text: # Natural Language Toolkit: Punkt sentence tokenizer
  #
  # Copyright (C) 2001-2011 NLTK Project
  # Algorithm: Kiss & Strunk (2006)
  # Author: Willy <willy@csse.unimelb.edu.au> (original Python port)
  #         Steven Bird <sb@csse.unimelb.edu.au> (additions)
  #         Edward Loper <edloper@gradient.cis.upenn.edu> (rewrite)
  #         Joel Nothman <jnothman@student.usyd.edu.au> (almost rewrite)
  # URL: <http://www.nltk.org/>
  # For license information, see LICENSE.TXT
  #
  # $Id: probability.py 4865 2007-07-11 22:6:07Z edloper $
  
  """
  The Punkt sentence tokenizer.  The algorithm for this tokenizer is
  described in Kiss & Strunk (2006)::
  
    Kiss, Tibor and Strunk, Jan (2006): Unsupervised Multilingual Sentence
      Boundary Detection.  Computational Linguistics 32: 485-525.
  """
  
  # TODO: Make orthographic heuristic less susceptible to overtraining
  # TODO: Frequent sentence starters optionally exclude always-capitalised words
  # FIXME: Problem with ending string with e.g. '!!!' -> '!! !'
  
  import re
  import math
  
  from nltk.compat import defaultdict
  from nltk.probability import FreqDist
  
  from api import TokenizerI
  
  ######################################################################
  #{ Orthographic Context Constants
  ######################################################################
  # The following constants are used to describe the orthographic
  # contexts in which a word can occur.  BEG=beginning, MID=middle,
  # UNK=unknown, UC=uppercase, LC=lowercase, NC=no case.
  
  _ORTHO_BEG_UC    = 1 << 1
  """Orthogaphic context: beginning of a sentence with upper case."""
  
  _ORTHO_MID_UC    = 1 << 2
  """Orthogaphic context: middle of a sentence with upper case."""
  
  _ORTHO_UNK_UC    = 1 << 3
  """Orthogaphic context: unknown position in a sentence with upper case."""
  
  _ORTHO_BEG_LC    = 1 << 4
  """Orthogaphic context: beginning of a sentence with lower case."""
  
  _ORTHO_MID_LC    = 1 << 5
  """Orthogaphic context: middle of a sentence with lower case."""
  
  _ORTHO_UNK_LC    = 1 << 6
  """Orthogaphic context: unknown position in a sentence with lower case."""
  
  _ORTHO_UC = _ORTHO_BEG_UC + _ORTHO_MID_UC + _ORTHO_UNK_UC
  """Orthogaphic context: occurs with upper case."""
  
  _ORTHO_LC = _ORTHO_BEG_LC + _ORTHO_MID_LC + _ORTHO_UNK_LC
  """Orthogaphic context: occurs with lower case."""
  
  _ORTHO_MAP = {
          ('initial',  'upper'): _ORTHO_BEG_UC,
          ('internal', 'upper'): _ORTHO_MID_UC,
          ('unknown',  'upper'): _ORTHO_UNK_UC,
          ('initial',  'lower'): _ORTHO_BEG_LC,
          ('internal', 'lower'): _ORTHO_MID_LC,
          ('unknown',  'lower'): _ORTHO_UNK_LC,
  }
  """A map from context position and first-letter case to the
  appropriate orthographic context flag."""
  
  #} (end orthographic context constants)
  ######################################################################
          
  ######################################################################
  #{ Language-dependent variables
  ######################################################################
  
  class PunktLanguageVars(object):
      """
      Stores variables, mostly regular expressions, which may be
      language-dependent for correct application of the algorithm.
      An extension of this class may modify its properties to suit
      a language other than English; an instance can then be passed
      as an argument to PunktSentenceTokenizer and PunktTrainer
      constructors.
      """
  
      __slots__ = ('_re_period_context', '_re_word_tokenizer')
  
      def __getstate__(self):
          # All modifications to the class are performed by inheritance.
          # Non-default parameters to be pickled must be defined in the inherited
          # class.
          return 1
  
      def __setstate__(self, state):
          return 1
  
      sent_end_chars = ('.', '?', '!')
      """Characters which are candidates for sentence boundaries"""
  
      @property
      def _re_sent_end_chars(self):
          return '[%s]' % re.escape(''.join(self.sent_end_chars))
  
      internal_punctuation = ',:;' # might want to extend this..
      """sentence internal punctuation, which indicates an abbreviation if
      preceded by a period-final token."""
  
      re_boundary_realignment = re.compile(r'["\')\]}]+?(?:\s+|(?=--)|$)',
              re.MULTILINE)
      """Used to realign punctuation that should be included in a sentence
      although it follows the period (or ?, !)."""
  
      _re_word_start    = r"[^\(\"\`{\[:;&\#\*@\)}\]\-,]"
      """Excludes some characters from starting word tokens"""
  
      _re_non_word_chars   = r"(?:[?!)\";}\]\*:@\'\({\[])"
      """Characters that cannot appear within words"""
  
      _re_multi_char_punct = r"(?:\-{2,}|\.{2,}|(?:\.\s){2,}\.)"
      """Hyphen and ellipsis are multi-character punctuation"""
  
      _word_tokenize_fmt = r'''(
          %(MultiChar)s
          |
          (?=%(WordStart)s)\S+?  # Accept word characters until end is found
          (?= # Sequences marking a word's end
              \s|                                 # White-space
              $|                                  # End-of-string
              %(NonWord)s|%(MultiChar)s|          # Punctuation
              ,(?=$|\s|%(NonWord)s|%(MultiChar)s) # Comma if at end of word
          )
          |
          \S
      )'''
      """Format of a regular expression to split punctuation from words,
      excluding period."""
  
      def _word_tokenizer_re(self):
          """Compiles and returns a regular expression for word tokenization"""
          try:
              return self._re_word_tokenizer
          except AttributeError:
              self._re_word_tokenizer = re.compile(
                  self._word_tokenize_fmt %
                  {
                      'NonWord':   self._re_non_word_chars,
                      'MultiChar': self._re_multi_char_punct,
                      'WordStart': self._re_word_start,
                  },
                  re.UNICODE | re.VERBOSE
              )
              return self._re_word_tokenizer
  
      def word_tokenize(self, s):
          """Tokenize a string to split of punctuation other than periods"""
          return self._word_tokenizer_re().findall(s)
  
