参考资料：
http://ir.dlut.edu.cn/NewsShow.aspx?ID=291
http://www.douban.com/note/298095260/
http://machinelearning.wustl.edu/mlpapers/paper_files/BengioDVJ03.pdf
https://code.google.com/p/word2vec/
word2vec是NLP领域的重要算法，它的功能是将word用K维的dense vector来表达，训练集是语料库，不含标点，以空格断句。因此可以看作是种特征处理方法。
主要优点：
- 加法操作。
- 高效。单机可处理1小时2千万词。
google的开源版本比较权威，地址（ http://word2vec.googlecode.com/svn/trunk/ ），不过我以spark版本学习的。
I.背景知识
Distributed representation，word的特征表达方式，通过训练将每个词映射成 K 维实数向量(K 一般为模型中的超参数)，通过词之间的距离(比如 cosine 相似度、欧氏距离等)来判断它们之间的语义相似度。
语言模型：n-gram等。
II.模型
0.word window构成context，对于一个单词i，以$u_i$表示，它作为别的单词的context时用$v_i$表示（也即它作为context的表示是不同的）。只有word window内的word才被认为是context，并且是顺序无关的。
1.概率模型为\[ P=\sum lot p(u_i) \]，i表示位置（或单词），也即各单词出现概率的累积函数。
2.以skip gram为例（CBOW条件概率反过来），则位置i的单词出现概率为
\[ p(u_i)=\sum_{-c\leq j\leq c,j\neq 0} p(v_{i+j}|u_{i}) \]
表示位置i只和其context有关。
3.条件概率$p(v_{i+j}|u_i)$ 通过softmax实现K维向量到概率的转化表达。
III.优化
最开始使用神经网络，后来用层次softmax来降低时间复杂度。还用了很多trick，比如ExpTable。
IV.spark源码分析
1 /**
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17
18 package org.apache.spark.mllib.feature
19
20 import java.lang.{Iterable => JavaIterable}
21
22 import com.github.fommil.netlib.BLAS.{getInstance => blas}
23 import org.apache.spark.Logging
24 import org.apache.spark.SparkContext._
25 import org.apache.spark.annotation.Experimental
26 import org.apache.spark.api.java.JavaRDD
27 import org.apache.spark.mllib.linalg.{Vector, Vectors}
28 import org.apache.spark.rdd.RDD
29 import org.apache.spark.util.Utils
30 import org.apache.spark.util.random.XORShiftRandom
31 import scala.collection.JavaConverters._
32 import scala.collection.mutable
33 import scala.collection.mutable.ArrayBuffer
34
35
36 /**
37 * Entry in vocabulary
38 */
39 private case class VocabWord(
40 var word: String,
41 var cn: Int,
42 var point: Array[Int],
43 var code: Array[Int],
44 var codeLen:Int
45 )
46
47 /**
48 * :: Experimental ::
49 * Word2Vec creates vector representation of words in a text corpus.
50 * The algorithm first constructs a vocabulary from the corpus
51 * and then learns vector representation of words in the vocabulary.
52 * The vector representation can be used as features in
53 * natural language processing and machine learning algorithms.
54 *
55 * We used skip-gram model in our implementation and hierarchical softmax
56 * method to train the model. The variable names in the implementation
57 * matches the original C implementation.
58 *
59 * For original C implementation, see https://code.google.com/p/word2vec/
60 * For research papers, see
61 * Efficient Estimation of Word Representations in Vector Space
62 * and
63 * Distributed Representations of Words and Phrases and their Compositionality.
64 */
65 @Experimental
66 class Word2VectorEX extends Serializable with Logging {
67
68 private var vectorSize = 100
69 private var startingAlpha = 0.025
70 private var numPartitions = 1
71 private var numIterations = 1
72 private var seed = Utils.random.nextLong()
73
74 /**
75 * Sets vector size (default: 100).
76 */
77 def setVectorSize(vectorSize: Int): this.type = {
78 this.vectorSize = vectorSize
79 this
80 }
81
82 /**
83 * Sets initial learning rate (default: 0.025).
84 */
85 def setLearningRate(learningRate: Double): this.type = {
86 this.startingAlpha = learningRate
87 this
88 }
89
90 /**
91 * Sets number of partitions (default: 1). Use a small number for accuracy.
92 */
93 def setNumPartitions(numPartitions: Int): this.type = {
94 require(numPartitions > 0, s"numPartitions must be greater than 0 but got $numPartitions")
95 this.numPartitions = numPartitions
96 this
97 }
98
99 /**
100 * Sets number of iterations (default: 1), which should be smaller than or equal to number of
101 * partitions.
