A histogram should have the same consistent interface whatever the dimension. Like std::vector it should just work, users shouldn't be forced to make a priori choices among several histogram classes and options everytime they encounter a new data set.

Python is a great language for data analysis, so the histogram needs Python bindings. The histogram should be usable as an interface between a complex simulation or data-storage system written in C++ and data-analysis/plotting in Python: define the histogram in Python, let it be filled on the C++ side, and then get it back for further data analysis or plotting.

Data analysis in Python is Numpy-based, so Numpy support is a must.

The histogram supports half a dozent different binning strategies, conveniently encapsulated in axis objects. There is the standard sorting of real-valued data into bins of equal or varying width, but also binning of angles or integer values.

Extra bins that count over- and underflow values are added by default. This feature can be turned off individually for each axis. The extra bins do not disturb normal bin counting. On an axis with n bins, the first bin has the index 0, the last bin n-1, while the under- and overflow bins are accessible at -1 and n, respectively.

Dense storage in memory is a must for high performance. Unfortunately, the curse of dimensionality quickly become a problem as the number of dimensions grows, leading to histograms which consume large amounts (up to GBs) of memory.

Fortunately, having many dimensions typically reduces the number of counts per bin, since counts get spread over many dimensions. The histogram uses an adaptive count size per bin to be as memory-efficient as possible, by starting with the smallest integer size per bin of 1 byte and increasing as needed to up to 8 byte. A std::vector grows in length as new elements are added, while the count storage grows in depth.

A histogram categorizes and counts, so the natural choice for the data type of the counts are integers. However, in particle physics, histograms are also often filled with weighted events, for example, to make sure that two histograms look the same in one variable, while the distribution of another, correlated variable is a subject of study.

The histogram can be filled with either weighted or unweighted counts. In the weighted case, the sum of weights is stored in a double. The histogram provides a variance estimate is both cases. In the unweighted case, the estimate is computed from the count itself, using Poisson-theory. In the weighted case, the sum of squared weights is stored alongside the sum of weights in a second double, and used to compute a variance estimate.

Serialization is implemented using boost::serialization. Pickling in Python is implemented based on the C++ serialization code. To ensure portability of the pickled histogram, the pickle string is an ASCII representation of the histogram, generated with the boost::archive::text_oarchive. It would be great to switch to a portable binary representation in the future, when that becomes available.

To reduce the size of the string, run-length encoding is applied (zero-suppression) to sequences of zeros. Partly filled histograms often contain large sequences of zeros.