      _period_context_fmt = r"""
          \S*                          # some word material
          %(SentEndChars)s             # a potential sentence ending
          (?=(?P<after_tok>
              %(NonWord)s              # either other punctuation
              |
              \s+(?P<next_tok>\S+)     # or whitespace and some other token
          ))"""
      """Format of a regular expression to find contexts including possible
      sentence boundaries. Matches token which the possible sentence boundary
      ends, and matches the following token within a lookahead expression."""
  
      def period_context_re(self):
          """Compiles and returns a regular expression to find contexts
          including possible sentence boundaries."""
          try:
              return self._re_period_context
          except:
              self._re_period_context = re.compile(
                  self._period_context_fmt %
                  {
                      'NonWord':      self._re_non_word_chars,
                      'SentEndChars': self._re_sent_end_chars,
                  },
                  re.UNICODE | re.VERBOSE)
              return self._re_period_context
  
  
  _re_non_punct = re.compile(r'[^\W\d]', re.UNICODE)
  """Matches token types that are not merely punctuation. (Types for
  numeric tokens are changed to ##number## and hence contain alpha.)"""
  
  #}
  ######################################################################
          
  
  ######################################################################
  #{ Punkt Word Tokenizer
  ######################################################################
  
  class PunktWordTokenizer(TokenizerI):
      # Retained for backward compatibility
      def __init__(self, lang_vars=PunktLanguageVars()):
          self._lang_vars = lang_vars
  
      def tokenize(self, text):
          return self._lang_vars.word_tokenize(text)
  
  #}
  ######################################################################
  
  
  #////////////////////////////////////////////////////////////
  #{ Helper Functions
  #////////////////////////////////////////////////////////////
  
  def _pair_iter(it):
      """
      Yields pairs of tokens from the given iterator such that each input
      token will appear as the first element in a yielded tuple. The last
      pair will have None as its second element.
      """
      it = iter(it)
      prev = it.next()
      for el in it:
          yield (prev, el)
          prev = el
      yield (prev, None)
  
  ######################################################################
  #{ Punkt Parameters
  ######################################################################
  
  class PunktParameters(object):
      """Stores data used to perform sentence boundary detection with punkt."""
  
      def __init__(self):
          self.abbrev_types = set()
          """A set of word types for known abbreviations."""
          
          self.collocations = set()
          """A set of word type tuples for known common collocations
          where the first word ends in a period.  E.g., ('S.', 'Bach')
          is a common collocation in a text that discusses 'Johann
          S. Bach'.  These count as negative evidence for sentence
          boundaries."""
          
          self.sent_starters = set()
          """A set of word types for words that often appear at the
          beginning of sentences."""
          
          self.ortho_context = defaultdict(int)
          """A dictionary mapping word types to the set of orthographic
          contexts that word type appears in.  Contexts are represented
          by adding orthographic context flags: ..."""
  
      def clear_abbrevs(self):
          self.abbrev_types = set()
  
      def clear_collocations(self):
          self.collocations = set()
  
      def clear_sent_starters(self):
          self.sent_starters = set()
  
      def clear_ortho_context(self):
          self.ortho_context = defaultdict(int)
  
      def add_ortho_context(self, typ, flag):
          self.ortho_context[typ] |= flag
  
  ######################################################################
  #{ PunktToken
  ######################################################################
  
  class PunktToken(object):
      """Stores a token of text with annotations produced during
      sentence boundary detection."""
  
      _properties = [
          'parastart', 'linestart',
          'sentbreak', 'abbr', 'ellipsis'
      ]
      __slots__ = ['tok', 'type', 'period_final'] + _properties
  
      def __init__(self, tok, **params):
          self.tok = tok
          self.type = self._get_type(tok)
          self.period_final = tok.endswith('.')
  
          for p in self._properties:
              setattr(self, p, None)
          for k, v in params.iteritems():
              setattr(self, k, v)
  
      #////////////////////////////////////////////////////////////
      #{ Regular expressions for properties
      #////////////////////////////////////////////////////////////
      # Note: [A-Za-z] is approximated by [^\W\d] in the general case.
      _RE_ELLIPSIS = re.compile(r'\.\.+$')
      _RE_NUMERIC = re.compile(r'^-?[\.,]?\d[\d,\.-]*\.?$')
      _RE_INITIAL = re.compile(r'[^\W\d]\.$', re.UNICODE)
      _RE_ALPHA = re.compile(r'[^\W\d]+$', re.UNICODE)
  
      #////////////////////////////////////////////////////////////
      #{ Derived properties
      #////////////////////////////////////////////////////////////
  
      def _get_type(self, tok):
          """Returns a case-normalized representation of the token."""
          return self._RE_NUMERIC.sub('##number##', tok.lower())
  
      @property
      def type_no_period(self):
          """
          The type with its final period removed if it has one.
          """
          if len(self.type) > 1 and self.type[-1] == '.':
              return self.type[:-1]
          return self.type
  
      @property
      def type_no_sentperiod(self):
          """
          The type with its final period removed if it is marked as a
          sentence break.
          """
          if self.sentbreak:
              return self.type_no_period
          return self.type
  
      @property
      def first_upper(self):
          """True if the token's first character is uppercase."""
          return self.tok[0].isupper()
  
      @property
      def first_lower(self):
          """True if the token's first character is lowercase."""
          return self.tok[0].islower()
  
      @property
      def first_case(self):
          if self.first_lower:
              return 'lower'
          elif self.first_upper:
              return 'upper'
          return 'none'
  
      @property
      def is_ellipsis(self):
          """True if the token text is that of an ellipsis."""
          return self._RE_ELLIPSIS.match(self.tok)
  
      @property
      def is_number(self):
          """True if the token text is that of a number."""
          return self.type.startswith('##number##')
  
      @property
      def is_initial(self):
          """True if the token text is that of an initial."""
          return self._RE_INITIAL.match(self.tok)
  
      @property
      def is_alpha(self):
          """True if the token text is all alphabetic."""
          return self._RE_ALPHA.match(self.tok)
  
      @property
      def is_non_punct(self):
          """True if the token is either a number or is alphabetic."""
          return _re_non_punct.search(self.type)
      
