# -*- coding: utf-8 -*-
import numpy as np
from numpy.linalg import eigh

from ase.optimize.optimize import Optimizer


class BFGS(Optimizer):
    def __init__(self, atoms, restart=None, logfile='-', trajectory=None,
                 maxstep=None):
        """BFGS optimizer.

        Parameters:

        restart: string
            Pickle file used to store hessian matrix. If set, file with 
            such a name will be searched and hessian matrix stored will
            be used, if the file exists.
        trajectory: string
            Pickle file used to store trajectory of atomic movement.
        maxstep: float
            Used to set the maximum distance an atom can move per
            iteration (default value is 0.04 Å).
        """
        
        Optimizer.__init__(self, atoms, restart, logfile, trajectory)

        if maxstep is not None:
            if maxstep > 1.0:
                raise ValueError('You are using a much too large value for ' +
                                 'the maximum step size: %.1f Å' % maxstep)
            self.maxstep = maxstep

    def initialize(self):
        self.H = None
        self.r0 = None
        self.f0 = None
        self.maxstep = 0.04

    def read(self):
        self.H, self.r0, self.f0, self.maxstep = self.load()

    def step(self, f):
        atoms = self.atoms
        r = atoms.get_positions()
        f = f.reshape(-1)
        self.update(r.flat, f, self.r0, self.f0)
        omega, V = eigh(self.H)
        dr = np.dot(V, np.dot(f, V) / np.fabs(omega)).reshape((-1, 3))
        steplengths = (dr**2).sum(1)**0.5
        dr = self.determine_step(dr, steplengths)
        atoms.set_positions(r + dr)
        self.r0 = r.flat.copy()
        self.f0 = f.copy()
        self.dump((self.H, self.r0, self.f0, self.maxstep))

    def determine_step(self, dr, steplengths):
        """Determine step to take according to maxstep
        
        Normalize all steps as the largest step. This way
        we still move along the eigendirection.
        """
        maxsteplength = np.max(steplengths)
        if maxsteplength >= self.maxstep:
            dr *= self.maxstep / maxsteplength
        
        return dr

    def update(self, r, f, r0, f0):
        if self.H is None:
            self.H = np.eye(3 * len(self.atoms)) * 70.0
            return
        dr = r - r0

        if np.abs(dr).max() < 1e-7:
            # Same configuration again (maybe a restart):
            return

        df = f - f0
        a = np.dot(dr, df)
        dg = np.dot(self.H, dr)
        b = np.dot(dr, dg)
        self.H -= np.outer(df, df) / a + np.outer(dg, dg) / b

    def replay_trajectory(self, traj):
        """Initialize hessian from old trajectory."""
        if isinstance(traj, str):
            from ase.io.trajectory import PickleTrajectory
            traj = PickleTrajectory(traj, 'r')
        self.H = None
        atoms = traj[0]
        r0 = atoms.get_positions().ravel()
        f0 = atoms.get_forces().ravel()
        for atoms in traj:
            r = atoms.get_positions().ravel()
            f = atoms.get_forces().ravel()
            self.update(r, f, r0, f0)
            r0 = r
            f0 = f

        self.r0 = r0
        self.f0 = f0

class oldBFGS(BFGS):
    def determine_step(self, dr, steplengths):
        """Old BFGS behaviour for scaling step lengths

        This keeps the behaviour of truncating individual steps. Some might
        depend of this as some absurd kind of stimulated annealing to find the
        global minimum.
        """
        dr /= np.maximum(steplengths / self.maxstep, 1.0).reshape(-1, 1)
        return dr
