5. The system of claim 1 , wherein the signature-generation means for generating a plurality of document signatures comprises: means for determining one or more document length features or values; means for identifying a set of features from each document, based on their corresponding inverse-document-frequency (idf) values; means for determining positions of the features in each document; means for concatenating the set of features and their determined positions to define a string; means for hashing the string to define a hash value for each document; means for forming a document signature based on the document length feature or values and the hash value for each document; and storing a document signature in a memory device.
5. The system of claim 1 , wherein the signature-generation means for generating a plurality of document signatures comprises: means for determining one or more document length features or values; means for identifying a set of features from each document, based on their corresponding inverse-document-frequency (idf) values; means for determining positions of the features in each document; means for concatenating the set of features and their determined positions to define a string; means for hashing the string to define a hash value for each document; means for forming a document signature based on the document length feature or values and the hash value for each document; and storing a document signature in a memory device. 6. The system of claim 5 , wherein the signature-generation means further comprises means for rounding the determined positions of each feature; and concatenating the set of features and their rounded determined positions to define a string.