      #////////////////////////////////////////////////////////////
      #{ String representation
      #////////////////////////////////////////////////////////////
  
      def __repr__(self):
          """
          A string representation of the token that can reproduce it
          with eval(), which lists all the token's non-default
          annotations.
          """
          if self.type != self.tok:
              typestr = ' type=%s,' % repr(self.type)
          else:
              typestr = ''
  
          propvals = ', '.join(
              '%s=%s' % (p, repr(getattr(self, p)))
              for p in self._properties
              if getattr(self, p)
          )
  
          return '%s(%s,%s %s)' % (self.__class__.__name__,
              repr(self.tok), typestr, propvals)
  
      def __str__(self):
          """
          A string representation akin to that used by Kiss and Strunk.
          """
          res = self.tok
          if self.abbr:
              res += '<A>'
          if self.ellipsis:
              res += '<E>'
          if self.sentbreak:
              res += '<S>'
          return res
  
  ######################################################################
  #{ Punkt base class
  ######################################################################
  
  class _PunktBaseClass(object):
      """
      Includes common components of PunktTrainer and PunktSentenceTokenizer.
      """
      
      def __init__(self, lang_vars=PunktLanguageVars(), token_cls=PunktToken,
              params=PunktParameters()):
          self._params = params
          self._lang_vars = lang_vars
          self._Token = token_cls
          """The collection of parameters that determines the behavior
          of the punkt tokenizer."""
  
      #////////////////////////////////////////////////////////////
      #{ Word tokenization
      #////////////////////////////////////////////////////////////
  
      def _tokenize_words(self, plaintext):
          """
          Divide the given text into tokens, using the punkt word
          segmentation regular expression, and generate the resulting list
          of tokens augmented as three-tuples with two boolean values for whether
          the given token occurs at the start of a paragraph or a new line,
          respectively.
          """
          parastart = False
          for line in plaintext.split('\n'):
              if line.strip():
                  line_toks = iter(self._lang_vars.word_tokenize(line))
  
                  yield self._Token(line_toks.next(),
                          parastart=parastart, linestart=True)
                  parastart = False
  
                  for t in line_toks:
                      yield self._Token(t)
              else:
                  parastart = True
  
  
      #////////////////////////////////////////////////////////////
      #{ Annotation Procedures
      #////////////////////////////////////////////////////////////
  
      def _annotate_first_pass(self, tokens):
          """
          Perform the first pass of annotation, which makes decisions
          based purely based on the word type of each word:
          
            - '?', '!', and '.' are marked as sentence breaks.
            - sequences of two or more periods are marked as ellipsis.
            - any word ending in '.' that's a known abbreviation is
              marked as an abbreviation.
            - any other word ending in '.' is marked as a sentence break.
  
          Return these annotations as a tuple of three sets:
          
            - sentbreak_toks: The indices of all sentence breaks.
            - abbrev_toks: The indices of all abbreviations.
            - ellipsis_toks: The indices of all ellipsis marks.
          """
          for aug_tok in tokens:
              self._first_pass_annotation(aug_tok)
              yield aug_tok
  
      def _first_pass_annotation(self, aug_tok):
          """
          Performs type-based annotation on a single token.
          """
  
          tok = aug_tok.tok
  
          if tok in self._lang_vars.sent_end_chars:
              aug_tok.sentbreak = True
          elif aug_tok.is_ellipsis:
              aug_tok.ellipsis = True
          elif aug_tok.period_final and not tok.endswith('..'):
              if (tok[:-1].lower() in self._params.abbrev_types or
                  tok[:-1].lower().split('-')[-1] in self._params.abbrev_types):
  
                  aug_tok.abbr = True
              else:
                  aug_tok.sentbreak = True
  
          return
  
  ######################################################################
  #{ Punkt Trainer
  ######################################################################
  
  
  class PunktTrainer(_PunktBaseClass):
      """Learns parameters used in Punkt sentence boundary detection."""
  
      def __init__(self, train_text=None, verbose=False,
              lang_vars=PunktLanguageVars(), token_cls=PunktToken):
  
          _PunktBaseClass.__init__(self, lang_vars=lang_vars,
                  token_cls=token_cls)
  
          self._type_fdist = FreqDist()
          """A frequency distribution giving the frequency of each
          case-normalized token type in the training data."""
          
          self._num_period_toks = 0
          """The number of words ending in period in the training data."""
          
          self._collocation_fdist = FreqDist()
          """A frequency distribution giving the frequency of all
          bigrams in the training data where the first word ends in a
          period.  Bigrams are encoded as tuples of word types.
          Especially common collocations are extracted from this
          frequency distribution, and stored in
          L{_params}.L{collocations <PunktParameters.collocations>}."""
          
          self._sent_starter_fdist = FreqDist()
          """A frequency distribution giving the frequency of all words
          that occur at the training data at the beginning of a sentence
          (after the first pass of annotation).  Especially common
          sentence starters are extracted from this frequency
          distribution, and stored in L{_params}.L{sent_starters
          <PunktParameters.sent_starters>}.
          """
  
          self._sentbreak_count = 0
          """The total number of sentence breaks identified in training, used for
          calculating the frequent sentence starter heuristic."""
  
          self._finalized = True
          """A flag as to whether the training has been finalized by finding
          collocations and sentence starters, or whether finalize_training()
          still needs to be called."""
  
          if train_text:
              self.train(train_text, verbose, finalize=True)
  
      def get_params(self):
          """
          Calculates and returns parameters for sentence boundary detection as
          derived from training."""
          if not self._finalized:
              self.finalize_training()
          return self._params
  
      #////////////////////////////////////////////////////////////
      #{ Customization Variables
      #////////////////////////////////////////////////////////////
  
      ABBREV = 0.3
      """cut-off value whether a 'token' is an abbreviation"""
  
      IGNORE_ABBREV_PENALTY = False
      """allows the disabling of the abbreviation penalty heuristic, which
      exponentially disadvantages words that are found at times without a
      final period."""
  
