338 lines
14 KiB
Python
338 lines
14 KiB
Python
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# -*- coding: utf-8 -*-
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# Natural Language Toolkit: IBM Model 3
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#
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# Copyright (C) 2001-2013 NLTK Project
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# Authors: Chin Yee Lee, Hengfeng Li, Ruxin Hou, Calvin Tanujaya Lim
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# URL: <http://nltk.org/>
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# For license information, see LICENSE.TXT
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"""
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Translation model that considers how a word can be aligned to
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multiple words in another language.
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IBM Model 3 improves on Model 2 by directly modeling the phenomenon
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where a word in one language may be translated into zero or more words
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in another. This is expressed by the fertility probability,
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n(phi | source word).
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If a source word translates into more than one word, it is possible to
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generate sentences that have the same alignment in multiple ways. This
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is modeled by a distortion step. The distortion probability, d(j|i,l,m),
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predicts a target word position, given its aligned source word's
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position. The distortion probability replaces the alignment probability
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of Model 2.
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The fertility probability is not applicable for NULL. Target words that
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align to NULL are assumed to be distributed uniformly in the target
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sentence. The existence of these words is modeled by p1, the probability
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that a target word produced by a real source word requires another
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target word that is produced by NULL.
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The EM algorithm used in Model 3 is:
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E step - In the training data, collect counts, weighted by prior
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probabilities.
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(a) count how many times a source language word is translated
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into a target language word
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(b) count how many times a particular position in the target
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sentence is aligned to a particular position in the source
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sentence
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(c) count how many times a source word is aligned to phi number
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of target words
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(d) count how many times NULL is aligned to a target word
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M step - Estimate new probabilities based on the counts from the E step
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Because there are too many possible alignments, only the most probable
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ones are considered. First, the best alignment is determined using prior
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probabilities. Then, a hill climbing approach is used to find other good
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candidates.
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Notations:
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i: Position in the source sentence
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Valid values are 0 (for NULL), 1, 2, ..., length of source sentence
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j: Position in the target sentence
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Valid values are 1, 2, ..., length of target sentence
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l: Number of words in the source sentence, excluding NULL
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m: Number of words in the target sentence
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s: A word in the source language
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t: A word in the target language
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phi: Fertility, the number of target words produced by a source word
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p1: Probability that a target word produced by a source word is
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accompanied by another target word that is aligned to NULL
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p0: 1 - p1
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References:
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Philipp Koehn. 2010. Statistical Machine Translation.
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Cambridge University Press, New York.
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Peter E Brown, Stephen A. Della Pietra, Vincent J. Della Pietra, and
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Robert L. Mercer. 1993. The Mathematics of Statistical Machine
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Translation: Parameter Estimation. Computational Linguistics, 19 (2),
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263-311.
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"""
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from __future__ import division
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from collections import defaultdict
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from math import factorial
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from nltk.translate import AlignedSent
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from nltk.translate import Alignment
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from nltk.translate import IBMModel
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from nltk.translate import IBMModel2
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from nltk.translate.ibm_model import Counts
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import warnings
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class IBMModel3(IBMModel):
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"""
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Translation model that considers how a word can be aligned to
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multiple words in another language
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>>> bitext = []
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>>> bitext.append(AlignedSent(['klein', 'ist', 'das', 'haus'], ['the', 'house', 'is', 'small']))
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>>> bitext.append(AlignedSent(['das', 'haus', 'war', 'ja', 'groß'], ['the', 'house', 'was', 'big']))
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>>> bitext.append(AlignedSent(['das', 'buch', 'ist', 'ja', 'klein'], ['the', 'book', 'is', 'small']))
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>>> bitext.append(AlignedSent(['ein', 'haus', 'ist', 'klein'], ['a', 'house', 'is', 'small']))
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>>> bitext.append(AlignedSent(['das', 'haus'], ['the', 'house']))
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>>> bitext.append(AlignedSent(['das', 'buch'], ['the', 'book']))
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>>> bitext.append(AlignedSent(['ein', 'buch'], ['a', 'book']))
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>>> bitext.append(AlignedSent(['ich', 'fasse', 'das', 'buch', 'zusammen'], ['i', 'summarize', 'the', 'book']))
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>>> bitext.append(AlignedSent(['fasse', 'zusammen'], ['summarize']))
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>>> ibm3 = IBMModel3(bitext, 5)
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>>> print(round(ibm3.translation_table['buch']['book'], 3))
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1.0
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>>> print(round(ibm3.translation_table['das']['book'], 3))
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0.0
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>>> print(round(ibm3.translation_table['ja'][None], 3))
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1.0
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>>> print(round(ibm3.distortion_table[1][1][2][2], 3))
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1.0
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>>> print(round(ibm3.distortion_table[1][2][2][2], 3))
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0.0
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>>> print(round(ibm3.distortion_table[2][2][4][5], 3))
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0.75
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>>> print(round(ibm3.fertility_table[2]['summarize'], 3))
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1.0
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>>> print(round(ibm3.fertility_table[1]['book'], 3))
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1.0
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>>> print(ibm3.p1)
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0.054...
