dn
        c_t = H.dot(dn) + c
        b_t = np.zeros_like(b)
        trust_radius_t = np.sqrt(trust_radius**2 - np.linalg.norm(dn)**2)
        lb_t = trust_lb - dn
        ub_t = trust_ub - dn
        dt, cg_info = projected_cg(H, c_t, Z, Y, b_t,
                                   trust_radius_t,
                                   lb_t, ub_t)

        # Compute update (normal + tangential steps).
        d = dn + dt

        # Compute second order model: 1/2 d H d + c.T d + f.
        quadratic_model = 1/2*(H.dot(d)).dot(d) + c.T.dot(d)
        # Compute linearized constraint: l = A d + b.
        linearized_constr = A.dot(d)+b
        # Compute new penalty parameter according to formula (3.52),
        # reference [2]_, p.891.
        vpred = norm(b) - norm(linearized_constr)
        # Guarantee `vpred` always positive,
        # regardless of roundoff errors.
        vpred = max(1e-16, vpred)
        previous_penalty = penalty
        if quadratic_model > 0:
            new_penalty = quadratic_model / ((1-PENALTY_FACTOR)*vpred)
            penalty = max(penalty, new_penalty)
        # Compute predicted reduction according to formula (3.52),
        # reference [2]_, p.891.
        predicted_reduction = -quadratic_model + penalty*vpred

        # Compute merit function at current point
        merit_function = f + penalty*norm(b)
        # Evaluate function and constraints at trial point
        x_next = x + S.dot(d)
        f_next, b_next = fun_and_constr(x_next)
        # Compute merit function at trial point
        merit_function_next = f_next + penalty*norm(b_next)
        # Compute actual reduction according to formula (3.54),
        # reference [2]_, p.892.
        actual_reduction = merit_function - merit_function_next
        # Compute reduction ratio
        reduction_ratio = actual_reduction / predicted_reduction

        # Second order correction (SOC), reference [2]_, p.892.
        if reduction_ratio < SUFFICIENT_REDUCTION_RATIO and \
           norm(dn) <= SOC_THRESHOLD * norm(dt):
            # Compute second order correction
            y = -Y.dot(b_next)
            # Make sure increment is inside box constraints
            _, t, intersect = box_intersections(d, y, trust_lb, trust_ub)
            # Compute tentative point
            x_soc = x + S.dot(d + t*y)
            f_soc, b_soc = fun_and_constr(x_soc)
            # Recompute actual reduction
            merit_function_soc = f_soc + penalty*norm(b_soc)
            actual_reduction_soc = merit_function - merit_function_soc
            # Recompute reduction ratio
            reduction_ratio_soc = actual_reduction_soc / predicted_reduction
            if intersect and reduction_ratio_soc >= SUFFICIENT_REDUCTION_RATIO:
                x_next = x_soc
                f_next = f_soc
                b_next = b_soc
                reduction_ratio = reduction_ratio_soc

        # Readjust trust region step, formula (3.55), reference [2]_, p.892.
        if reduction_ratio >= LARGE_REDUCTION_RATIO:
            trust_radius = max(TRUST_ENLARGEMENT_FACTOR_L * norm(d),
                               trust_radius)
        elif reduction_ratio >= INTERMEDIARY_REDUCTION_RATIO:
            trust_radius = max(TRUST_ENLARGEMENT_FACTOR_S * norm(d),
                               trust_radius)
        # Reduce trust region step, according to reference [3]_, p.696.
        elif reduction_ratio < SUFFICIENT_REDUCTION_RATIO:
            trust_reduction = ((1-SUFFICIENT_REDUCTION_RATIO) /
                               (1-reduction_ratio))
            new_trust_radius = trust_reduction * norm(d)
            if new_trust_radius >= MAX_TRUST_REDUCTION * trust_radius:
                trust_radius *= MAX_TRUST_REDUCTION
            elif new_trust_radius >= MIN_TRUST_REDUCTION * trust_radius:
                trust_radius = new_trust_radius
            else:
                trust_radius *= MIN_TRUST_REDUCTION

        # Update iteration
        if reduction_ratio >= SUFFICIENT_REDUCTION_RATIO:
            x = x_next
            f, b = f_next, b_next
            c, A = grad_and_jac(x)
            S = scaling(x)
            # Get projections
            Z, LS, Y = projections(A, factorization_method)
            # Compute least-square lagrange multipliers
            v = -LS.dot(c)
            # Compute Hessian
            H = lagr_hess(x, v)
            # Set Flag
            last_iteration_failed = False
            # Optimality values
            optimality = norm(c + A.T.dot(v), np.inf)
            constr_violation = norm(b, np.inf) if len(b) > 0 else 0
        else:
            penalty = previous_penalty
            last_iteration_failed = True

    return x, state