      ABBREV_BACKOFF = 5
      """upper cut-off for Mikheev's(2002) abbreviation detection algorithm"""
  
      COLLOCATION = 7.88
      """minimal log-likelihood value that two tokens need to be considered
      as a collocation"""
  
      SENT_STARTER = 30
      """minimal log-likelihood value that a token requires to be considered
      as a frequent sentence starter"""
  
      INCLUDE_ALL_COLLOCS = False
      """this includes as potential collocations all word pairs where the first
      word ends in a period. It may be useful in corpora where there is a lot
      of variation that makes abbreviations like Mr difficult to identify."""
      
      INCLUDE_ABBREV_COLLOCS = False
      """this includes as potential collocations all word pairs where the first
      word is an abbreviation. Such collocations override the orthographic
      heuristic, but not the sentence starter heuristic. This is overridden by
      INCLUDE_ALL_COLLOCS, and if both are false, only collocations with initials
      and ordinals are considered."""
      """"""
  
      MIN_COLLOC_FREQ = 1
      """this sets a minimum bound on the number of times a bigram needs to
      appear before it can be considered a collocation, in addition to log
      likelihood statistics. This is useful when INCLUDE_ALL_COLLOCS is True."""
  
      #////////////////////////////////////////////////////////////
      #{ Training..
      #////////////////////////////////////////////////////////////
  
      def train(self, text, verbose=False, finalize=True):
          """
          Collects training data from a given text. If finalize is True, it
          will determine all the parameters for sentence boundary detection. If
          not, this will be delayed until get_params() or finalize_training() is
          called. If verbose is True, abbreviations found will be listed.
          """
          # Break the text into tokens; record which token indices correspond to
          # line starts and paragraph starts; and determine their types.
          self._train_tokens(self._tokenize_words(text), verbose)
          if finalize:
              self.finalize_training(verbose)
  
      def train_tokens(self, tokens, verbose=False, finalize=True):
          """
          Collects training data from a given list of tokens.
          """
          self._train_tokens((self._Token(t) for t in tokens), verbose)
          if finalize:
              self.finalize_training(verbose)
  
      def _train_tokens(self, tokens, verbose):
          self._finalized = False
  
          # Ensure tokens are a list
          tokens = list(tokens)
  
          # Find the frequency of each case-normalized type.  (Don't
          # strip off final periods.)  Also keep track of the number of
          # tokens that end in periods.
          for aug_tok in tokens:
              self._type_fdist.inc(aug_tok.type)
              if aug_tok.period_final:
                  self._num_period_toks += 1
  
          # Look for new abbreviations, and for types that no longer are
          unique_types = self._unique_types(tokens)
          for abbr, score, is_add in self._reclassify_abbrev_types(unique_types):
              if score >= self.ABBREV:
                  if is_add:
                      self._params.abbrev_types.add(abbr)
                      if verbose:
                          print ('  Abbreviation: [%6.4f] %s' %
                                 (score, abbr))
              else:
                  if not is_add:
                      self._params.abbrev_types.remove(abbr)
                      if verbose:
                          print ('  Removed abbreviation: [%6.4f] %s' %
                                 (score, abbr))
  
          # Make a preliminary pass through the document, marking likely
          # sentence breaks, abbreviations, and ellipsis tokens.
          tokens = list(self._annotate_first_pass(tokens))
  
          # Check what contexts each word type can appear in, given the
          # case of its first letter.
          self._get_orthography_data(tokens)
  
          # We need total number of sentence breaks to find sentence starters
          self._sentbreak_count += self._get_sentbreak_count(tokens)
  
          # The remaining heuristics relate to pairs of tokens where the first
          # ends in a period.
          for aug_tok1, aug_tok2 in _pair_iter(tokens):
              if not aug_tok1.period_final or not aug_tok2:
                  continue
  
              # Is the first token a rare abbreviation?
              if self._is_rare_abbrev_type(aug_tok1, aug_tok2):
                  self._params.abbrev_types.add(aug_tok1.type_no_period)
                  if verbose:
                      print ('  Rare Abbrev: %s' % aug_tok1.type)
  
              # Does second token have a high likelihood of starting a sentence?
              if self._is_potential_sent_starter(aug_tok2, aug_tok1):
                  self._sent_starter_fdist.inc(aug_tok2.type)
  
              # Is this bigram a potential collocation?
              if self._is_potential_collocation(aug_tok1, aug_tok2):
                  self._collocation_fdist.inc(
                      (aug_tok1.type_no_period, aug_tok2.type_no_sentperiod))
  
      def _unique_types(self, tokens):
          return set(aug_tok.type for aug_tok in tokens)
          
      def finalize_training(self, verbose=False):
          """
          Uses data that has been gathered in training to determine likely
          collocations and sentence starters.
          """
          self._params.clear_sent_starters()
          for typ, ll in self._find_sent_starters():
              self._params.sent_starters.add(typ)
              if verbose:
                  print ('  Sent Starter: [%6.4f] %r' % (ll, typ))
  
          self._params.clear_collocations()
          for (typ1, typ2), ll in self._find_collocations():
              self._params.collocations.add( (typ1,typ2) )
              if verbose:
                  print ('  Collocation: [%6.4f] %r+%r' %
                         (ll, typ1, typ2))
  
          self._finalized = True
  
      #////////////////////////////////////////////////////////////
      #{ Overhead reduction
      #////////////////////////////////////////////////////////////
  
      def freq_threshold(self, ortho_thresh=2, type_thresh=2, colloc_thres=2,
              sentstart_thresh=2):
          """
          Allows memory use to be reduced after much training by removing data
          about rare tokens that are unlikely to have a statistical effect with
          further training. Entries occurring above the given thresholds will be
          retained.
          """
          if ortho_thresh > 1:
              old_oc = self._params.ortho_context
              self._params.clear_ortho_context()
              for tok, count in self._type_fdist.iteritems():
                  if count >= ortho_thresh:
                      self._params.ortho_context[tok] = old_oc[tok]
  
          self._type_fdist = self._freq_threshold(self._type_fdist, type_thresh)
          self._collocation_fdist = self._freq_threshold(
                  self._collocation_fdist, colloc_thres)
          self._sent_starter_fdist = self._freq_threshold(
                  self._sent_starter_fdist, sentstart_thresh)
  
      def _freq_threshold(self, fdist, threshold):
          """
          Returns a FreqDist containing only data with counts below a given
          threshold, as well as a mapping (None -> count_removed).
          """
          # We assume that there is more data below the threshold than above it
          # and so create a new FreqDist rather than working in place.
          res = FreqDist()
          num_removed = 0
          for tok, count in fdist.iteritems():
              if count < threshold:
                  num_removed += 1
              else:
                  res.inc(tok, count)
          res.inc(None, num_removed)
          return res
  