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>>> test_sentence = bitext[2]
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>>> test_sentence.words
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['das', 'buch', 'ist', 'ja', 'klein']
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>>> test_sentence.mots
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['the', 'book', 'is', 'small']
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>>> test_sentence.alignment
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Alignment([(0, 0), (1, 1), (2, 2), (3, None), (4, 3)])
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"""
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def __init__(self, sentence_aligned_corpus, iterations,
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probability_tables=None):
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"""
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Train on ``sentence_aligned_corpus`` and create a lexical
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translation model, a distortion model, a fertility model, and a
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model for generating NULL-aligned words.
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Translation direction is from ``AlignedSent.mots`` to
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``AlignedSent.words``.
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:param sentence_aligned_corpus: Sentence-aligned parallel corpus
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:type sentence_aligned_corpus: list(AlignedSent)
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:param iterations: Number of iterations to run training algorithm
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:type iterations: int
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:param probability_tables: Optional. Use this to pass in custom
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probability values. If not specified, probabilities will be
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set to a uniform distribution, or some other sensible value.
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If specified, all the following entries must be present:
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``translation_table``, ``alignment_table``,
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``fertility_table``, ``p1``, ``distortion_table``.
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See ``IBMModel`` for the type and purpose of these tables.
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:type probability_tables: dict[str]: object
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"""
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super(IBMModel3, self).__init__(sentence_aligned_corpus)
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self.reset_probabilities()
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if probability_tables is None:
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# Get translation and alignment probabilities from IBM Model 2
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ibm2 = IBMModel2(sentence_aligned_corpus, iterations)
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self.translation_table = ibm2.translation_table
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self.alignment_table = ibm2.alignment_table
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self.set_uniform_probabilities(sentence_aligned_corpus)
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else:
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# Set user-defined probabilities
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self.translation_table = probability_tables['translation_table']
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self.alignment_table = probability_tables['alignment_table']
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self.fertility_table = probability_tables['fertility_table']
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self.p1 = probability_tables['p1']
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self.distortion_table = probability_tables['distortion_table']
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for n in range(0, iterations):
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self.train(sentence_aligned_corpus)
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def reset_probabilities(self):
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super(IBMModel3, self).reset_probabilities()
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self.distortion_table = defaultdict(
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lambda: defaultdict(lambda: defaultdict(lambda: defaultdict(
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lambda: self.MIN_PROB))))
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"""
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dict[int][int][int][int]: float. Probability(j | i,l,m).
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Values accessed as ``distortion_table[j][i][l][m]``.
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"""
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def set_uniform_probabilities(self, sentence_aligned_corpus):
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# d(j | i,l,m) = 1 / m for all i, j, l, m
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l_m_combinations = set()
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for aligned_sentence in sentence_aligned_corpus:
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l = len(aligned_sentence.mots)
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m = len(aligned_sentence.words)
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if (l, m) not in l_m_combinations:
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l_m_combinations.add((l, m))
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initial_prob = 1 / m
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if initial_prob < IBMModel.MIN_PROB:
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warnings.warn("A target sentence is too long (" + str(m) +
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" words). Results may be less accurate.")
