Implementation of Time Series in Clojure - a learning exercise.
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README.md
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README.md

Object-Oriented Time Series

At one time, I implemented data sets that contained historical time series. Each data set was composed of a set of individual time series modeling one measurement. Not only did each data set have a name but each individual series had a similar name.

Given this problem, the following Java code models an interface for a time series::

public interface TimeSeries {
    public String getName();
    public Calendar getStartTime();
    public TimePeriod getPeriod();
    public ArrayList<DataPoint> getSamples();
    public void AddPoint (DataPoint aNewPoint);
    public DataPoint GetPoint(int k);
};

public interface DataPoint {
    public double getValue();
    public Calendar getSampleTime();
};

Implementing this interface does not produce a particularly object-oriented solution. One can envision a single class that simply implements this interface. The solution becomes much more interesting when we consider how to create these series programatically.

Historically, data was available in a sanitized form. In this form, data had a single period and every measurement had a time stamp that was a exact multiple of that period. This regularity allowed programmers to compress the data. In addition, although we had a measurement at every time period, some of those values were "magic." That is, legacy data used a sentinel value to indicating that we actually had no data.

Current technology in this particular domain now allows us to take advantage of the actual sample times; in other words, we can take advantage of the "jiggles" in the actual sample times. In addition, current technology allows us to take advantage of invalid or missing data. As a result, the specification preferred storing actual data instead of the sanitized form.

Finally, we have some additional time series that we capture irregularly. It still is a time series but it can be thought of as "sparsely" or "nominally" regular. An example are laboratory samples taken from a factory floor. Although the samples are taken "every six hours", the variation from this period is measured in minutes and hours instead of seconds and milliseconds.

These different "input types" produce the object-oriented characteristics of our solution. When we handle legacy data, we want to take advantage of the regularity of the data. When we create a time series from data captured now, we want "jiggly" data. Finally, we want our code to handle our "irregular data." Most importantly, we want to allow client objects to handle time series from these different sources in the same way.

Given these constraints, you can probably already design a class hierarchy:

  • TimeSeries (client interface)
    • AbstractTimeSeries (common implementation)
      • RegularTimeSeries (for regular, sanitized legacy data)
      • AlmostRegTimeSeries (for data collected with today's technology)
      • IrregularTimeSeries (for our sparse, "irregular" data)

My question, and hopefully yours, is, "How would we solve this problem using Clojure?"

"Instructions"

The file test/chug_time_series/time_series.clj contains unit tests. Because I created these tests from my idea of how to build it, they probably contain some "bias" on how to implement a solution.

This solution contains four different tags representing four different stages of development.

 * ts_constructor - A very simple implementation of the time
   series constructor.
 * ts_ctor_refactor - Refactor the constructor implementation to
   remove duplicate code.
 * add_point - Added feature to add points to the time series.
 * add_point_refactor - Refactored code to eliminate duplicate
   code by introducing an ad hoc hierarchy.

In addition, the tip revision contains a file named CrossParadigmProgramming.pdf. This file is a presentation that I gave at the Clojure Houston User Group.