      #////////////////////////////////////////////////////////////
      #{ Orthographic data
      #////////////////////////////////////////////////////////////
  
      def _get_orthography_data(self, tokens):
          """
          Collect information about whether each token type occurs
          with different case patterns (i) overall, (ii) at
          sentence-initial positions, and (iii) at sentence-internal
          positions.
          """
          # 'initial' or 'internal' or 'unknown'
          context = 'internal'
          tokens = list(tokens)
          
          for aug_tok in tokens:
              # If we encounter a paragraph break, then it's a good sign
              # that it's a sentence break.  But err on the side of
              # caution (by not positing a sentence break) if we just
              # saw an abbreviation.
              if aug_tok.parastart and context != 'unknown':
                  context = 'initial'
  
              # If we're at the beginning of a line, then err on the
              # side of calling our context 'initial'.
              if aug_tok.linestart and context == 'internal':
                  context = 'unknown'
  
              # Find the case-normalized type of the token.  If it's a
              # sentence-final token, strip off the period.
              typ = aug_tok.type_no_sentperiod
  
              # Update the orthographic context table.
              flag = _ORTHO_MAP.get((context, aug_tok.first_case), 0)
              if flag:
                  self._params.add_ortho_context(typ, flag)
  
              # Decide whether the next word is at a sentence boundary.
              if aug_tok.sentbreak:
                  if not (aug_tok.is_number or aug_tok.is_initial):
                      context = 'initial'
                  else:
                      context = 'unknown'
              elif aug_tok.ellipsis or aug_tok.abbr:
                  context = 'unknown'
              else:
                  context = 'internal'
          
      #////////////////////////////////////////////////////////////
      #{ Abbreviations
      #////////////////////////////////////////////////////////////
  
      def _reclassify_abbrev_types(self, types):
          """
          (Re)classifies each given token if
            - it is period-final and not a known abbreviation; or
            - it is not period-final and is otherwise a known abbreviation
          by checking whether its previous classification still holds according
          to the heuristics of section 3.
          Yields triples (abbr, score, is_add) where abbr is the type in question,
          score is its log-likelihood with penalties applied, and is_add specifies
          whether the present type is a candidate for inclusion or exclusion as an
          abbreviation, such that:
            - (is_add and score >= 0.3)    suggests a new abbreviation; and
            - (not is_add and score < 0.3) suggests excluding an abbreviation.
          """
          # (While one could recalculate abbreviations from all .-final tokens at
          # every iteration, in cases requiring efficiency, the number of tokens
          # in the present training document will be much less.)
  
          for typ in types:
              # Check some basic conditions, to rule out words that are
              # clearly not abbrev_types.
              if not _re_non_punct.search(typ) or typ == '##number##':
                  continue
              
              if typ.endswith('.'):
                  if typ in self._params.abbrev_types:
                      continue
                  typ = typ[:-1]
                  is_add = True
              else:
                  if typ not in self._params.abbrev_types:
                      continue
                  is_add = False
  
              # Count how many periods & nonperiods are in the
              # candidate.
              num_periods = typ.count('.') + 1
              num_nonperiods = len(typ) - num_periods + 1
              
              # Let <a> be the candidate without the period, and <b>
              # be the period.  Find a log likelihood ratio that
              # indicates whether <ab> occurs as a single unit (high
              # value of ll), or as two independent units <a> and
              # <b> (low value of ll).
              count_with_period = self._type_fdist[typ + '.']
              count_without_period = self._type_fdist[typ]
              ll = self._dunning_log_likelihood(
                  count_with_period + count_without_period,
                  self._num_period_toks, count_with_period,
                  self._type_fdist.N())
  
              # Apply three scaling factors to 'tweak' the basic log
              # likelihood ratio:
              #   F_length: long word -> less likely to be an abbrev
              #   F_periods: more periods -> more likely to be an abbrev
              #   F_penalty: penalize occurances w/o a period
              f_length = math.exp(-num_nonperiods)
              f_periods = num_periods
              f_penalty = (int(self.IGNORE_ABBREV_PENALTY)
                      or math.pow(num_nonperiods, -count_without_period))
              score = ll * f_length * f_periods * f_penalty
  
              yield typ, score, is_add
  
      def find_abbrev_types(self):
          """
          Recalculates abbreviations given type frequencies, despite no prior
          determination of abbreviations.
          This fails to include abbreviations otherwise found as "rare".
          """
          self._params.clear_abbrevs()
          tokens = (typ for typ in self._type_fdist if typ and typ.endswith('.'))
          for abbr, score, is_add in self._reclassify_abbrev_types(tokens):
              if score >= self.ABBREV:
                  self._params.abbrev_types.add(abbr)
  
      # This function combines the work done by the original code's
      # functions `count_orthography_context`, `get_orthography_count`,
      # and `get_rare_abbreviations`.
      def _is_rare_abbrev_type(self, cur_tok, next_tok):
          """
          A word type is counted as a rare abbreviation if...
            - it's not already marked as an abbreviation
            - it occurs fewer than ABBREV_BACKOFF times
            - either it is followed by a sentence-internal punctuation
              mark, *or* it is followed by a lower-case word that
              sometimes appears with upper case, but never occurs with
              lower case at the beginning of sentences.
          """
          if cur_tok.abbr or not cur_tok.sentbreak:
              return False
  
          # Find the case-normalized type of the token.  If it's
          # a sentence-final token, strip off the period.
          typ = cur_tok.type_no_sentperiod
          
          # Proceed only if the type hasn't been categorized as an
          # abbreviation already, and is sufficiently rare...
          count = self._type_fdist[typ] + self._type_fdist[typ[:-1]]
          if (typ in self._params.abbrev_types or count >= self.ABBREV_BACKOFF):
              return False
  
          # Record this token as an abbreviation if the next
          # token is a sentence-internal punctuation mark.
          # [XX] :1 or check the whole thing??
          if next_tok.tok[:1] in self._lang_vars.internal_punctuation:
              return True
  
