from __future__ import print_function

import string
import sys
from collections import deque

import numpy as np
from scipy.misc import logsumexp
from sklearn.base import BaseEstimator, _pprint
from sklearn.utils import check_array, check_random_state
from sklearn.utils.validation import check_is_fitted

from . import _hmmc
from .utils import normalize, log_normalize, iter_from_X_lengths, log_mask_zero


#: Supported decoder algorithms.
DECODER_ALGORITHMS = frozenset(("viterbi", "map"))


class ConvergenceMonitor(object):
    """Monitors and reports convergence to :data:`sys.stderr`.

    Parameters
    ----------
    tol : double
        Convergence threshold. EM has converged either if the maximum
        number of iterations is reached or the log probability
        improvement between the two consecutive iterations is less
        than threshold.

    n_iter : int
        Maximum number of iterations to perform.

    verbose : bool
        If ``True`` then per-iteration convergence reports are printed,
        otherwise the monitor is mute.

    Attributes
    ----------
    history : deque
        The log probability of the data for the last two training
        iterations. If the values are not strictly increasing, the
        model did not converge.

    iter : int
        Number of iterations performed while training the model.
    """
    _template = "{iter:>10d} {logprob:>16.4f} {delta:>+16.4f}"

    def __init__(self, tol, n_iter, verbose):
        self.tol = tol
        self.n_iter = n_iter
        self.verbose = verbose
        self.history = deque(maxlen=2)
        self.iter = 0

    def __repr__(self):
        class_name = self.__class__.__name__
        params = dict(vars(self), history=list(self.history))
        return "{0}({1})".format(
            class_name, _pprint(params, offset=len(class_name)))

    def report(self, logprob):
        """Reports convergence to :data:`sys.stderr`.

        The output consists of three columns: iteration number, log
        probability of the data at the current iteration and convergence
        rate.  At the first iteration convergence rate is unknown and
        is thus denoted by NaN.

        Parameters
        ----------
        logprob : float
            The log probability of the data as computed by EM algorithm
            in the current iteration.
        """
        if self.verbose:
            delta = logprob - self.history[-1] if self.history else np.nan
            message = self._template.format(
                iter=self.iter + 1, logprob=logprob, delta=delta)
            print(message, file=sys.stderr)

        self.history.append(logprob)
        self.iter += 1

    @property
    def converged(self):
        """``True`` if the EM algorithm converged and ``False`` otherwise."""
        # XXX we might want to check that ``logprob`` is non-decreasing.
        return (self.iter == self.n_iter or
                (len(self.history) == 2 and
                 self.history[1] - self.history[0] < self.tol))


class _BaseHMM(BaseEstimator):
    """Base class for Hidden Markov Models.

    This class allows for easy evaluation of, sampling from, and
    maximum-likelihood estimation of the parameters of a HMM.

    See the instance documentation for details specific to a
    particular object.

    Parameters
    ----------
    n_components : int
        Number of states in the model.

    startprob_prior : array, shape (n_components, )
        Initial state occupation prior distribution.

    transmat_prior : array, shape (n_components, n_components)
        Matrix of prior transition probabilities between states.

    algorithm : string
        Decoder algorithm. Must be one of "viterbi" or "map".
        Defaults to "viterbi".

    random_state: RandomState or an int seed
        A random number generator instance.

    n_iter : int, optional
        Maximum number of iterations to perform.

    tol : float, optional
        Convergence threshold. EM will stop if the gain in log-likelihood
        is below this value.

    verbose : bool, optional
        When ``True`` per-iteration convergence reports are printed
        to :data:`sys.stderr`. You can diagnose convergence via the
        :attr:`monitor_` attribute.

    params : string, optional
        Controls which parameters are updated in the training
        process.  Can contain any combination of 's' for startprob,
        't' for transmat, and other characters for subclass-specific
        emission parameters. Defaults to all parameters.

    init_params : string, optional
        Controls which parameters are initialized prior to
        training.  Can contain any combination of 's' for
        startprob, 't' for transmat, and other characters for
        subclass-specific emission parameters. Defaults to all
        parameters.

