# -*- coding: utf-8 -*-

"""Provide OptGP sampler."""

from __future__ import absolute_import, division

from multiprocessing import Pool

import numpy as np
import pandas

from cobra.core.configuration import Configuration
from cobra.sampling.hr_sampler import HRSampler, shared_np_array, step


__all__ = ("OptGPSampler",)


CONFIGURATION = Configuration()


def mp_init(obj):
    """Initialize the multiprocessing pool."""

    global sampler
    sampler = obj


# Unfortunately this has to be outside the class to be usable with
# multiprocessing :()
def _sample_chain(args):
    """Sample a single chain for OptGPSampler.

    center and n_samples are updated locally and forgotten afterwards.

    """

    n, idx = args       # has to be this way to work in Python 2.7
    center = sampler.center
    np.random.seed((sampler._seed + idx) % np.iinfo(np.int32).max)
    pi = np.random.randint(sampler.n_warmup)

    prev = sampler.warmup[pi, ]
    prev = step(sampler, center, prev - center, 0.95)

    n_samples = max(sampler.n_samples, 1)
    samples = np.zeros((n, center.shape[0]))

    for i in range(1, sampler.thinning * n + 1):
        pi = np.random.randint(sampler.n_warmup)
        delta = sampler.warmup[pi, ] - center

        prev = step(sampler, prev, delta)

        if sampler.problem.homogeneous and (
                n_samples * sampler.thinning % sampler.nproj == 0):
            prev = sampler._reproject(prev)
            center = sampler._reproject(center)

        if i % sampler.thinning == 0:
            samples[i//sampler.thinning - 1, ] = prev

        center = ((n_samples * center) / (n_samples + 1) +
                  prev / (n_samples + 1))
        n_samples += 1

    return (sampler.retries, samples)


class OptGPSampler(HRSampler):
    """A parallel optimized sampler.

    A parallel sampler with fast convergence and parallel execution. See [1]_
    for details.

    Parameters
    ----------
    model : cobra.Model
        The cobra model from which to generate samples.
    processes: int, optional (default Configuration.processes)
        The number of processes used during sampling.
    thinning : int, optional
        The thinning factor of the generated sampling chain. A thinning of 10
        means samples are returned every 10 steps.
    nproj : int > 0, optional
        How often to reproject the sampling point into the feasibility space.
        Avoids numerical issues at the cost of lower sampling. If you observe
        many equality constraint violations with `sampler.validate` you should
        lower this number.
    seed : int > 0, optional
        Sets the random number seed. Initialized to the current time stamp if
        None.

    Attributes
    ----------
    model : cobra.Model
        The cobra model from which the samples get generated.
    thinning : int
        The currently used thinning factor.
    n_samples : int
        The total number of samples that have been generated by this
        sampler instance.
    problem : collections.namedtuple
        A python object whose attributes define the entire sampling problem in
        matrix form. See docstring of `Problem`.
    warmup : numpy.matrix
        A matrix of with as many columns as reactions in the model and more
        than 3 rows containing a warmup sample in each row. None if no warmup
        points have been generated yet.
    retries : int
        The overall of sampling retries the sampler has observed. Larger
        values indicate numerical instabilities.
    seed : int > 0, optional
        Sets the random number seed. Initialized to the current time stamp if
        None.
    nproj : int
        How often to reproject the sampling point into the feasibility space.
    fwd_idx : numpy.array
        Has one entry for each reaction in the model containing the index of
        the respective forward variable.
    rev_idx : numpy.array
        Has one entry for each reaction in the model containing the index of
        the respective reverse variable.
    prev : numpy.array
        The current/last flux sample generated.
    center : numpy.array
        The center of the sampling space as estimated by the mean of all
        previously generated samples.

    Notes
    -----
    The sampler is very similar to artificial centering where each process
    samples its own chain. Initial points are chosen randomly from the warmup
    points followed by a linear transformation that pulls the points a little
    bit towards the center of the sampling space.

    If the number of processes used is larger than the one requested,
    number of samples is adjusted to the smallest multiple of the number of
    processes larger than the requested sample number. For instance, if you
    have 3 processes and request 8 samples you will receive 9.

    Memory usage is roughly in the order of (2 * number reactions)^2
    due to the required nullspace matrices and warmup points. So large
    models easily take up a few GB of RAM. However, most of the large matrices
    are kept in shared memory. So the RAM usage is independent of the number
    of processes.

    References
    ----------
    .. [1] Megchelenbrink W, Huynen M, Marchiori E (2014)
       optGpSampler: An Improved Tool for Uniformly Sampling the Solution-Space
       of Genome-Scale Metabolic Networks.
       PLoS ONE 9(2): e86587.
       https://doi.org/10.1371/journal.pone.0086587

    """

    def __init__(self, model, processes=None, thinning=100, nproj=None,
                 seed=None):
        """Initialize a new OptGPSampler."""

        super(OptGPSampler, self).__init__(model, thinning, seed=seed)
        self.generate_fva_warmup()

        if processes is None:
            self.processes = CONFIGURATION.processes
        else:
            self.processes = processes

        # This maps our saved center into shared memory,
        # meaning they are synchronized across processes
        self.center = shared_np_array((len(model.variables), ),
                                      self.warmup.mean(axis=0))

    def sample(self, n, fluxes=True):
        """Generate a set of samples.

        This is the basic sampling function for all hit-and-run samplers.

        Parameters
        ---------
        n : int
            The minimum number of samples that are generated at once
            (see Notes).
        fluxes : boolean
            Whether to return fluxes or the internal solver variables. If set
            to False will return a variable for each forward and backward flux
            as well as all additional variables you might have defined in the
            model.

        Returns
        -------
        numpy.matrix
            Returns a matrix with `n` rows, each containing a flux sample.

        Notes
        -----
        Performance of this function linearly depends on the number
        of reactions in your model and the thinning factor.

        If the number of processes is larger than one, computation is split
        across as the CPUs of your machine. This may shorten computation time.
        However, there is also overhead in setting up parallel computation so
        we recommend to calculate large numbers of samples at once
        (`n` > 1000).

        """

        if self.processes > 1:
            n_process = np.ceil(n / self.processes).astype(int)
            n = n_process * self.processes

            # The cast to list is weird but not doing it gives recursion
            # limit errors, something weird going on with multiprocessing
            args = list(zip(
                [n_process] * self.processes, range(self.processes)))

            # No with statement or starmap here since Python 2.x
            # does not support it :(
            mp = Pool(self.processes, initializer=mp_init, initargs=(self,))
            results = mp.map(_sample_chain, args, chunksize=1)
            mp.close()
            mp.join()

            chains = np.vstack([r[1] for r in results])
            self.retries += sum(r[0] for r in results)
        else:
            mp_init(self)
            results = _sample_chain((n, 0))
            chains = results[1]

        # Update the global center
        self.center = (self.n_samples * self.center +
                       np.atleast_2d(chains).sum(0)) / (self.n_samples + n)
        self.n_samples += n

        if fluxes:
            names = [r.id for r in self.model.reactions]

            return pandas.DataFrame(
                chains[:, self.fwd_idx] - chains[:, self.rev_idx],
                columns=names)
        else:
            names = [v.name for v in self.model.variables]

            return pandas.DataFrame(chains, columns=names)

    # Models can be large so don't pass them around during multiprocessing
    def __getstate__(self):
        """Return the object for serialization."""
        d = dict(self.__dict__)
        del d['model']
        return d