          # Record this type as an abbreviation if the next
          # token...  (i) starts with a lower case letter,
          # (ii) sometimes occurs with an uppercase letter,
          # and (iii) never occus with an uppercase letter
          # sentence-internally.
          # [xx] should the check for (ii) be modified??
          elif next_tok.first_lower:
              typ2 = next_tok.type_no_sentperiod
              typ2ortho_context = self._params.ortho_context[typ2]
              if ( (typ2ortho_context & _ORTHO_BEG_UC) and
                   not (typ2ortho_context & _ORTHO_MID_UC) ):
                  return True
  
      #////////////////////////////////////////////////////////////
      #{ Log Likelihoods
      #////////////////////////////////////////////////////////////
  
      # helper for _reclassify_abbrev_types:
      @staticmethod
      def _dunning_log_likelihood(count_a, count_b, count_ab, N):
          """
          A function that calculates the modified Dunning log-likelihood
          ratio scores for abbreviation candidates.  The details of how
          this works is available in the paper.
          """
          p1 = float(count_b) / N
          p2 = 0.99
          
          null_hypo = (float(count_ab) * math.log(p1) +
                       (count_a - count_ab) * math.log(1.0 - p1))
          alt_hypo  = (float(count_ab) * math.log(p2) +
                       (count_a - count_ab) * math.log(1.0 - p2))
          
          likelihood = null_hypo - alt_hypo
          
          return (-2.0 * likelihood)
  
      @staticmethod
      def _col_log_likelihood(count_a, count_b, count_ab, N):
          """
          A function that will just compute log-likelihood estimate, in
          the original paper it's decribed in algorithm 6 and 7.
  
          This *should* be the original Dunning log-likelihood values,
          unlike the previous log_l function where it used modified
          Dunning log-likelihood values
          """
          import math
          
          p = 1.0 * count_b / N
          p1 = 1.0 * count_ab / count_a
          p2 = 1.0 * (count_b - count_ab) / (N - count_a)
  
          summand1 = (count_ab * math.log(p) +
                      (count_a - count_ab) * math.log(1.0 - p))
  
          summand2 = ((count_b - count_ab) * math.log(p) +
                      (N - count_a - count_b + count_ab) * math.log(1.0 - p))
      
          if count_a == count_ab:
              summand3 = 0
          else:
              summand3 = (count_ab * math.log(p1) +
                          (count_a - count_ab) * math.log(1.0 - p1))
      
          if count_b == count_ab:
              summand4 = 0
          else:
              summand4 = ((count_b - count_ab) * math.log(p2) +
                          (N - count_a - count_b + count_ab) * math.log(1.0 - p2))
      
          likelihood = summand1 + summand2 - summand3 - summand4
      
          return (-2.0 * likelihood)
      
      #////////////////////////////////////////////////////////////
      #{ Collocation Finder
      #////////////////////////////////////////////////////////////
  
      def _is_potential_collocation(self, aug_tok1, aug_tok2):
          """
          Returns True if the pair of tokens may form a collocation given
          log-likelihood statistics.
          """
          return ((self.INCLUDE_ALL_COLLOCS or
                  (self.INCLUDE_ABBREV_COLLOCS and aug_tok1.abbr) or
                  (aug_tok1.sentbreak and
                      (aug_tok1.is_number or aug_tok1.is_initial)))
                  and aug_tok1.is_non_punct
                  and aug_tok2.is_non_punct)
  
      def _find_collocations(self):
          """
          Generates likely collocations and their log-likelihood.
          """
          for types, col_count in self._collocation_fdist.iteritems():
              try:
                  typ1, typ2 = types
              except TypeError:
                  # types may be None after calling freq_threshold()
                  continue
              if typ2 in self._params.sent_starters:
                  continue
  
              typ1_count = self._type_fdist[typ1]+self._type_fdist[typ1+'.']
              typ2_count = self._type_fdist[typ2]+self._type_fdist[typ2+'.']
              if (typ1_count > 1 and typ2_count > 1
                      and self.MIN_COLLOC_FREQ <
                          col_count <= min(typ1_count, typ2_count)):
  
                  ll = self._col_log_likelihood(typ1_count, typ2_count,
                                                col_count, self._type_fdist.N())
                  # Filter out the not-so-collocative
                  if (ll >= self.COLLOCATION and 
                      (float(self._type_fdist.N())/typ1_count >
                       float(typ2_count)/col_count)):
                      yield (typ1, typ2), ll
  
      #////////////////////////////////////////////////////////////
      #{ Sentence-Starter Finder
      #////////////////////////////////////////////////////////////
  
      def _is_potential_sent_starter(self, cur_tok, prev_tok):
          """
          Returns True given a token and the token that preceds it if it
          seems clear that the token is beginning a sentence.
          """
          # If a token (i) is preceeded by a sentece break that is
          # not a potential ordinal number or initial, and (ii) is
          # alphabetic, then it is a a sentence-starter.
          return ( prev_tok.sentbreak and 
               not (prev_tok.is_number or prev_tok.is_initial) and
               cur_tok.is_alpha )
  
      def _find_sent_starters(self):
          """
          Uses collocation heuristics for each candidate token to
          determine if it frequently starts sentences.
          """
          for (typ, typ_at_break_count) in self._sent_starter_fdist.iteritems():
              if not typ:
                  continue
  
              typ_count = self._type_fdist[typ]+self._type_fdist[typ+'.']
              if typ_count < typ_at_break_count:
                  # needed after freq_threshold
                  continue
  
              ll = self._col_log_likelihood(self._sentbreak_count, typ_count,
                                           typ_at_break_count,
                                            self._type_fdist.N())
  
              if (ll >= self.SENT_STARTER and
                  float(self._type_fdist.N())/self._sentbreak_count >
                  float(typ_count)/typ_at_break_count):
  
                  yield typ, ll
  
      def _get_sentbreak_count(self, tokens):
          """
          Returns the number of sentence breaks marked in a given set of
          augmented tokens.
          """
          return sum(1 for aug_tok in tokens if aug_tok.sentbreak)
  