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for j in range(1, m + 1):
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for i in range(0, l + 1):
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self.distortion_table[j][i][l][m] = initial_prob
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# simple initialization, taken from GIZA++
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self.fertility_table[0] = defaultdict(lambda: 0.2)
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self.fertility_table[1] = defaultdict(lambda: 0.65)
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self.fertility_table[2] = defaultdict(lambda: 0.1)
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self.fertility_table[3] = defaultdict(lambda: 0.04)
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MAX_FERTILITY = 10
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initial_fert_prob = 0.01 / (MAX_FERTILITY - 4)
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for phi in range(4, MAX_FERTILITY):
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self.fertility_table[phi] = defaultdict(lambda: initial_fert_prob)
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self.p1 = 0.5
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def train(self, parallel_corpus):
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counts = Model3Counts()
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for aligned_sentence in parallel_corpus:
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l = len(aligned_sentence.mots)
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m = len(aligned_sentence.words)
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# Sample the alignment space
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sampled_alignments, best_alignment = self.sample(aligned_sentence)
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# Record the most probable alignment
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aligned_sentence.alignment = Alignment(
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best_alignment.zero_indexed_alignment())
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# E step (a): Compute normalization factors to weigh counts
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total_count = self.prob_of_alignments(sampled_alignments)
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# E step (b): Collect counts
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for alignment_info in sampled_alignments:
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count = self.prob_t_a_given_s(alignment_info)
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normalized_count = count / total_count
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for j in range(1, m + 1):
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counts.update_lexical_translation(
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normalized_count, alignment_info, j)
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counts.update_distortion(
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normalized_count, alignment_info, j, l, m)
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counts.update_null_generation(normalized_count, alignment_info)
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counts.update_fertility(normalized_count, alignment_info)
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# M step: Update probabilities with maximum likelihood estimates
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# If any probability is less than MIN_PROB, clamp it to MIN_PROB
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existing_alignment_table = self.alignment_table
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self.reset_probabilities()
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self.alignment_table = existing_alignment_table # don't retrain
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self.maximize_lexical_translation_probabilities(counts)
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self.maximize_distortion_probabilities(counts)
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self.maximize_fertility_probabilities(counts)
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self.maximize_null_generation_probabilities(counts)
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def maximize_distortion_probabilities(self, counts):
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MIN_PROB = IBMModel.MIN_PROB
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for j, i_s in counts.distortion.items():
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for i, src_sentence_lengths in i_s.items():
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for l, trg_sentence_lengths in src_sentence_lengths.items():
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for m in trg_sentence_lengths:
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estimate = (counts.distortion[j][i][l][m] /
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counts.distortion_for_any_j[i][l][m])
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self.distortion_table[j][i][l][m] = max(estimate,
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MIN_PROB)
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def prob_t_a_given_s(self, alignment_info):
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"""
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Probability of target sentence and an alignment given the
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source sentence
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"""
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src_sentence = alignment_info.src_sentence
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trg_sentence = alignment_info.trg_sentence
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l = len(src_sentence) - 1 # exclude NULL
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m = len(trg_sentence) - 1
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p1 = self.p1
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p0 = 1 - p1
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probability = 1.0
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MIN_PROB = IBMModel.MIN_PROB
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# Combine NULL insertion probability
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null_fertility = alignment_info.fertility_of_i(0)
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probability *= (pow(p1, null_fertility) *
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pow(p0, m - 2 * null_fertility))
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if probability < MIN_PROB:
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return MIN_PROB
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# Compute combination (m - null_fertility) choose null_fertility
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for i in range(1, null_fertility + 1):
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probability *= (m - null_fertility - i + 1) / i
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if probability < MIN_PROB:
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return MIN_PROB
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# Combine fertility probabilities
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for i in range(1, l + 1):
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fertility = alignment_info.fertility_of_i(i)
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probability *= (factorial(fertility) *
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self.fertility_table[fertility][src_sentence[i]])
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if probability < MIN_PROB:
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return MIN_PROB
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# Combine lexical and distortion probabilities
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for j in range(1, m + 1):
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t = trg_sentence[j]
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i = alignment_info.alignment[j]
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s = src_sentence[i]
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probability *= (self.translation_table[t][s] *
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self.distortion_table[j][i][l][m])
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if probability < MIN_PROB:
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return MIN_PROB
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return probability
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class Model3Counts(Counts):
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"""
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Data object to store counts of various parameters during training.
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Includes counts for distortion.
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"""
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def __init__(self):
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super(Model3Counts, self).__init__()
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self.distortion = defaultdict(
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lambda: defaultdict(lambda: defaultdict(lambda: defaultdict(
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lambda: 0.0))))
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self.distortion_for_any_j = defaultdict(
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lambda: defaultdict(lambda: defaultdict(lambda: 0.0)))
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def update_distortion(self, count, alignment_info, j, l, m):
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i = alignment_info.alignment[j]
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self.distortion[j][i][l][m] += count
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self.distortion_for_any_j[i][l][m] += count
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