1. A method for identifying test content representing visual or aural stimuli, the method comprising: (a) advancing a selection window along a sequence of reference signatures by an interval equal to one or more reference signatures, wherein the reference signatures represent a sequence of segments of reference content representing visual or aural stimuli, and wherein the reference signatures within the selection window constitute a group of selected reference signatures; (b) modifying nodes and links connecting the nodes in a rooted tree structure to reflect a changing membership of reference signature sets in a collection of reference signature sets, wherein each reference signature set in the collection of reference signature sets comprises a series of reference signatures within the group of selected reference signatures, the nodes in the rooted tree structure are associated with the reference signatures in the series of reference signatures, and some of the nodes are leaf nodes that each correspond to a respective reference signature set and wherein the nodes and links are modified by a process that comprises: identifying a new reference signature in the sequence of reference signatures that is newly within the selection window as a result of advancing the selection window; adding the new reference signature to the group of selected reference signatures; adding a new reference signature set to the collection of reference signature sets, wherein the new reference signature set comprises a series of reference signatures in the group of selected reference signatures that ends with the new reference signature; adding one or more new nodes including a new leaf node and one or more new links to the rooted tree structure, wherein the new leaf node corresponds to the new reference signature set and the one or more new links connect other nodes in the rooted tree structure to the one or more new nodes; identifying an old reference signature in the sequence of reference signatures that is newly outside the selection window as a result of advancing the selection window; removing the old reference signature from the group of selected reference signatures; removing an old reference signature set from the collection of reference signature sets, wherein the old reference signature set comprises a series of reference signatures that begins with the old reference signature; and removing one or more old nodes including an old leaf node and one or more old links from the rooted tree structure, wherein the old leaf node corresponds to the old reference signature set and the one or more old links connect other nodes in the rooted tree structure to the one or more old nodes; (c) obtaining a test signature set comprising a series of test signatures that represent a series of segments of the test content; (d) traversing the links to determine if a matching leaf node exists in the rooted tree structure that corresponds to a reference signature set and provides a level of similarity with the test signature set that exceeds a threshold; and (e) generating a match-status signal that indicates whether the matching node exists in the rooted tree structure.