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- /**
- * A function to calculate the inverse document frequency for
- * a posting. This is shared between the builder and the index
- *
- * @private
- * @param {object} posting - The posting for a given term
- * @param {number} documentCount - The total number of documents.
- */
- lunr.idf = function (posting, documentCount) {
- var documentsWithTerm = 0
- for (var fieldName in posting) {
- if (fieldName == '_index') continue // Ignore the term index, its not a field
- documentsWithTerm += Object.keys(posting[fieldName]).length
- }
- var x = (documentCount - documentsWithTerm + 0.5) / (documentsWithTerm + 0.5)
- return Math.log(1 + Math.abs(x))
- }
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