  
  ######################################################################
  #{ Punkt Sentence Tokenizer
  ######################################################################
  
  
  class PunktSentenceTokenizer(_PunktBaseClass,TokenizerI):
      """
      A sentence tokenizer which uses an unsupervised algorithm to build
      a model for abbreviation words, collocations, and words that start
      sentences; and then uses that model to find sentence boundaries.
      This approach has been shown to work well for many European
      languages.
      """
      def __init__(self, train_text=None, verbose=False,
              lang_vars=PunktLanguageVars(), token_cls=PunktToken):
          """
          train_text can either be the sole training text for this sentence
          boundary detector, or can be a PunktParameters object.
          """
          _PunktBaseClass.__init__(self, lang_vars=lang_vars,
                  token_cls=token_cls)
          
          if train_text:
              self._params = self.train(train_text, verbose)
  
      def train(self, train_text, verbose=False):
          """
          Derives parameters from a given training text, or uses the parameters
          given. Repeated calls to this method destroy previous parameters. For
          incremental training, instantiate a separate PunktTrainer instance.
          """
          if type(train_text) not in (type(''), type(u'')):
              return train_text
          return PunktTrainer(train_text, lang_vars=self._lang_vars,
                  token_cls=self._Token).get_params()
      
      #////////////////////////////////////////////////////////////
      #{ Tokenization
      #////////////////////////////////////////////////////////////
  
      def tokenize(self, text, realign_boundaries=False):
          """
          Given a text, returns a list of the sentences in that text.
          """
          return list(self.sentences_from_text(text, realign_boundaries))
  
      def span_tokenize(self, text):
          """
          Given a text, returns a list of the (start, end) spans of sentences
          in the text.
          """
          return [(sl.start, sl.stop) for sl in self._slices_from_text(text)]
  
      def sentences_from_text(self, text, realign_boundaries=False):
          """
          Given a text, generates the sentences in that text by only
          testing candidate sentence breaks. If realign_boundaries is
          True, includes in the sentence closing punctuation that
          follows the period.
          """
          sents = [text[sl] for sl in self._slices_from_text(text)]
          if realign_boundaries:
              sents = self._realign_boundaries(sents)
          return sents
  
      def _slices_from_text(self, text):
          last_break = 0
          for match in self._lang_vars.period_context_re().finditer(text):
              context = match.group() + match.group('after_tok')
              if self.text_contains_sentbreak(context):
                  yield slice(last_break, match.end())
                  if match.group('next_tok'):
                      # next sentence starts after whitespace
                      last_break = match.start('next_tok')
                  else:
                      # next sentence starts at following punctuation
                      last_break = match.end()
          yield slice(last_break, len(text))
  
      def _realign_boundaries(self, sents):
          """
          Attempts to realign punctuation that falls after the period but
          should otherwise be included in the same sentence.
  
          For example: "(Sent1.) Sent2." will otherwise be split as::
          
              ["(Sent1.", ") Sent1."].
              
          This method will produce::
          
              ["(Sent1.)", "Sent2."].
          """
          realign = 0
          for s1, s2 in _pair_iter(sents):
              s1 = s1[realign:]
              if not s2:
                  if s1:
                      yield s1
                  continue
  
              m = self._lang_vars.re_boundary_realignment.match(s2)
              if m:
                  yield s1 + m.group(0).strip()
                  realign = m.end()
              else:
                  realign = 0
                  if s1:
                      yield s1
  
      def text_contains_sentbreak(self, text):
          """
          Returns True if the given text includes a sentence break.
          """
          found = False # used to ignore last token
          for t in self._annotate_tokens(self._tokenize_words(text)):
              if found:
                  return True
              if t.sentbreak:
                  found = True
          return False
     
      def sentences_from_text_legacy(self, text):
          """
          Given a text, generates the sentences in that text. Annotates all
          tokens, rather than just those with possible sentence breaks. Should
          produce the same results as L{sentences_from_text}.
          """
          tokens = self._annotate_tokens(self._tokenize_words(text))
          return self._build_sentence_list(text, tokens)
  
      def sentences_from_tokens(self, tokens):
          """
          Given a sequence of tokens, generates lists of tokens, each list
          corresponding to a sentence.
          """
          tokens = iter(self._annotate_tokens(self._Token(t) for t in tokens))
          sentence = []
          for aug_tok in tokens:
              sentence.append(aug_tok.tok)
              if aug_tok.sentbreak:
                  yield sentence
                  sentence = []
          if sentence:
              yield sentence
              
      def _annotate_tokens(self, tokens):
          """
          Given a set of tokens augmented with markers for line-start and
          paragraph-start, returns an iterator through those tokens with full
          annotation including predicted sentence breaks.
          """
          # Make a preliminary pass through the document, marking likely
          # sentence breaks, abbreviations, and ellipsis tokens.
          tokens = self._annotate_first_pass(tokens)
  
          # Make a second pass through the document, using token context
          # information to change our preliminary decisions about where
          # sentence breaks, abbreviations, and ellipsis occurs.
          tokens = self._annotate_second_pass(tokens)
  
          ## [XX] TESTING
          #tokens = list(tokens)
          #self.dump(tokens)
  
          return tokens
  
      def _build_sentence_list(self, text, tokens):
          """
          Given the original text and the list of augmented word tokens,
          construct and return a tokenized list of sentence strings.
          """
          # Most of the work here is making sure that we put the right
          # pieces of whitespace back in all the right places.
  
