Source code for

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"""Candidate samplers"""

__all__ = ['UnigramCandidateSampler']

import mxnet as mx
import numpy as np

[docs]class UnigramCandidateSampler(mx.gluon.HybridBlock): """Unigram Candidate Sampler Draw random samples from a unigram distribution with specified weights using the alias method. Parameters ---------- weights : mx.nd.NDArray Unnormalized class probabilities. Samples are drawn and returned on the same context as weights.context. dtype : str or np.dtype, default 'float32' Data type of the candidates. Make sure that the dtype precision is large enough to represent the size of your weights array precisely. For example, float32 can not distinguish 2**24 from 2**24 + 1. """ def __init__(self, weights, dtype='float32'): super(UnigramCandidateSampler, self).__init__() self._dtype = dtype self.N = weights.size if (np.dtype(dtype) == np.float32 and weights.size > 2**24) or \ (np.dtype(dtype) == np.float16 and weights.size > 2**11): s = 'dtype={dtype} can not represent all weights' raise ValueError(s.format(dtype=dtype)) total_weights = weights.sum() prob = (weights * self.N / total_weights).asnumpy().tolist() alias = [0] * self.N # sort the data into the outcomes with probabilities # that are high and low than 1/N. low = [] high = [] for i in range(self.N): if prob[i] < 1.0: low.append(i) else: high.append(i) # pair low with high while len(low) > 0 and len(high) > 0: l = low.pop() h = high.pop() alias[l] = h prob[h] = prob[h] - (1.0 - prob[l]) if prob[h] < 1.0: low.append(h) else: high.append(h) for i in low + high: prob[i] = 1 alias[i] = i # store prob = mx.nd.array(prob, dtype='float64') alias = mx.nd.array(alias, dtype='float64') self.prob = self.params.get_constant('prob', prob) self.alias = self.params.get_constant('alias', alias) def __repr__(self): s = '{block_name}({len_weights}, {dtype})' return s.format(block_name=self.__class__.__name__, len_weights=self.N, dtype=self._dtype) # pylint: disable=arguments-differ, unused-argument
[docs] def hybrid_forward(self, F, candidates_like, prob, alias): """Draw samples from uniform distribution and return sampled candidates. Parameters ---------- candidates_like: mxnet.nd.NDArray or mxnet.sym.Symbol This input specifies the shape of the to be sampled candidates. # Returns ------- samples: mxnet.nd.NDArray or mxnet.sym.Symbol The sampled candidates of shape candidates_like.shape. Candidates are sampled based on the weights specified on creation of the UnigramCandidateSampler. """ candidates_flat = candidates_like.reshape((-1, )).astype('float64') idx = F.random.uniform_like(candidates_flat, low=0, high=self.N).floor() prob = F.gather_nd(prob, idx.reshape((1, -1))) alias = F.gather_nd(alias, idx.reshape((1, -1))) where = F.random.uniform_like(candidates_flat) < prob hit = idx * where alt = alias * (1 - where) candidates = (hit + alt).reshape_like(candidates_like) return candidates.astype(self._dtype)