IsoTree

Fast and multi-threaded implementation of Extended Isolation Forest, Fair-Cut Forest, SCiForest (a.k.a. Split-Criterion iForest), and regular Isolation Forest, for outlier/anomaly detection, plus additions for imputation of missing values, distance/similarity calculation between observations, and handling of categorical data. Written in C++ with interfaces for Python and R. An additional wrapper for Ruby can be found here.

The new concepts in this software are described in:

• Revisiting randomized choices in isolation forests
• Distance approximation using Isolation Forests
• Imputing missing values with unsupervised random trees

Description

Isolation Forest is an algorithm originally developed for outlier detection that consists in splitting sub-samples of the data according to some attribute/feature/column at random. The idea is that, the rarer the observation, the more likely it is that a random uniform split on some feature would put outliers alone in one branch, and the fewer splits it will take to isolate an outlier observation like this. The concept is extended to splitting hyperplanes in the extended model (i.e. splitting by more than one column at a time), and to guided (not entirely random) splits in the SCiForest model that aim at isolating outliers faster and finding clustered outliers.

Note that this is a black-box model that will not produce explanations or importances - for a different take on explainable outlier detection see OutlierTree.

(Code to produce these plots can be found in the R examples in the documentation)

Comparison against other libraries

The folder timings contains a speed comparison against other Isolation Forest implementations in Python (SciKit-Learn, EIF) and R (IsolationForest, isofor, solitude). From the benchmarks, IsoTree tends to be at least 1 order of magnitude faster than the libraries compared against in both single-threaded and multi-threaded mode.

Example timings for 100 trees and different sample sizes, CovType dataset - see the link above for full benchmark and details:

Example AUC as outlier detector in typical datasets (notebook to produce results here):

• Satellite dataset:

• Annthyroid dataset:

(Disclaimer: these are rather small datasets and thus these AUC estimates have high variance)

Non-random splits

While the original idea behind isolation forests consisted in deciding splits uniformly at random, it's possible to get better performance at detecting outliers in some datasets (particularly those with multimodal distributions) by determining splits according to an information gain criterion instead. The idea is described in "Revisiting randomized choices in isolation forests" along with some comparisons of different split guiding criteria.

Distance / similarity calculations

General idea was extended to produce distance (alternatively, similarity) between observations according to how many random splits it takes to separate them - idea is described in "Distance approximation using Isolation Forests".

Imputation of missing values

The model can also be used to impute missing values in a similar fashion as kNN, by taking the values from observations in the terminal nodes of each tree in which an observation with missing values falls at prediction time, combining the non-missing values of the other observations as a weighted average according to the depth of the node and the number of observations that fall there. This is not related to how the model handles missing values internally, but is rather meant as a faster way of imputing by similarity. Quality is usually not as good as chained equations, but the method is a lot faster and more scalable. Recommended to use non-random splits when used as an imputer. Details are described in "Imputing missing values with unsupervised random trees".

Highlights

There's already many available implementations of isolation forests for both Python and R (such as the one from the original paper's authors' or the one in SciKit-Learn), but at the time of writing, all of them are lacking some important functionality and/or offer sub-optimal speed. This particular implementation offers the following:

• Implements the extended model (with splitting hyperplanes) and split-criterion model (with non-random splits).
• Can handle missing values (but performance with them is not so good).
• Can handle categorical variables (one-hot/dummy encoding does not produce the same result).
• Can use a mixture of random and non-random splits, and can split by weighted/pooled gain (in addition to simple average).
• Can produce approximated pairwise distances between observations according to how many steps it takes on average to separate them down the tree.
• Can produce missing value imputations according to observations that fall on each terminal node.
• Can work with sparse matrices.
• Supports sample/observation weights, either as sampling importance or as distribution density measurement.
• Supports user-provided column sample weights.
• Can sample columns randomly with weights given by kurtosis.
• Uses exact formula (not approximation as others do) for harmonic numbers at lower sample and remainder sizes, and a higher-order approximation for larger sizes.
• Can fit trees incrementally to user-provided data samples.
• Produces serializable model objects with reasonable file sizes.
• Can convert the models to treelite format (Python-only and depending on the parameters that are used) (example here).
• Can translate the generated trees into SQL statements.
• Fast and multi-threaded C++ code with an ISO C interface, which is architecture-agnostic, multi-platform, and with the only external dependency (Robin-Map) being optional. Can be wrapped in languages other than Python/R/Ruby.

(Note that categoricals, NAs, and density-like sample weights, are treated heuristically with different options as there is no single logical extension of the original idea to them, and having them present might degrade performance/accuracy for regular numerical non-missing observations)

Installation

• Python:

pip install isotree

or if that fails:

pip install --no-use-pep517 isotree

Note for macOS users: on macOS, the Python version of this package might compile without multi-threading capabilities. In order to enable multi-threading support, first install OpenMP:

brew install libomp

And then reinstall this package: pip install --force-reinstall isotree.

• R:

install.packages("isotree")

• C and C++:

git clone --recursive https://www.github.com/david-cortes/isotree.git
cd isotree
mkdir build
cd build
cmake -DUSE_MARCH_NATIVE=1 ..
cmake --build .

