1. A computing-system-implemented method for calculating a covariance for two adjusted computation windows on a computing-device-based computing system which comprises one or more computing devices and one or more storage media, each of the one or more computing devices comprising one or more processors, the method comprising: initializing, by the computing-device-based computing system, a sum or a mean or both for each of two pre-adjusted computation windows and one or more other components of a covariance for the two pre-adjusted computation windows wherein the two pre-adjusted computation windows contain a specified number n (n≥6) pairs of data elements of one or two data buffers which are on at least one of the one or more storage media and for storing data elements from one or two data streams; receiving, by the computing-device-based computing system, a pair of data elements to be added to the two pre-adjusted computation windows; storing, by the computing-device-based computing system, the received pair of data elements in the one or two buffers; adjusting, by the computing-device-based computing system, the two pre-adjusted computation windows by: removing a least recently received pair of data elements from the two pre-adjusted computation windows; and adding the to-be-added pair of data elements to the two pre-adjusted computation windows; iteratively deriving, by the computing-device-based computing system, a sum or a mean or both for each of the two adjusted computation windows; directly iteratively deriving, by the computing-device-based computing system and based at least in part on the one or more components other than a sum and a mean for the two pre-adjusted computation windows, one or more components of a covariance other than a sum and a mean for the two adjusted computation windows, wherein the directly iteratively deriving includes: accessing the removed pair of data elements and the added pair of data elements without accessing all data elements in the two adjusted computation windows to reduce data accessing latency thereby saving computing resources and reducing the computing system's power consumption; accessing the one or more components other than a sum and a mean; and removing any contribution of the removed pair of data elements from each of the accessed components mathematically and adding any contribution of the added pair of data elements to each of the accessed components mathematically wherein not all data elements in the two adjusted computation windows are used during the iteratively deriving one or more components to reduce operations performed by the computing system thereby increasing calculation efficiency; and generating, by the computing-device-based computing system, the covariance for the two adjusted computation windows based on one or more of the iteratively derived components.