102 */
103 def setNumIterations(numIterations: Int): this.type = {
104 this.numIterations = numIterations
105 this
106 }
107
108 /**
109 * Sets random seed (default: a random long integer).
110 */
111 def setSeed(seed: Long): this.type = {
112 this.seed = seed
113 this
114 }
115
116 private val EXP_TABLE_SIZE = 1000
117 private val MAX_EXP = 6
118 private val MAX_CODE_LENGTH = 40
119 private val MAX_SENTENCE_LENGTH = 1000
120
121 /** context words from [-window, window] */
122 private val window = 5 //context 范围限定
123
124 /** minimum frequency to consider a vocabulary word */
125 private val minCount = 5 //过滤单词阈值
126
127 private var trainWordsCount = 0 //语料库总共词量（计重复出现）
128 private var vocabSize = 0 //词表内单词总数
129 private var vocab: Array[VocabWord] = null //词表
130 private var vocabHash = mutable.HashMap.empty[String, Int] //词表反查索引
131
132 private def learnVocab(words: RDD[String]): Unit = { //构造词表，统计更新上面四个量
133 vocab = words.map(w => (w, 1))
134 .reduceByKey(_ + _)
135 .map(x => VocabWord(
136 x._1,
137 x._2,
138 new Array[Int](MAX_CODE_LENGTH),
139 new Array[Int](MAX_CODE_LENGTH),
140 0))
141 .filter(_.cn >= minCount)
142 .collect()
143 .sortWith((a, b) => a.cn > b.cn)
144
145 vocabSize = vocab.length
146 var a = 0
147 while (a < vocabSize) {
148 vocabHash += vocab(a).word -> a
149 trainWordsCount += vocab(a).cn
150 a += 1
151 }
152 logInfo("trainWordsCount = " + trainWordsCount)
153 }
154
155 private def createExpTable(): Array[Float] = { //指数运算查表
156 val expTable = new Array[Float](EXP_TABLE_SIZE)
157 var i = 0
158 while (i < EXP_TABLE_SIZE) {
159 val tmp = math.exp((2.0 * i / EXP_TABLE_SIZE - 1.0) * MAX_EXP)
160 expTable(i) = (tmp / (tmp + 1.0)).toFloat
161 i += 1
162 }
163 expTable
164 }
165
166 private def createBinaryTree(): Unit = {
167 val count = new Array[Long](vocabSize * 2 + 1)
168 val binary = new Array[Int](vocabSize * 2 + 1)
169 val parentNode = new Array[Int](vocabSize * 2 + 1)
170 val code = new Array[Int](MAX_CODE_LENGTH)
171 val point = new Array[Int](MAX_CODE_LENGTH)
172 var a = 0
173 while (a < vocabSize) {
174 count(a) = vocab(a).cn
175 a += 1
176 }
177 while (a < 2 * vocabSize) {
178 count(a) = 1e9.toInt
179 a += 1
180 }
181 var pos1 = vocabSize - 1
182 var pos2 = vocabSize
183
184 var min1i = 0
185 var min2i = 0
186
187 a = 0
188 while (a < vocabSize - 1) {
189 if (pos1 >= 0) {
190 if (count(pos1) < count(pos2)) {
191 min1i = pos1
192 pos1 -= 1
193 } else {
194 min1i = pos2
195 pos2 += 1
196 }
197 } else {
198 min1i = pos2
199 pos2 += 1
200 }
201 if (pos1 >= 0) {
202 if (count(pos1) < count(pos2)) {
203 min2i = pos1
204 pos1 -= 1
205 } else {
206 min2i = pos2
207 pos2 += 1
208 }
209 } else {
210 min2i = pos2
211 pos2 += 1
212 }
213 count(vocabSize + a) = count(min1i) + count(min2i)
214 parentNode(min1i) = vocabSize + a
215 parentNode(min2i) = vocabSize + a
216 binary(min2i) = 1
217 a += 1
218 }
219 // Now assign binary code to each vocabulary word
220 var i = 0
221 a = 0
222 while (a < vocabSize) {
223 var b = a
224 i = 0
225 while (b != vocabSize * 2 - 2) {
226 code(i) = binary(b)
227 point(i) = b
228 i += 1
229 b = parentNode(b)
230 }
231 vocab(a).codeLen = i
232 vocab(a).point(0) = vocabSize - 2
233 b = 0
234 while (b < i) {
235 vocab(a).code(i - b - 1) = code(b)
236 vocab(a).point(i - b) = point(b) - vocabSize
237 b += 1
238 }
239 a += 1
240 }
241 }
242
243 /**
244 * Computes the vector representation of each word in vocabulary.