### for a system-wide install in linux
sudo make install
sudo ldconfig

(Will build as a shared object - linkage is then done with -lisotree)

Be aware that the snippet above includes option -DUSE_MARCH_NATIVE=1, which will make it use the highest-available CPU instruction set (e.g. AVX2) and will produces objects that might not run on older CPUs - to build more "portable" objects, remove this option from the cmake command.

The package has an optional dependency on the Robin-Map library, which is added to this repository as a linked submodule. If this library is not found under /src, will use the compiler's own hashmaps, which are less optimal.

• Ruby:

See external repository with wrapper.

Sample usage

Warning: default parameters in this implementation are very different from default parameters in others such as SciKit-Learn's, and these defaults won't scale to large datasets (see documentation for details).

• Python:

(Library is SciKit-Learn compatible)

import numpy as np
from isotree import IsolationForest

### Random data from a standard normal distribution
np.random.seed(1)
n = 100
m = 2
X = np.random.normal(size = (n, m))

### Will now add obvious outlier point (3, 3) to the data
X = np.r_[X, np.array([3, 3]).reshape((1, m))]

### Fit a small isolation forest model
iso = IsolationForest(ntrees = 10, nthreads = 1)
iso.fit(X)

### Check which row has the highest outlier score
pred = iso.predict(X)
print("Point with highest outlier score: ",
      X[np.argsort(-pred)[0], ])

• R:

(see documentation for more examples - help(isotree::isolation.forest))

### Random data from a standard normal distribution
library(isotree)
set.seed(1)
n <- 100
m <- 2
X <- matrix(rnorm(n * m), nrow = n)

### Will now add obvious outlier point (3, 3) to the data
X <- rbind(X, c(3, 3))

### Fit a small isolation forest model
iso <- isolation.forest(X, ntrees = 10, nthreads = 1)

### Check which row has the highest outlier score
pred <- predict(iso, X)
cat("Point with highest outlier score: ",
    X[which.max(pred), ], "\n")

• C++:

The package comes with two different C++ interfaces: (a) a struct-based interface which exposes the full library's functionalities but makes little checks on the inputs it receives and is perhaps a bit difficult to use due to the large number of arguments that functions require; and (b) a scikit-learn-like interface in which the model exposes a single class with methods like 'fit' and 'predict', which is less flexible than the struct-based interface but easier to use and the function signatures disallow some potential errors due to invalid parameter combinations.

See files: isotree_cpp_ex.cpp for an example with the struct-based interface; and isotree_cpp_oop_ex.cpp for an example with the scikit-learn-like interface.

Note that the second interface does not expose all the functionalities - for example, it only supports inputs of classes 'double' and 'int', while the struct-based interface also supports 'float'/'size_t'.

• C:

See file isotree_c_ex.c.

Note that the C interface is a simple wrapper over the scikit-learn-like C++ interface, but using only ISO C bindings for better compatibility and easier wrapping in other languages.

• Ruby

See external repository with wrapper.

Examples

• Python: example notebook here, (also example as imputer in sklearn pipeline here, and example converting to treelite here).
• R: examples available in the documentation (help(isotree::isolation.forest), link to CRAN).
• C and C++: see short examples in the section above.
• Ruby: see external repository with wrapper.

Documentation

• Python: documentation is available at ReadTheDocs.
• R: documentation is available internally in the package (e.g. help(isolation.forest)) and in CRAN.
• C++: documentation is available in the public header (include/isotree.hpp) and in the source files. See also the header for the scikit-learn-like interface (include/isotree_oop.hpp).
• C: interface is not documented per-se, but the same documentation from the C++ header applies to it. See also its header for some non-comprehensive comments about the parameters that functions take (include/isotree_c.h).
• Ruby: see external repository with wrapper for the syntax and the Python docs for details about the parameters.

Help wanted

The package does not currenly have any functionality for visualizing trees. Pull requests adding such functionality would be welcome.

References

• Liu, Fei Tony, Kai Ming Ting, and Zhi-Hua Zhou. "Isolation forest." 2008 Eighth IEEE International Conference on Data Mining. IEEE, 2008.
• Liu, Fei Tony, Kai Ming Ting, and Zhi-Hua Zhou. "Isolation-based anomaly detection." ACM Transactions on Knowledge Discovery from Data (TKDD) 6.1 (2012): 3.
• Hariri, Sahand, Matias Carrasco Kind, and Robert J. Brunner. "Extended Isolation Forest." arXiv preprint arXiv:1811.02141 (2018).
• Liu, Fei Tony, Kai Ming Ting, and Zhi-Hua Zhou. "On detecting clustered anomalies using SCiForest." Joint European Conference on Machine Learning and Knowledge Discovery in Databases. Springer, Berlin, Heidelberg, 2010.
• https://sourceforge.net/projects/iforest/
• https://math.stackexchange.com/questions/3388518/expected-number-of-paths-required-to-separate-elements-in-a-binary-tree
• Quinlan, J. Ross. C4. 5: programs for machine learning. Elsevier, 2014.
• Cortes, David. "Distance approximation using Isolation Forests." arXiv preprint arXiv:1910.12362 (2019).
• Cortes, David. "Imputing missing values with unsupervised random trees." arXiv preprint arXiv:1911.06646 (2019).
• Cortes, David. "Revisiting randomized choices in isolation forests." arXiv preprint arXiv:2110.13402 (2021).