    Attributes
    ----------
    monitor\_ : ConvergenceMonitor
        Monitor object used to check the convergence of EM.

    startprob\_ : array, shape (n_components, )
        Initial state occupation distribution.

    transmat\_ : array, shape (n_components, n_components)
        Matrix of transition probabilities between states.
    """
    def __init__(self, n_components=1,
                 startprob_prior=1.0, transmat_prior=1.0,
                 algorithm="viterbi", random_state=None,
                 n_iter=10, tol=1e-2, verbose=False,
                 params=string.ascii_letters,
                 init_params=string.ascii_letters):
        self.n_components = n_components
        self.params = params
        self.init_params = init_params
        self.startprob_prior = startprob_prior
        self.transmat_prior = transmat_prior
        self.algorithm = algorithm
        self.random_state = random_state
        self.n_iter = n_iter
        self.tol = tol
        self.verbose = verbose

    def score_samples(self, X, lengths=None):
        """Compute the log probability under the model and compute posteriors.

        Parameters
        ----------
        X : array-like, shape (n_samples, n_features)
            Feature matrix of individual samples.

        lengths : array-like of integers, shape (n_sequences, ), optional
            Lengths of the individual sequences in ``X``. The sum of
            these should be ``n_samples``.

        Returns
        -------
        logprob : float
            Log likelihood of ``X``.

        posteriors : array, shape (n_samples, n_components)
            State-membership probabilities for each sample in ``X``.

        See Also
        --------
        score : Compute the log probability under the model.
        decode : Find most likely state sequence corresponding to ``X``.
        """
        check_is_fitted(self, "startprob_")
        self._check()

        X = check_array(X)
        n_samples = X.shape[0]
        logprob = 0
        posteriors = np.zeros((n_samples, self.n_components))
        for i, j in iter_from_X_lengths(X, lengths):
            framelogprob = self._compute_log_likelihood(X[i:j])
            logprobij, fwdlattice = self._do_forward_pass(framelogprob)
            logprob += logprobij

            bwdlattice = self._do_backward_pass(framelogprob)
            posteriors[i:j] = self._compute_posteriors(fwdlattice, bwdlattice)
        return logprob, posteriors

    def score(self, X, lengths=None):
        """Compute the log probability under the model.

        Parameters
        ----------
        X : array-like, shape (n_samples, n_features)
            Feature matrix of individual samples.

        lengths : array-like of integers, shape (n_sequences, ), optional
            Lengths of the individual sequences in ``X``. The sum of
            these should be ``n_samples``.

        Returns
        -------
        logprob : float
            Log likelihood of ``X``.

        See Also
        --------
        score_samples : Compute the log probability under the model and
            posteriors.
        decode : Find most likely state sequence corresponding to ``X``.
        """
        check_is_fitted(self, "startprob_")
        self._check()

        X = check_array(X)
        # XXX we can unroll forward pass for speed and memory efficiency.
        logprob = 0
        for i, j in iter_from_X_lengths(X, lengths):
            framelogprob = self._compute_log_likelihood(X[i:j])
            logprobij, _fwdlattice = self._do_forward_pass(framelogprob)
            logprob += logprobij
        return logprob

    def _decode_viterbi(self, X):
        framelogprob = self._compute_log_likelihood(X)
        return self._do_viterbi_pass(framelogprob)

    def _decode_map(self, X):
        _, posteriors = self.score_samples(X)
        logprob = np.max(posteriors, axis=1).sum()
        state_sequence = np.argmax(posteriors, axis=1)
        return logprob, state_sequence

    def decode(self, X, lengths=None, algorithm=None):
        """Find most likely state sequence corresponding to ``X``.

        Parameters
        ----------
        X : array-like, shape (n_samples, n_features)
            Feature matrix of individual samples.

        lengths : array-like of integers, shape (n_sequences, ), optional
            Lengths of the individual sequences in ``X``. The sum of
            these should be ``n_samples``.

        algorithm : string
            Decoder algorithm. Must be one of "viterbi" or "map".
            If not given, :attr:`decoder` is used.