245 * @param dataset an RDD of words
246 * @return a Word2VecModel
247 */
248 def fit[S <: Iterable[String]](dataset: RDD[S]): Word2VectorModel = {
249
250 val words = dataset.flatMap(x => x) //拉成词序列，句话断点通过Iterable来表征
251
252 learnVocab(words) //学习词库
253
254 createBinaryTree()
255
256 val sc = dataset.context
257
258 val expTable = sc.broadcast(createExpTable())
259 val bcVocab = sc.broadcast(vocab)
260 val bcVocabHash = sc.broadcast(vocabHash)
261
262 val sentences: RDD[Array[Int]] = words.mapPartitions { iter => //按句子划分，单词以Int表征
263 new Iterator[Array[Int]] {
264 def hasNext: Boolean = iter.hasNext
265
266 def next(): Array[Int] = {
267 var sentence = new ArrayBuffer[Int]
268 var sentenceLength = 0
269 while (iter.hasNext && sentenceLength < MAX_SENTENCE_LENGTH) {
270 val word = bcVocabHash.value.get(iter.next())
271 word match {
272 case Some(w) =>
273 sentence += w
274 sentenceLength += 1
275 case None =>
276 }
277 }
278 sentence.toArray
279 }
280 }
281 }
282
283 //Hierarchical Softmax
284 val newSentences = sentences.repartition(numPartitions).cache()
285 val initRandom = new XORShiftRandom(seed)
286 val syn0Global =
287 Array.fill[Float](vocabSize * vectorSize)((initRandom.nextFloat() - 0.5f) / vectorSize)
288 val syn1Global = new Array[Float](vocabSize * vectorSize)
289 var alpha = startingAlpha
290 for (k <- 1 to numIterations) {
291 val partial = newSentences.mapPartitionsWithIndex { case (idx, iter) =>
292 val random = new XORShiftRandom(seed ^ ((idx + 1) << 16) ^ ((-k - 1) << 8)) //随机梯度下降
293 val syn0Modify = new Array[Int](vocabSize)
294 val syn1Modify = new Array[Int](vocabSize)
295 val model = iter.foldLeft((syn0Global, syn1Global, 0, 0)) {
296 case ((syn0, syn1, lastWordCount, wordCount), sentence) =>
297 var lwc = lastWordCount
298 var wc = wordCount
299 if (wordCount - lastWordCount > 10000) {
300 lwc = wordCount
301 // TODO: discount by iteration?
302 alpha =
303 startingAlpha * (1 - numPartitions * wordCount.toDouble / (trainWordsCount + 1))
304 if (alpha < startingAlpha * 0.0001) alpha = startingAlpha * 0.0001
305 logInfo("wordCount = " + wordCount + ", alpha = " + alpha)
306 }
307 wc += sentence.size
308 var pos = 0
309 while (pos < sentence.size) {
310 val word = sentence(pos)
311 val b = random.nextInt(window)
312 // Train Skip-gram
313 var a = b
314 while (a < window * 2 + 1 - b) {
315 if (a != window) {
316 val c = pos - window + a
317 if (c >= 0 && c < sentence.size) {
318 val lastWord = sentence(c)
319 val l1 = lastWord * vectorSize
320 val neu1e = new Array[Float](vectorSize)
321 // Hierarchical softmax
322 var d = 0
323 while (d < bcVocab.value(word).codeLen) {
324 val inner = bcVocab.value(word).point(d)
325 val l2 = inner * vectorSize
326 // Propagate hidden -> output
327 var f = blas.sdot(vectorSize, syn0, l1, 1, syn1, l2, 1)
328 if (f > -MAX_EXP && f < MAX_EXP) {
329 val ind = ((f + MAX_EXP) * (EXP_TABLE_SIZE / MAX_EXP / 2.0)).toInt
330 f = expTable.value(ind)
331 val g = ((1 - bcVocab.value(word).code(d) - f) * alpha).toFloat
332 blas.saxpy(vectorSize, g, syn1, l2, 1, neu1e, 0, 1)
333 blas.saxpy(vectorSize, g, syn0, l1, 1, syn1, l2, 1)
334 syn1Modify(inner) += 1
335 }
336 d += 1
337 }
338 blas.saxpy(vectorSize, 1.0f, neu1e, 0, 1, syn0, l1, 1)
339 syn0Modify(lastWord) += 1
340 }
341 }
342 a += 1
343 }
344 pos += 1
345 }
346 (syn0, syn1, lwc, wc)
347 }
348 val syn0Local = model._1
349 val syn1Local = model._2
350 // Only output modified vectors.
