Posterior decoding with a hidden Markov model
Hmmus has some C implementations of HMM algorithms with Python bindings, and it is meant to be useful under the following conditions:
- The sequence of observations to be analyzed is so long that it does not fit conveniently in RAM.
- Likelihoods per hidden state per position have been precalculated.
- Numerical stability is important, but is not so important that error bounds on the output are required.
- Speed is important.
- The number of hidden states is small.
- The matrix of probabilities of transitions between hidden states is dense.
- Binary data files are acceptable as input and output.
This project would be especially useless in the following cases:
- User friendly or pedagogically informative software is desired.
- All of the data can fit in RAM and numerical stability is not an issue.
- The hidden state transitions are defined by a large sparse graph.
- The emission distributions are uncomplicated (e.g. finite or normal).
- A variable number of observations are emitted per hidden state.
- Silent states other than start and stop states are used.
Operating system requirements:
- This project was developed using Ubuntu, so it will probably work on Debian-based Linux distributions.
- It might work with non-Debian-based Unix variants.
- It probably will not work on Windows.
- A recent version of Python-2.x (2.6+).
- A C compiler which is not too different from gcc.
Python package and module dependencies:
Setting up virtualenv and pip
These programs have been packaged for Ubuntu and probably Debian, and can be installed from the Linux distribution package repository as follows:
$ sudo apt-get install python-virtualenv $ sudo apt-get install python-pip
Alternatively the development version can be downloaded:
$ hg clone http://bitbucket.org/ianb/virtualenv
To use a binary installation of virtualenv to create a virtual python environment:
$ virtualenv /path/to/myenv
Or to use the source installation of virtualenv to create a virtual python environment:
$ /go/to/virtualenv.py --distribute --python=/go/to/python /go/to/myenv
Now activate the virtual environment:
$ . /path/to/myenv/bin/activate
Installing required Python modules and packages
The following packages and modules should be installed:
- The numpy package should be installed by sudo apt-get install python-numpy on Debian and Ubuntu. Or to get a newer version, install from subversion.
- The argparse module can be installed by pip install argparse in the activated virtual environment.
The easiest way to install hmmus is from the python package index pypi as follows:
$ pip install hmmus
If pypi is inaccessible for some reason, then hmmus can alternatively be installed directly from its github repository as follows:
$ pip install git+git://github.com/argriffing/hmmus
If you are developing hmmus or have cloned the git repo as ~/repos/hmmus for some other reason, hmmus can be installed from this local repository as follows:
$ pip install -e ~/repos/hmmus
It is easy to uninstall hmmus using pip:
$ pip uninstall hmmus
If this fails for some reason and you really want to get rid of hmmus, then you can delete the virtual environment into which hmmus was installed.
In its current incarnation hmmus provides some scripts for doing posterior decoding, using unfriendly binary files for input and output. The following commands create an empty directory and then fill it with some sample input files:
$ mkdir mydemo $ cd mydemo $ hmm-demo smith
This creates the files distribution.bin, transitions.bin, and likelihoods.bin from a numerical example in the paper http://www.cs.cmu.edu/~nasmith/papers/smith.tut04a.pdf which explains posterior decoding. The first two binary files define the initial distribution and the transition matrix of the HMM. The third binary file defines the sequence of likelihoods at each position conditional on each hidden state.
To get the position specific posterior distributions of hidden states, run these three commands:
$ hmm-forward $ hmm-backward $ hmm-posterior
This should create four more binary files in the mydemo directory, including one named posterior.bin which has the distributions of interest. To look at this binary file, use the octal display utility with a format of 8-byte floating point numbers and a width of 24 bytes per row:
$ od --format=f8 --width=24 posterior.bin
Until better documentation is written, information about the usage of the hmmus-associated scripts can be found using commands like this:
$ hmm-backward --help
For now, the only interface to the posterior decoding is through the binary files.
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