1. A computing device-implemented method, comprising: receiving a document; comparing the document with a prior version of the document; determining, based on the comparison, at least one first portion of the document that is common with the prior version of the document and at least one second portion of the document that is uncommon with the prior version of the document; determining an age of the at least one first portion and the at least one second portion; determining whether the age of the at least one first portion is greater than or equal to an age threshold and that the age of the at least one second portion is less than the age threshold; calculating a checksum of the document based on the at least one first portion but not the at least one second portion when the age of the at least one first portion is greater than or equal to the age threshold and the age of the at least one second portion is less than the age threshold; and storing the calculated checksum.