        Returns
        -------
        logprob : float
            Log probability of the produced state sequence.

        state_sequence : array, shape (n_samples, )
            Labels for each sample from ``X`` obtained via a given
            decoder ``algorithm``.

        See Also
        --------
        score_samples : Compute the log probability under the model and
            posteriors.
        score : Compute the log probability under the model.
        """
        check_is_fitted(self, "startprob_")
        self._check()

        algorithm = algorithm or self.algorithm
        if algorithm not in DECODER_ALGORITHMS:
            raise ValueError("Unknown decoder {0!r}".format(algorithm))

        decoder = {
            "viterbi": self._decode_viterbi,
            "map": self._decode_map
        }[algorithm]

        X = check_array(X)
        n_samples = X.shape[0]
        logprob = 0
        state_sequence = np.empty(n_samples, dtype=int)
        for i, j in iter_from_X_lengths(X, lengths):
            # XXX decoder works on a single sample at a time!
            logprobij, state_sequenceij = decoder(X[i:j])
            logprob += logprobij
            state_sequence[i:j] = state_sequenceij

        return logprob, state_sequence

    def predict(self, X, lengths=None):
        """Find most likely state sequence corresponding to ``X``.

        Parameters
        ----------
        X : array-like, shape (n_samples, n_features)
            Feature matrix of individual samples.

        lengths : array-like of integers, shape (n_sequences, ), optional
            Lengths of the individual sequences in ``X``. The sum of
            these should be ``n_samples``.

        Returns
        -------
        state_sequence : array, shape (n_samples, )
            Labels for each sample from ``X``.
        """
        _, state_sequence = self.decode(X, lengths)
        return state_sequence

    def predict_proba(self, X, lengths=None):
        """Compute the posterior probability for each state in the model.

        X : array-like, shape (n_samples, n_features)
            Feature matrix of individual samples.

        lengths : array-like of integers, shape (n_sequences, ), optional
            Lengths of the individual sequences in ``X``. The sum of
            these should be ``n_samples``.

        Returns
        -------
        posteriors : array, shape (n_samples, n_components)
            State-membership probabilities for each sample from ``X``.
        """
        _, posteriors = self.score_samples(X, lengths)
        return posteriors

    def sample(self, n_samples=1, random_state=None):
        """Generate random samples from the model.

        Parameters
        ----------
        n_samples : int
            Number of samples to generate.

        random_state : RandomState or an int seed
            A random number generator instance. If ``None``, the object's
            ``random_state`` is used.

        Returns
        -------
        X : array, shape (n_samples, n_features)
            Feature matrix.

        state_sequence : array, shape (n_samples, )
            State sequence produced by the model.
        """
        check_is_fitted(self, "startprob_")

        if random_state is None:
            random_state = self.random_state
        random_state = check_random_state(random_state)

        startprob_cdf = np.cumsum(self.startprob_)
        transmat_cdf = np.cumsum(self.transmat_, axis=1)

        currstate = (startprob_cdf > random_state.rand()).argmax()
        state_sequence = [currstate]
        X = [self._generate_sample_from_state(
            currstate, random_state=random_state)]

        for t in range(n_samples - 1):
            currstate = (transmat_cdf[currstate] > random_state.rand()) \
                .argmax()
            state_sequence.append(currstate)
            X.append(self._generate_sample_from_state(
                currstate, random_state=random_state))

        return np.atleast_2d(X), np.array(state_sequence, dtype=int)

    def fit(self, X, lengths=None):
        """Estimate model parameters.

        An initialization step is performed before entering the
        EM algorithm. If you want to avoid this step for a subset of
        the parameters, pass proper ``init_params`` keyword argument
        to estimator's constructor.

        Parameters
        ----------
        X : array-like, shape (n_samples, n_features)
            Feature matrix of individual samples.

        lengths : array-like of integers, shape (n_sequences, )
            Lengths of the individual sequences in ``X``. The sum of
            these should be ``n_samples``.

