Source code for nltk.metrics.segmentation

# Natural Language Toolkit: Text Segmentation Metrics
#
# Copyright (C) 2001-2018 NLTK Project
# Author: Edward Loper <edloper@gmail.com>
#         Steven Bird <stevenbird1@gmail.com>
#         David Doukhan <david.doukhan@gmail.com>
# URL: <http://nltk.org/>
# For license information, see LICENSE.TXT



"""
Text Segmentation Metrics

1. Windowdiff

Pevzner, L., and Hearst, M., A Critique and Improvement of
  an Evaluation Metric for Text Segmentation,
Computational Linguistics 28, 19-36


2. Generalized Hamming Distance

Bookstein A., Kulyukin V.A., Raita T.
Generalized Hamming Distance
Information Retrieval 5, 2002, pp 353-375

Baseline implementation in C++
http://digital.cs.usu.edu/~vkulyukin/vkweb/software/ghd/ghd.html

Study describing benefits of Generalized Hamming Distance Versus
WindowDiff for evaluating text segmentation tasks
Begsten, Y.  Quel indice pour mesurer l'efficacite en segmentation de textes ?
TALN 2009


3. Pk text segmentation metric

Beeferman D., Berger A., Lafferty J. (1999)
Statistical Models for Text Segmentation
Machine Learning, 34, 177-210
"""

try:
    import numpy as np
except ImportError:
    pass

from six.moves import range


[docs]def windowdiff(seg1, seg2, k, boundary="1", weighted=False): """ Compute the windowdiff score for a pair of segmentations. A segmentation is any sequence over a vocabulary of two items (e.g. "0", "1"), where the specified boundary value is used to mark the edge of a segmentation. >>> s1 = "000100000010" >>> s2 = "000010000100" >>> s3 = "100000010000" >>> '%.2f' % windowdiff(s1, s1, 3) '0.00' >>> '%.2f' % windowdiff(s1, s2, 3) '0.30' >>> '%.2f' % windowdiff(s2, s3, 3) '0.80' :param seg1: a segmentation :type seg1: str or list :param seg2: a segmentation :type seg2: str or list :param k: window width :type k: int :param boundary: boundary value :type boundary: str or int or bool :param weighted: use the weighted variant of windowdiff :type weighted: boolean :rtype: float """ if len(seg1) != len(seg2): raise ValueError("Segmentations have unequal length") if k > len(seg1): raise ValueError("Window width k should be smaller or equal than segmentation lengths") wd = 0 for i in range(len(seg1) - k + 1): ndiff = abs(seg1[i:i+k].count(boundary) - seg2[i:i+k].count(boundary)) if weighted: wd += ndiff else: wd += min(1, ndiff) return wd / (len(seg1) - k + 1.)
# Generalized Hamming Distance def _init_mat(nrows, ncols, ins_cost, del_cost): mat = np.empty((nrows, ncols)) mat[0, :] = ins_cost * np.arange(ncols) mat[:, 0] = del_cost * np.arange(nrows) return mat def _ghd_aux(mat, rowv, colv, ins_cost, del_cost, shift_cost_coeff): for i, rowi in enumerate(rowv): for j, colj in enumerate(colv): shift_cost = shift_cost_coeff * abs(rowi - colj) + mat[i, j] if rowi == colj: # boundaries are at the same location, no transformation required tcost = mat[i, j] elif rowi > colj: # boundary match through a deletion tcost = del_cost + mat[i, j + 1] else: # boundary match through an insertion tcost = ins_cost + mat[i + 1, j] mat[i + 1, j + 1] = min(tcost, shift_cost)
[docs]def ghd(ref, hyp, ins_cost=2.0, del_cost=2.0, shift_cost_coeff=1.0, boundary='1'): """ Compute the Generalized Hamming Distance for a reference and a hypothetical segmentation, corresponding to the cost related to the transformation of the hypothetical segmentation into the reference segmentation through boundary insertion, deletion and shift operations. A segmentation is any sequence over a vocabulary of two items (e.g. "0", "1"), where the specified boundary value is used to mark the edge of a segmentation. Recommended parameter values are a shift_cost_coeff of 2. Associated with a ins_cost, and del_cost equal to the mean segment length in the reference segmentation. >>> # Same examples as Kulyukin C++ implementation >>> ghd('1100100000', '1100010000', 1.0, 1.0, 0.5) 0.5 >>> ghd('1100100000', '1100000001', 1.0, 1.0, 0.5) 2.0 >>> ghd('011', '110', 1.0, 1.0, 0.5) 1.0 >>> ghd('1', '0', 1.0, 1.0, 0.5) 1.0 >>> ghd('111', '000', 1.0, 1.0, 0.5) 3.0 >>> ghd('000', '111', 1.0, 2.0, 0.5) 6.0 :param ref: the reference segmentation :type ref: str or list :param hyp: the hypothetical segmentation :type hyp: str or list :param ins_cost: insertion cost :type ins_cost: float :param del_cost: deletion cost :type del_cost: float :param shift_cost_coeff: constant used to compute the cost of a shift. shift cost = shift_cost_coeff * |i - j| where i and j are the positions indicating the shift :type shift_cost_coeff: float :param boundary: boundary value :type boundary: str or int or bool :rtype: float """ ref_idx = [i for (i, val) in enumerate(ref) if val == boundary] hyp_idx = [i for (i, val) in enumerate(hyp) if val == boundary] nref_bound = len(ref_idx) nhyp_bound = len(hyp_idx) if nref_bound == 0 and nhyp_bound == 0: return 0.0 elif nref_bound > 0 and nhyp_bound == 0: return nref_bound * ins_cost elif nref_bound == 0 and nhyp_bound > 0: return nhyp_bound * del_cost mat = _init_mat(nhyp_bound + 1, nref_bound + 1, ins_cost, del_cost) _ghd_aux(mat, hyp_idx, ref_idx, ins_cost, del_cost, shift_cost_coeff) return mat[-1, -1]
# Beeferman's Pk text segmentation evaluation metric
[docs]def pk(ref, hyp, k=None, boundary='1'): """ Compute the Pk metric for a pair of segmentations A segmentation is any sequence over a vocabulary of two items (e.g. "0", "1"), where the specified boundary value is used to mark the edge of a segmentation. >>> '%.2f' % pk('0100'*100, '1'*400, 2) '0.50' >>> '%.2f' % pk('0100'*100, '0'*400, 2) '0.50' >>> '%.2f' % pk('0100'*100, '0100'*100, 2) '0.00' :param ref: the reference segmentation :type ref: str or list :param hyp: the segmentation to evaluate :type hyp: str or list :param k: window size, if None, set to half of the average reference segment length :type boundary: str or int or bool :param boundary: boundary value :type boundary: str or int or bool :rtype: float """ if k is None: k = int(round(len(ref) / (ref.count(boundary) * 2.))) err = 0 for i in range(len(ref)-k +1): r = ref[i:i+k].count(boundary) > 0 h = hyp[i:i+k].count(boundary) > 0 if r != h: err += 1 return err / (len(ref)-k +1.)
# skip doctests if numpy is not installed
[docs]def setup_module(module): from nose import SkipTest try: import numpy except ImportError: raise SkipTest("numpy is required for nltk.metrics.segmentation")