jupytersketcher/Lib/site-packages/nltk/test/unit/translate/test_stack_decoder.py

# -*- coding: utf-8 -*-
# Natural Language Toolkit: Stack decoder
#
# Copyright (C) 2001-2020 NLTK Project
# Author: Tah Wei Hoon <hoon.tw@gmail.com>
# URL: <http://nltk.org/>
# For license information, see LICENSE.TXT

"""
Tests for stack decoder
"""

import unittest
from collections import defaultdict
from math import log
from nltk.translate import PhraseTable
from nltk.translate import StackDecoder
from nltk.translate.stack_decoder import _Hypothesis, _Stack


class TestStackDecoder(unittest.TestCase):
    def test_find_all_src_phrases(self):
        # arrange
        phrase_table = TestStackDecoder.create_fake_phrase_table()
        stack_decoder = StackDecoder(phrase_table, None)
        sentence = ('my', 'hovercraft', 'is', 'full', 'of', 'eels')

        # act
        src_phrase_spans = stack_decoder.find_all_src_phrases(sentence)

        # assert
        self.assertEqual(src_phrase_spans[0], [2])  # 'my hovercraft'
        self.assertEqual(src_phrase_spans[1], [2])  # 'hovercraft'
        self.assertEqual(src_phrase_spans[2], [3])  # 'is'
        self.assertEqual(src_phrase_spans[3], [5, 6])  # 'full of', 'full of eels'
        self.assertFalse(src_phrase_spans[4])  # no entry starting with 'of'
        self.assertEqual(src_phrase_spans[5], [6])  # 'eels'

    def test_distortion_score(self):
        # arrange
        stack_decoder = StackDecoder(None, None)
        stack_decoder.distortion_factor = 0.5
        hypothesis = _Hypothesis()
        hypothesis.src_phrase_span = (3, 5)

        # act
        score = stack_decoder.distortion_score(hypothesis, (8, 10))

        # assert
        expected_score = log(stack_decoder.distortion_factor) * (8 - 5)
        self.assertEqual(score, expected_score)

    def test_distortion_score_of_first_expansion(self):
        # arrange
        stack_decoder = StackDecoder(None, None)
        stack_decoder.distortion_factor = 0.5
        hypothesis = _Hypothesis()

        # act
        score = stack_decoder.distortion_score(hypothesis, (8, 10))

        # assert
        # expansion from empty hypothesis always has zero distortion cost
        self.assertEqual(score, 0.0)

    def test_compute_future_costs(self):
        # arrange
        phrase_table = TestStackDecoder.create_fake_phrase_table()
        language_model = TestStackDecoder.create_fake_language_model()
        stack_decoder = StackDecoder(phrase_table, language_model)
        sentence = ('my', 'hovercraft', 'is', 'full', 'of', 'eels')

        # act
        future_scores = stack_decoder.compute_future_scores(sentence)

        # assert
        self.assertEqual(
            future_scores[1][2],
            (
                phrase_table.translations_for(('hovercraft',))[0].log_prob
                + language_model.probability(('hovercraft',))
            ),
        )
        self.assertEqual(
            future_scores[0][2],
            (
                phrase_table.translations_for(('my', 'hovercraft'))[0].log_prob
                + language_model.probability(('my', 'hovercraft'))
            ),
        )

    def test_compute_future_costs_for_phrases_not_in_phrase_table(self):
        # arrange
        phrase_table = TestStackDecoder.create_fake_phrase_table()
        language_model = TestStackDecoder.create_fake_language_model()
        stack_decoder = StackDecoder(phrase_table, language_model)
        sentence = ('my', 'hovercraft', 'is', 'full', 'of', 'eels')

        # act
        future_scores = stack_decoder.compute_future_scores(sentence)

        # assert
        self.assertEqual(
            future_scores[1][3],  # 'hovercraft is' is not in phrase table
            future_scores[1][2] + future_scores[2][3],
        )  # backoff

    def test_future_score(self):
        # arrange: sentence with 8 words; words 2, 3, 4 already translated
        hypothesis = _Hypothesis()
        hypothesis.untranslated_spans = lambda _: [(0, 2), (5, 8)]  # mock
        future_score_table = defaultdict(lambda: defaultdict(float))
        future_score_table[0][2] = 0.4
        future_score_table[5][8] = 0.5
        stack_decoder = StackDecoder(None, None)

        # act
        future_score = stack_decoder.future_score(hypothesis, future_score_table, 8)

        # assert
        self.assertEqual(future_score, 0.4 + 0.5)

    def test_valid_phrases(self):
        # arrange
        hypothesis = _Hypothesis()
        # mock untranslated_spans method
        hypothesis.untranslated_spans = lambda _: [(0, 2), (3, 6)]
        all_phrases_from = [[1, 4], [2], [], [5], [5, 6, 7], [], [7]]

