'''@file target_coder.py
a file containing the target coders which can be used to encode and decode text,
alignments etc. '''

from abc import ABCMeta, abstractmethod
import numpy as np

class TargetCoder(object):
    '''an abstract class for a target coder which can encode and decode target
    sequences'''

    __metaclass__ = ABCMeta

    def __init__(self, target_normalizer):
        '''
        TargetCoder constructor

        Args:
            target_normalizer: a target normalizer function
        '''

        #save the normalizer
        self.target_normalizer = target_normalizer

        #create an alphabet of possible targets
        alphabet = self.create_alphabet()

        #create a lookup dictionary for fast encoding
        self.lookup = {character:index for index, character
                       in enumerate(alphabet)}

    @abstractmethod
    def create_alphabet(self):
        '''create the alphabet for the coder'''

    def encode(self, targets):
        '''
        encode a target sequence

        Args:
            targets: a string containing the target sequence

        Returns:
            A numpy array containing the encoded targets
        '''

        #normalize the targets
        normalized_targets = self.target_normalizer(targets, self.lookup.keys())

        encoded_targets = []

        for target in normalized_targets.split(' '):
            encoded_targets.append(self.lookup[target])

        return np.array(encoded_targets, dtype=np.uint32)

    def decode(self, encoded_targets):
        '''
        decode an encoded target sequence

        Args:
            encoded_targets: A numpy array containing the encoded targets

        Returns:
            A string containing the decoded target sequence
        '''

        targets = [self.lookup.keys()[encoded_target]
                   for encoded_target in encoded_targets]

        return ' '.join(targets)

    @property
    def num_labels(self):
        '''the number of possible labels'''

        return len(self.lookup)

class TextCoder(TargetCoder):
    '''a coder for text'''

    def create_alphabet(self):
        '''create the alphabet of characters'''

        alphabet = []

        # end of sentence token
        alphabet.append('<eos>')

        #start of sentence token
        alphabet.append('<sos>')

        #space
        alphabet.append('<space>')

        #comma
        alphabet.append(',')

        #period
        alphabet.append('.')

        #apostrophy
        alphabet.append('\'')

        #hyphen
        alphabet.append('-')

        #question mark
        alphabet.append('?')

        #unknown character
        alphabet.append('<unk>')

        #letters in the alphabet
        for letter in range(ord('a'), ord('z')+1):
            alphabet.append(chr(letter))

        return alphabet

class AlignmentCoder(TargetCoder):
    '''a coder for state alignments'''

    def __init__(self, target_normalizer, num_targets):
        '''
        AlignmentCoder constructor

        Args:
            target_normalizer: a target normalizer function
            num_targets: total number of targets
        '''

        self.num_targets = num_targets
        super(AlignmentCoder, self).__init__(target_normalizer)

    def create_alphabet(self):
        '''
        create the alphabet of alignment targets
        '''

        alphabet = [str(target) for target in range(self.num_targets)]

        return alphabet