          # Our position in the source text, used to keep track of which
          # whitespace to add:
          pos = 0
  
          # A regular expression that finds pieces of whitespace:
          WS_REGEXP = re.compile(r'\s*')
          
          sentence = ''
          for aug_tok in tokens:
              tok = aug_tok.tok
  
              # Find the whitespace before this token, and update pos.
              ws = WS_REGEXP.match(text, pos).group()
              pos += len(ws)
  
              # Some of the rules used by the punkt word tokenizer
              # strip whitespace out of the text, resulting in tokens
              # that contain whitespace in the source text.  If our
              # token doesn't match, see if adding whitespace helps.
              # If so, then use the version with whitespace.
              if text[pos:pos+len(tok)] != tok:
                  pat = '\s*'.join(re.escape(c) for c in tok)
                  m = re.compile(pat).match(text,pos)
                  if m: tok = m.group()
  
              # Move our position pointer to the end of the token.
              assert text[pos:pos+len(tok)] == tok
              pos += len(tok)
  
              # Add this token.  If it's not at the beginning of the
              # sentence, then include any whitespace that separated it
              # from the previous token.
              if sentence:
                  sentence += ws + tok
              else:
                  sentence += tok
  
              # If we're at a sentence break, then start a new sentence.
              if aug_tok.sentbreak:
                  yield sentence
                  sentence = ''
  
          # If the last sentence is emtpy, discard it.
          if sentence:
              yield sentence
  
      # [XX] TESTING
      def dump(self, tokens):
          print 'writing to /tmp/punkt.new...'
          out = open('/tmp/punkt.new', 'w')
          for aug_tok in tokens:
              if aug_tok.parastart:
                  out.write('\n\n')
              elif aug_tok.linestart:
                  out.write('\n')
              else:
                  out.write(' ')
  
              out.write(str(aug_tok))
          out.close()
  
      #////////////////////////////////////////////////////////////
      #{ Customization Variables
      #////////////////////////////////////////////////////////////
  
      PUNCTUATION = tuple(';:,.!?')
  
      #////////////////////////////////////////////////////////////
      #{ Annotation Procedures
      #////////////////////////////////////////////////////////////
  
      def _annotate_second_pass(self, tokens):
          """
          Performs a token-based classification (section 4) over the given
          tokens, making use of the orthographic heuristic (4.1.1), collocation
          heuristic (4.1.2) and frequent sentence starter heuristic (4.1.3).
          """
          for t1, t2 in _pair_iter(tokens):
              self._second_pass_annotation(t1, t2)
              yield t1
  
      def _second_pass_annotation(self, aug_tok1, aug_tok2):
          """
          Performs token-based classification over a pair of contiguous tokens
          returning an updated augmented token for the first of them.
          """
          # Is it the last token? We can't do anything then.
          if not aug_tok2:
              return
  
          tok = aug_tok1.tok
          if not aug_tok1.period_final:
              # We only care about words ending in periods.
              return
  
          typ = aug_tok1.type_no_period
          next_tok = aug_tok2.tok
          next_typ = aug_tok2.type_no_sentperiod
          tok_is_initial = aug_tok1.is_initial
  
          # [4.1.2. Collocational Heuristic] If there's a
          # collocation between the word before and after the
          # period, then label tok as an abbreviation and NOT
          # a sentence break. Note that collocations with
          # frequent sentence starters as their second word are
          # excluded in training.
          if (typ, next_typ) in self._params.collocations:
              aug_tok1.sentbreak = False
              aug_tok1.abbr = True
              return
  
          # [4.2. Token-Based Reclassification of Abbreviations] If
          # the token is an abbreviation or an ellipsis, then decide
          # whether we should *also* classify it as a sentbreak.
          if ( (aug_tok1.abbr or aug_tok1.ellipsis) and
               (not tok_is_initial) ):
              # [4.1.1. Orthographic Heuristic] Check if there's
              # orthogrpahic evidence about whether the next word
              # starts a sentence or not.
              is_sent_starter = self._ortho_heuristic(aug_tok2)
              if is_sent_starter == True:
                  aug_tok1.sentbreak = True
                  return
  
              # [4.1.3. Frequent Sentence Starter Heruistic] If the
              # next word is capitalized, and is a member of the
              # frequent-sentence-starters list, then label tok as a
              # sentence break.
              if ( aug_tok2.first_upper and
                   next_typ in self._params.sent_starters):
                  aug_tok1.sentbreak = True
                  return
  
          # [4.3. Token-Based Detection of Initials and Ordinals]
          # Check if any initials or ordinals tokens that are marked
          # as sentbreaks should be reclassified as abbreviations.
          if tok_is_initial or typ == '##number##':
  
              # [4.1.1. Orthographic Heuristic] Check if there's
              # orthogrpahic evidence about whether the next word
              # starts a sentence or not.
              is_sent_starter = self._ortho_heuristic(aug_tok2)
  
              if is_sent_starter == False:
                  aug_tok1.sentbreak = False
                  aug_tok1.abbr = True
                  return
  
              # Special heuristic for initials: if orthogrpahic
              # heuristc is unknown, and next word is always
              # capitalized, then mark as abbrev (eg: J. Bach).
              if ( is_sent_starter == 'unknown' and tok_is_initial and
                   aug_tok2.first_upper and
                   not (self._params.ortho_context[next_typ] & _ORTHO_LC) ):
                  aug_tok1.sentbreak = False
                  aug_tok1.abbr = True
                  return
  
          return
  
      def _ortho_heuristic(self, aug_tok):
          """
          Decide whether the given token is the first token in a sentence.
          """
          # Sentences don't start with punctuation marks:
          if aug_tok.tok in self.PUNCTUATION:
              return False
  
          ortho_context = self._params.ortho_context[aug_tok.type_no_sentperiod]
  
          # If the word is capitalized, occurs at least once with a
          # lower case first letter, and never occurs with an upper case
          # first letter sentence-internally, then it's a sentence starter.
          if ( aug_tok.first_upper and
               (ortho_context & _ORTHO_LC) and
               not (ortho_context & _ORTHO_MID_UC) ):
              return True
  
          # If the word is lower case, and either (a) we've seen it used
          # with upper case, or (b) we've never seen it used
          # sentence-initially with lower case, then it's not a sentence
          # starter.
          if ( aug_tok.first_lower and
               ((ortho_context & _ORTHO_UC) or
                not (ortho_context & _ORTHO_BEG_LC)) ):
              return False
  
          # Otherwise, we're not sure.
          return 'unknown'
  
  
  def main(text, tok_cls=PunktSentenceTokenizer, train_cls=PunktTrainer):
      """Builds a punkt model and applies it to the same text"""
      cleanup = lambda s: re.compile(r'(?:\r|^\s+)', re.MULTILINE).sub('', s).replace('\n', ' ')
      trainer = train_cls()
      trainer.INCLUDE_ALL_COLLOCS = True
      trainer.train(text)
      sbd = tok_cls(trainer.get_params())
      for l in sbd.sentences_from_text(text, realign_boundaries=True):
          print cleanup(l)
  
  
  if __name__ == '__main__':
      import sys
      main(sys.stdin.read())
  