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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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