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# Calculates the TF*IDF score of words.
class Treat::Workers::Extractors::TfIdf::Native
DefaultOptions = {
:tf => :natural,
:idf => :logarithm,
:remove_common_words => true,
:precision => 4
}
Algorithms = {
:tf => {
:natural => lambda { |tf| tf },
:logarithm => lambda { |tf| Math.log(1 + tf) },
:sqrt =>lambda { |tf| Math.sqrt(tf) }
},
:idf => {
:logarithm => lambda { |n,df| Math.log(n/(1 + df)) },
:none => lambda { |n,idf| 1 }
}
}
# Optimization caches for tf idf.
@@n = {} # Number of documents in the collection (n).
@@df= {} # Number of documents that have a given value (document count).
@@f = {} # Number of times a word appears in a given document (term count).
@@wc = {} # Number of words in a given document (word count).
@@cw = {} # Common words to filter out.
def self.tf_idf(entity, options={})
l = Treat.languages[entity.language]
if l.respond_to?(:stop_words)
@@cw[entity.language] = l.stop_words
return 0 if @@cw[entity.language].include?(entity.value)
end
return 0 if entity.value.length <= 2
options = DefaultOptions.merge(options)
lambdas = options.partition do |k,v|
[:tf, :idf, :normalization].include?(k)
end[0]
lambdas.each do |opt,val|
if opt.is_a?(Symbol)
if Algorithms[opt][val]
options[opt] = Algorithms[opt][val]
else
raise Treat::Exception,
"The specified algorithm '#{val}' "+
"to calculate #{opt} does not exist."
end
end
end
collection = entity.parent_collection
unless collection
raise Treat::Exception, "Cannot get the TF*IDF scores " +
"for a document that is not in a collection."
end
document = entity.parent_document
dc = collection.document_count
if !collection || !document
raise Treat::Exception,
"Tf*Idf requires a collection with documents."
end
val = entity.value.downcase
@@n[collection.id] = dc if @@n[collection.id].nil?
@@df[collection.id] ||= {}
if @@df[collection.id][val].nil?
df = 0
collection.each_document do |doc|
@@f[doc.id] ||= {}
if @@f[doc.id][val].nil?
@@f[doc.id][val] =
doc.frequency_of(val)
end
df += 1 if @@f[doc.id][val] > 0
end
@@df[collection.id][val] = df
end
f = @@f[document.id][entity.value].to_f
df = @@df[collection.id][entity.value].to_f
tf = options[:tf].call(f).to_f
if options[:normalize_word_count]
@@wc[document.id] ||= document.word_count
tf /= @@wc[document.id]
end
n = @@n[collection.id].to_f
idf = options[:idf].call(n, df)
tf_idf = tf * idf
tf_idf.abs.round(options[:precision])
end
end
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