351 Iterator.tabulate(vocabSize) { index =>
352 if (syn0Modify(index) > 0) {
353 Some((index, syn0Local.slice(index * vectorSize, (index + 1) * vectorSize)))
354 } else {
355 None
356 }
357 }.flatten ++ Iterator.tabulate(vocabSize) { index =>
358 if (syn1Modify(index) > 0) {
359 Some((index + vocabSize, syn1Local.slice(index * vectorSize, (index + 1) * vectorSize)))
360 } else {
361 None
362 }
363 }.flatten
364 }
365 val synAgg = partial.reduceByKey { case (v1, v2) =>
366 blas.saxpy(vectorSize, 1.0f, v2, 1, v1, 1)
367 v1
368 }.collect()
369 var i = 0
370 while (i < synAgg.length) {
371 val index = synAgg(i)._1
372 if (index < vocabSize) {
373 Array.copy(synAgg(i)._2, 0, syn0Global, index * vectorSize, vectorSize)
374 } else {
375 Array.copy(synAgg(i)._2, 0, syn1Global, (index - vocabSize) * vectorSize, vectorSize)
376 }
377 i += 1
378 }
379 }
380 newSentences.unpersist()
381
382 val word2VecMap = mutable.HashMap.empty[String, Array[Float]]
383 var i = 0
384 while (i < vocabSize) {
385 val word = bcVocab.value(i).word
386 val vector = new Array[Float](vectorSize)
387 Array.copy(syn0Global, i * vectorSize, vector, 0, vectorSize)
388 word2VecMap += word -> vector
389 i += 1
390 }
391
392 new Word2VectorModel(word2VecMap.toMap)
393 }
394
395 /**
396 * Computes the vector representation of each word in vocabulary (Java version).
397 * @param dataset a JavaRDD of words
398 * @return a Word2VecModel
399 */
400 def fit[S <: JavaIterable[String]](dataset: JavaRDD[S]): Word2VectorModel = {
401 fit(dataset.rdd.map(_.asScala))
402 }
403
404 }
405
406 /**
407 * :: Experimental ::
408 * Word2Vec model
409 */
410 @Experimental
411 class Word2VectorModel private[mllib] (
412 private val model: Map[String, Array[Float]]) extends Serializable {
413
414 private def cosineSimilarity(v1: Array[Float], v2: Array[Float]): Double = {
415 require(v1.length == v2.length, "Vectors should have the same length")
416 val n = v1.length
417 val norm1 = blas.snrm2(n, v1, 1)
418 val norm2 = blas.snrm2(n, v2, 1)
419 if (norm1 == 0 || norm2 == 0) return 0.0
420 blas.sdot(n, v1, 1, v2,1) / norm1 / norm2
421 }
422
423 /**
424 * Transforms a word to its vector representation
425 * @param word a word
426 * @return vector representation of word
427 */
428 def transform(word: String): Vector = {
429 model.get(word) match {
430 case Some(vec) =>
431 Vectors.dense(vec.map(_.toDouble))
432 case None =>
433 throw new IllegalStateException(s"$word not in vocabulary")
434 }
435 }
436
437 /**
438 * Find synonyms of a word
439 * @param word a word
440 * @param num number of synonyms to find
441 * @return array of (word, similarity)
442 */
443 def findSynonyms(word: String, num: Int): Array[(String, Double)] = {
444 val vector = transform(word)
445 findSynonyms(vector,num)
446 }
447
448 /**
449 * Find synonyms of the vector representation of a word
450 * @param vector vector representation of a word
451 * @param num number of synonyms to find
452 * @return array of (word, cosineSimilarity)
453 */
454 def findSynonyms(vector: Vector, num: Int): Array[(String, Double)] = {
455 require(num > 0, "Number of similar words should > 0")
456 // TODO: optimize top-k
457 val fVector = vector.toArray.map(_.toFloat)
458 model.mapValues(vec => cosineSimilarity(fVector, vec))
459 .toSeq
460 .sortBy(- _._2)
461 .take(num + 1)
462 .tail
463 .toArray
464 }
465
466
467 def getModel(): Map[String, Array[Float]] = {
468 model
469 }
470
471
472 }
时间： 2024-10-11 11:42:20