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CONTENTS iii

3.6 Summary ............................................................. 68

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Inverted Indexing for Text Retrieval.........................................704.1 Web Crawling.........................................................714.2 Inverted Indexes ...................................................... 734.3 Inverted Indexing: Baseline Implementation ........................... 754.4 Inverted Indexing: Revised Implementation............................774.5 Index Compression....................................................794.5.1 Byte-Aligned and Word-Aligned Codes 804.5.2 Bit-Aligned Codes 824.5.3 Postings Compression 844.6 What About Retrieval? ............................................... 864.7 Summary and Additional Readings....................................89

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Graph Algorithms...........................................................915.1 Graph Representations................................................935.2 Parallel Breadth-First Search..........................................945.3 PageRank............................................................1025.4 Issues with Graph Processing ........................................ 1085.5 Summary and Additional Readings...................................110

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EM Algorithms for Text Processing........................................1126.1 Expectation Maximization ........................................... 1156.1.1 Maximum Likelihood Estimation 1156.1.2 A Latent Variable Marble Game 1176.1.3 MLE with Latent Variables 1186.1.4 Expectation Maximization 1196.1.5 An EM Example 1206.2 Hidden Markov Models .............................................. 1216.2.1 Three Questions for Hidden Markov Models 1236.2.2 The Forward Algorithm 1256.2.3 The Viterbi Algorithm 126