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# -*- coding: utf-8 -*-
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# Natural Language Toolkit: GDFA word alignment symmetrization
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#
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# Copyright (C) 2001-2019 NLTK Project
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# Authors: Liling Tan
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# URL: <http://nltk.org/>
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# For license information, see LICENSE.TXT
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from collections import defaultdict
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def grow_diag_final_and(srclen, trglen, e2f, f2e):
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"""
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This module symmetrisatizes the source-to-target and target-to-source
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word alignment output and produces, aka. GDFA algorithm (Koehn, 2005).
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Step 1: Find the intersection of the bidirectional alignment.
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Step 2: Search for additional neighbor alignment points to be added, given
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these criteria: (i) neighbor alignments points are not in the
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intersection and (ii) neighbor alignments are in the union.
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Step 3: Add all other alignment points thats not in the intersection, not in
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the neighboring alignments that met the criteria but in the original
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foward/backward alignment outputs.
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>>> forw = ('0-0 2-1 9-2 21-3 10-4 7-5 11-6 9-7 12-8 1-9 3-10 '
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... '4-11 17-12 17-13 25-14 13-15 24-16 11-17 28-18')
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>>> back = ('0-0 1-9 2-9 3-10 4-11 5-12 6-6 7-5 8-6 9-7 10-4 '
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... '11-6 12-8 13-12 15-12 17-13 18-13 19-12 20-13 '
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... '21-3 22-12 23-14 24-17 25-15 26-17 27-18 28-18')
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>>> srctext = ("この よう な ハロー 白色 わい 星 の Ｌ 関数 "
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... "は Ｌ と 共 に 不連続 に 増加 する こと が "
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... "期待 さ れる こと を 示し た 。")
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>>> trgtext = ("Therefore , we expect that the luminosity function "
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... "of such halo white dwarfs increases discontinuously "
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... "with the luminosity .")
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>>> srclen = len(srctext.split())
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>>> trglen = len(trgtext.split())
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>>>
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>>> gdfa = grow_diag_final_and(srclen, trglen, forw, back)
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>>> gdfa == sorted(set([(28, 18), (6, 6), (24, 17), (2, 1), (15, 12), (13, 12),
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... (2, 9), (3, 10), (26, 17), (25, 15), (8, 6), (9, 7), (20,
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... 13), (18, 13), (0, 0), (10, 4), (13, 15), (23, 14), (7, 5),
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... (25, 14), (1, 9), (17, 13), (4, 11), (11, 17), (9, 2), (22,
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... 12), (27, 18), (24, 16), (21, 3), (19, 12), (17, 12), (5,
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... 12), (11, 6), (12, 8)]))
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True
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References:
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Koehn, P., A. Axelrod, A. Birch, C. Callison, M. Osborne, and D. Talbot.
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2005. Edinburgh System Description for the 2005 IWSLT Speech
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Translation Evaluation. In MT Eval Workshop.
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:type srclen: int
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:param srclen: the number of tokens in the source language
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:type trglen: int
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:param trglen: the number of tokens in the target language
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:type e2f: str
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:param e2f: the forward word alignment outputs from source-to-target
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language (in pharaoh output format)
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:type f2e: str
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:param f2e: the backward word alignment outputs from target-to-source
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language (in pharaoh output format)
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:rtype: set(tuple(int))
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:return: the symmetrized alignment points from the GDFA algorithm
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"""
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# Converts pharaoh text format into list of tuples.
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e2f = [tuple(map(int, a.split('-'))) for a in e2f.split()]
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f2e = [tuple(map(int, a.split('-'))) for a in f2e.split()]
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neighbors = [(-1, 0), (0, -1), (1, 0), (0, 1), (-1, -1), (-1, 1), (1, -1), (1, 1)]
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alignment = set(e2f).intersection(set(f2e)) # Find the intersection.
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union = set(e2f).union(set(f2e))
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# *aligned* is used to check if neighbors are aligned in grow_diag()
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aligned = defaultdict(set)
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for i, j in alignment:
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aligned['e'].add(i)
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aligned['f'].add(j)
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def grow_diag():
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"""
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Search for the neighbor points and them to the intersected alignment
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points if criteria are met.
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"""
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prev_len = len(alignment) - 1
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# iterate until no new points added
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while prev_len < len(alignment):
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no_new_points = True
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# for english word e = 0 ... en
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for e in range(srclen):
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# for foreign word f = 0 ... fn
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for f in range(trglen):
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# if ( e aligned with f)
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if (e, f) in alignment:
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# for each neighboring point (e-new, f-new)
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for neighbor in neighbors:
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neighbor = tuple(i + j for i, j in zip((e, f), neighbor))
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e_new, f_new = neighbor
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# if ( ( e-new not aligned and f-new not aligned)
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# and (e-new, f-new in union(e2f, f2e) )
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if (
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e_new not in aligned and f_new not in aligned
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) and neighbor in union:
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alignment.add(neighbor)
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aligned['e'].add(e_new)
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aligned['f'].add(f_new)
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prev_len += 1
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no_new_points = False
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# iterate until no new points added
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if no_new_points:
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break
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def final_and(a):
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"""
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Adds remaining points that are not in the intersection, not in the
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neighboring alignments but in the original *e2f* and *f2e* alignments
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"""
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# for english word e = 0 ... en
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for e_new in range(srclen):
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# for foreign word f = 0 ... fn
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for f_new in range(trglen):
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# if ( ( e-new not aligned and f-new not aligned)
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# and (e-new, f-new in union(e2f, f2e) )
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if (
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e_new not in aligned
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and f_new not in aligned
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and (e_new, f_new) in union
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):
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alignment.add((e_new, f_new))
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aligned['e'].add(e_new)
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aligned['f'].add(f_new)
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grow_diag()
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final_and(e2f)
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final_and(f2e)
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return sorted(alignment)
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