        # act
        phrase_spans = StackDecoder.valid_phrases(all_phrases_from, hypothesis)

        # assert
        self.assertEqual(phrase_spans, [(0, 1), (1, 2), (3, 5), (4, 5), (4, 6)])

    @staticmethod
    def create_fake_phrase_table():
        phrase_table = PhraseTable()
        phrase_table.add(('hovercraft',), ('',), 0.8)
        phrase_table.add(('my', 'hovercraft'), ('', ''), 0.7)
        phrase_table.add(('my', 'cheese'), ('', ''), 0.7)
        phrase_table.add(('is',), ('',), 0.8)
        phrase_table.add(('is',), ('',), 0.5)
        phrase_table.add(('full', 'of'), ('', ''), 0.01)
        phrase_table.add(('full', 'of', 'eels'), ('', '', ''), 0.5)
        phrase_table.add(('full', 'of', 'spam'), ('', ''), 0.5)
        phrase_table.add(('eels',), ('',), 0.5)
        phrase_table.add(('spam',), ('',), 0.5)
        return phrase_table

    @staticmethod
    def create_fake_language_model():
        # nltk.model should be used here once it is implemented
        language_prob = defaultdict(lambda: -999.0)
        language_prob[('my',)] = log(0.1)
        language_prob[('hovercraft',)] = log(0.1)
        language_prob[('is',)] = log(0.1)
        language_prob[('full',)] = log(0.1)
        language_prob[('of',)] = log(0.1)
        language_prob[('eels',)] = log(0.1)
        language_prob[('my', 'hovercraft')] = log(0.3)
        language_model = type(
            '', (object,), {'probability': lambda _, phrase: language_prob[phrase]}
        )()
        return language_model


class TestHypothesis(unittest.TestCase):
    def setUp(self):
        root = _Hypothesis()
        child = _Hypothesis(
            raw_score=0.5,
            src_phrase_span=(3, 7),
            trg_phrase=('hello', 'world'),
            previous=root,
        )
        grandchild = _Hypothesis(
            raw_score=0.4,
            src_phrase_span=(1, 2),
            trg_phrase=('and', 'goodbye'),
            previous=child,
        )
        self.hypothesis_chain = grandchild

    def test_translation_so_far(self):
        # act
        translation = self.hypothesis_chain.translation_so_far()

        # assert
        self.assertEqual(translation, ['hello', 'world', 'and', 'goodbye'])

    def test_translation_so_far_for_empty_hypothesis(self):
        # arrange
        hypothesis = _Hypothesis()

        # act
        translation = hypothesis.translation_so_far()

        # assert
        self.assertEqual(translation, [])

    def test_total_translated_words(self):
        # act
        total_translated_words = self.hypothesis_chain.total_translated_words()

        # assert
        self.assertEqual(total_translated_words, 5)

    def test_translated_positions(self):
        # act
        translated_positions = self.hypothesis_chain.translated_positions()

        # assert
        translated_positions.sort()
        self.assertEqual(translated_positions, [1, 3, 4, 5, 6])

    def test_untranslated_spans(self):
        # act
        untranslated_spans = self.hypothesis_chain.untranslated_spans(10)

        # assert
        self.assertEqual(untranslated_spans, [(0, 1), (2, 3), (7, 10)])

    def test_untranslated_spans_for_empty_hypothesis(self):
        # arrange
        hypothesis = _Hypothesis()

        # act
        untranslated_spans = hypothesis.untranslated_spans(10)

        # assert
        self.assertEqual(untranslated_spans, [(0, 10)])


class TestStack(unittest.TestCase):
    def test_push_bumps_off_worst_hypothesis_when_stack_is_full(self):
        # arrange
        stack = _Stack(3)
        poor_hypothesis = _Hypothesis(0.01)

        # act
        stack.push(_Hypothesis(0.2))
        stack.push(poor_hypothesis)
        stack.push(_Hypothesis(0.1))
        stack.push(_Hypothesis(0.3))

        # assert
        self.assertFalse(poor_hypothesis in stack)

    def test_push_removes_hypotheses_that_fall_below_beam_threshold(self):
        # arrange
        stack = _Stack(3, 0.5)
        poor_hypothesis = _Hypothesis(0.01)
        worse_hypothesis = _Hypothesis(0.009)

        # act
        stack.push(poor_hypothesis)
        stack.push(worse_hypothesis)
        stack.push(_Hypothesis(0.9))  # greatly superior hypothesis

        # assert
        self.assertFalse(poor_hypothesis in stack)
        self.assertFalse(worse_hypothesis in stack)

    def test_push_does_not_add_hypothesis_that_falls_below_beam_threshold(self):
        # arrange
        stack = _Stack(3, 0.5)
        poor_hypothesis = _Hypothesis(0.01)

        # act
        stack.push(_Hypothesis(0.9))  # greatly superior hypothesis
        stack.push(poor_hypothesis)

        # assert
        self.assertFalse(poor_hypothesis in stack)

    def test_best_returns_the_best_hypothesis(self):
        # arrange
        stack = _Stack(3)
        best_hypothesis = _Hypothesis(0.99)

        # act
        stack.push(_Hypothesis(0.0))
        stack.push(best_hypothesis)
        stack.push(_Hypothesis(0.5))

        # assert
        self.assertEqual(stack.best(), best_hypothesis)

    def test_best_returns_none_when_stack_is_empty(self):
        # arrange
        stack = _Stack(3)

        # assert
        self.assertEqual(stack.best(), None)