        Returns
        -------
        self : object
            Returns self.
        """
        X = check_array(X)
        self._init(X, lengths=lengths)
        self._check()

        self.monitor_ = ConvergenceMonitor(self.tol, self.n_iter, self.verbose)
        for iter in range(self.n_iter):
            stats = self._initialize_sufficient_statistics()
            curr_logprob = 0
            for i, j in iter_from_X_lengths(X, lengths):
                framelogprob = self._compute_log_likelihood(X[i:j])
                logprob, fwdlattice = self._do_forward_pass(framelogprob)
                curr_logprob += logprob
                bwdlattice = self._do_backward_pass(framelogprob)
                posteriors = self._compute_posteriors(fwdlattice, bwdlattice)
                self._accumulate_sufficient_statistics(
                    stats, X[i:j], framelogprob, posteriors, fwdlattice,
                    bwdlattice)

            # XXX must be before convergence check, because otherwise
            #     there won't be any updates for the case ``n_iter=1``.
            self._do_mstep(stats)

            self.monitor_.report(curr_logprob)
            if self.monitor_.converged:
                break

        return self

    def _do_viterbi_pass(self, framelogprob):
        n_samples, n_components = framelogprob.shape
        state_sequence, logprob = _hmmc._viterbi(
            n_samples, n_components, log_mask_zero(self.startprob_),
            log_mask_zero(self.transmat_), framelogprob)
        return logprob, state_sequence

    def _do_forward_pass(self, framelogprob):
        n_samples, n_components = framelogprob.shape
        fwdlattice = np.zeros((n_samples, n_components))
        _hmmc._forward(n_samples, n_components,
                       log_mask_zero(self.startprob_),
                       log_mask_zero(self.transmat_),
                       framelogprob, fwdlattice)
        return logsumexp(fwdlattice[-1]), fwdlattice

    def _do_backward_pass(self, framelogprob):
        n_samples, n_components = framelogprob.shape
        bwdlattice = np.zeros((n_samples, n_components))
        _hmmc._backward(n_samples, n_components,
                        log_mask_zero(self.startprob_),
                        log_mask_zero(self.transmat_),
                        framelogprob, bwdlattice)
        return bwdlattice

    def _compute_posteriors(self, fwdlattice, bwdlattice):
        # gamma is guaranteed to be correctly normalized by logprob at
        # all frames, unless we do approximate inference using pruning.
        # So, we will normalize each frame explicitly in case we
        # pruned too aggressively.
        log_gamma = fwdlattice + bwdlattice
        log_normalize(log_gamma, axis=1)
        return np.exp(log_gamma)

    def _init(self, X, lengths):
        """Initializes model parameters prior to fitting.

        Parameters
        ----------
        X : array-like, shape (n_samples, n_features)
            Feature matrix of individual samples.

        lengths : array-like of integers, shape (n_sequences, )
            Lengths of the individual sequences in ``X``. The sum of
            these should be ``n_samples``.
        """
        init = 1. / self.n_components
        if 's' in self.init_params or not hasattr(self, "startprob_"):
            self.startprob_ = np.full(self.n_components, init)
        if 't' in self.init_params or not hasattr(self, "transmat_"):
            self.transmat_ = np.full((self.n_components, self.n_components),
                                     init)

    def _check(self):
        """Validates model parameters prior to fitting.

        Raises
        ------

        ValueError
            If any of the parameters are invalid, e.g. if :attr:`startprob_`
            don't sum to 1.
        """
        self.startprob_ = np.asarray(self.startprob_)
        if len(self.startprob_) != self.n_components:
            raise ValueError("startprob_ must have length n_components")
        if not np.allclose(self.startprob_.sum(), 1.0):
            raise ValueError("startprob_ must sum to 1.0 (got {0:.4f})"
                             .format(self.startprob_.sum()))

        self.transmat_ = np.asarray(self.transmat_)
        if self.transmat_.shape != (self.n_components, self.n_components):
            raise ValueError(
                "transmat_ must have shape (n_components, n_components)")
        if not np.allclose(self.transmat_.sum(axis=1), 1.0):
            raise ValueError("rows of transmat_ must sum to 1.0 (got {0})"
                             .format(self.transmat_.sum(axis=1)))

    def _compute_log_likelihood(self, X):
        """Computes per-component log probability under the model.

