I wouldn't call that a cache. It's certainly computed state, of the kind that can be thrown away and recomputed ala Crash-Only programming, but in my experience a "cache" is supposed to be transparent: to decorate an file/device/socket/RPC service/object/whatever and expose the same semantics as that whatever, but more performantly. Your indexed set doesn't expose the same semantics as the data file it was constructed from.