# -*- coding: utf-8 -*-
# coding=utf-8
# Copyright 2019 The SGNMT Authors.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Implementation of beam search with explicit limits on culmulative
predictor scores at each node expansion.
"""
from cam.sgnmt import utils
from cam.sgnmt.decoding.beam import BeamDecoder
import logging
[docs]class PredLimitBeamDecoder(BeamDecoder):
"""Beam search variant with explicit limits on the culmulative
predictor scores at each node expansion.
"""
def __init__(self, decoder_args):
"""Creates a new beam decoder with culmulative predictor score
limits. In addition to the constructor of `BeamDecoder`, the
following values are fetched from `decoder_args`:
pred_limits (string): Comma-separated list of predictor
score limits.
"""
super(PredLimitBeamDecoder, self).__init__(decoder_args)
self.pred_limits = []
for l in utils.split_comma(decoder_args.pred_limits):
try:
self.pred_limits.append(float(l))
except:
self.pred_limits.append(utils.NEG_INF)
logging.info("Cumulative predictor score limits: %s" % self.pred_limits)
def _expand_hypo(self, hypo):
n_preds = len(self.pred_limits)
all_hypos = super(PredLimitBeamDecoder, self)._expand_hypo(hypo)
next_hypos = []
all_accs = [0.0] * n_preds
for hypo in sorted(all_hypos, key=lambda h: -h.score):
valid = True
for limit, acc, breakdown in zip(self.pred_limits,
all_accs,
hypo.score_breakdown[-1]):
if acc + breakdown[0] < limit:
valid = False
break
if valid:
for i in range(n_preds):
all_accs[i] += hypo.score_breakdown[-1][i][0]
next_hypos.append(hypo)
return next_hypos