        Parameters
        ----------
        X : array-like, shape (n_samples, n_features)
            Feature matrix of individual samples.

        Returns
        -------
        logprob : array, shape (n_samples, n_components)
            Log probability of each sample in ``X`` for each of the
            model states.
        """

    def _generate_sample_from_state(self, state, random_state=None):
        """Generates a random sample from a given component.

        Parameters
        ----------
        state : int
            Index of the component to condition on.

        random_state: RandomState or an int seed
            A random number generator instance. If ``None``, the object's
            ``random_state`` is used.

        Returns
        -------
        X : array, shape (n_features, )
            A random sample from the emission distribution corresponding
            to a given component.
        """

    # Methods used by self.fit()

    def _initialize_sufficient_statistics(self):
        """Initializes sufficient statistics required for M-step.

        The method is *pure*, meaning that it doesn't change the state of
        the instance.  For extensibility computed statistics are stored
        in a dictionary.

        Returns
        -------
        nobs : int
            Number of samples in the data.

        start : array, shape (n_components, )
            An array where the i-th element corresponds to the posterior
            probability of the first sample being generated by the i-th
            state.

        trans : array, shape (n_components, n_components)
            An array where the (i, j)-th element corresponds to the
            posterior probability of transitioning between the i-th to j-th
            states.
        """
        stats = {'nobs': 0,
                 'start': np.zeros(self.n_components),
                 'trans': np.zeros((self.n_components, self.n_components))}
        return stats

    def _accumulate_sufficient_statistics(self, stats, X, framelogprob,
                                          posteriors, fwdlattice, bwdlattice):
        """Updates sufficient statistics from a given sample.

        Parameters
        ----------
        stats : dict
            Sufficient statistics as returned by
            :meth:`~base._BaseHMM._initialize_sufficient_statistics`.

        X : array, shape (n_samples, n_features)
            Sample sequence.

        framelogprob : array, shape (n_samples, n_components)
            Log-probabilities of each sample under each of the model states.

        posteriors : array, shape (n_samples, n_components)
            Posterior probabilities of each sample being generated by each
            of the model states.

        fwdlattice, bwdlattice : array, shape (n_samples, n_components)
            Log-forward and log-backward probabilities.
        """
        stats['nobs'] += 1
        if 's' in self.params:
            stats['start'] += posteriors[0]
        if 't' in self.params:
            n_samples, n_components = framelogprob.shape
            # when the sample is of length 1, it contains no transitions
            # so there is no reason to update our trans. matrix estimate
            if n_samples <= 1:
                return

            lneta = np.zeros((n_samples - 1, n_components, n_components))
            _hmmc._compute_lneta(n_samples, n_components, fwdlattice,
                                 log_mask_zero(self.transmat_),
                                 bwdlattice, framelogprob, lneta)
            stats['trans'] += np.exp(logsumexp(lneta, axis=0))

    def _do_mstep(self, stats):
        """Performs the M-step of EM algorithm.

        Parameters
        ----------
        stats : dict
            Sufficient statistics updated from all available samples.
        """
        # The ``np.where`` calls guard against updating forbidden states
        # or transitions in e.g. a left-right HMM.
        if 's' in self.params:
            startprob_ = self.startprob_prior - 1.0 + stats['start']
            self.startprob_ = np.where(self.startprob_ == 0.0,
                                       self.startprob_, startprob_)
            normalize(self.startprob_)
        if 't' in self.params:
            transmat_ = self.transmat_prior - 1.0 + stats['trans']
            self.transmat_ = np.where(self.transmat_ == 0.0,
                                      self.transmat_, transmat_)
            normalize(self.transmat_, axis=1)
