SlidingBuffer provides local caching of a data stream, but a DetectorBank operates by being initialised with audio data then, on successive calls, producing arrays of complex data.
Each row in such array contains the state locus of a Hopf system's complex variable. For an onset detector to work consistently and reliably, once a note is detected, the output of a channel has to backtrack to the note start by following the rising edge of a detector's output back to its origin.
detectorcache::DetectorCache wraps a sliding buffer to permit easy access to individual channels, and provides the method detectorcache::Producer to interface between a DetectorBank that has already been initialised with audio samples and the DetectorCache's segments as they require more data. DetectorBank::getZ is called to produce the complex data, then the absolute values of the returned results are taken and used to populate the segments.
To retain compatibility with the Python's numerical libraries, 2D arrays are allocated in C++ as a single area containing (channels*blocksize) result_t values. The API presented by a DetectorCache needs to change this view to individual channels, caching a given number of samples in each.
The type of SlidingBuffer used to achieve this is defined extending slidingbuffer::SlidingBuffer like this:
Note that each segment of the siding buffer is an instance of detectorcache::Segment (specialised Segment class in the detectorbank namespace) and its size is the number of channels supported by the detectorcache::Producer (determined in turn from the DetectorBank with which the Producer was constructed).
A consequence of performing fixed-length block requests to getZ is that the SlidingBuffer base class can not in fact storing channels; it is instead storing pointers to blocks of result_t data representing a single block of one channel as shown in the following diagram.
Now that the data accessed through a DetectorCache is essentially 2D, the [] operator can no longer be used, at least without the overhead of a proxy class (C++11 mandates a single argument). Two access methods are therefore provided. getResultItem returns the value of a single item at a given sample position n in channel c; getPreviousResults fills a given 1D array by copying historical data from the DetectorCache. The resulting array holds consecutive samples of data in contiguous memory locations even if the original data is spread across the blocks associated with different segments. The specified data ends with the currentSample, and the given number of previous results are copied.
