20 lines
1.3 KiB
Markdown
20 lines
1.3 KiB
Markdown
# Lexicon
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The lexicon contains a mapping for words to identifiers.
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To ease index construction, it makes calculations easier if the domain of word identifiers is dense, that is, there is no gaps between ids; if there are 100 words, they're indexed 0-99 and not 5, 23, 107, 9999, 819235 etc. The lexicon exists to create such a mapping.
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This lexicon is populated from a journal. The actual word data isn't mapped, but rather a 64 bit hash. As a result of the <a href="https://en.wikipedia.org/wiki/Birthday_problem">birthday paradox</a>, colissions will be rare up until about to 2<sup>32</sup> words.
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The lexicon is constructed by [processes/loading-process](../../processes/loading-process) and read when
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[services-core/index-service](../../services-core/index-service) interprets queries.
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## Central Classes
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* [KeywordLexicon](src/main/java/nu/marginalia/lexicon/KeywordLexicon.java)
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* [KeywordLexiconJournal](src/main/java/nu/marginalia/lexicon/journal/KeywordLexiconJournal.java)
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* [DictionaryMap](src/main/java/nu/marginalia/dict/DictionaryMap.java) comes in two versions
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* * [OnHeapDictionaryMap](src/main/java/nu/marginalia/dict/OnHeapDictionaryMap.java) - basically just a fastutil Long2IntOpenHashMap
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* * [OffHeapDictionaryHashMap](src/main/java/nu/marginalia/dict/OffHeapDictionaryHashMap.java) - a heavily modified trove TLongIntHashMap that uses off heap memory
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