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Matlab to Python converter

Project description

``SMOP`` is Small Matlab and Octave to Python compiler.
``SMOP`` translates matlab to python. Despite obvious
similarities between matlab and numeric python, there
are enough differences to make manual translation
infeasible in real life. ``SMOP`` generates
human-readable python, which also appears to be faster
than octave. How much faster? Timing results for the
"working example" below are summarized in Table 1, and
additional speedup is achieved by compiling ``SMOP``
run-time library ``runtime.py`` to C, using `cython`.

+-------------------------------------+--------+
| octave-3.8.1 | 190 ms |
+-------------------------------------+--------+
| smop+python-2.7 | 80 ms |
+-------------------------------------+--------+
| smop+cython-0.20.1 | 40 ms |
+-------------------------------------+--------+
| Table 1. ``SMOP`` performance, measured on |
| fujitsu AH552 running linux 3.8.0-19 |
+----------------------------------------------+

There is a price, too. The generated sources are
`matlabic`, rather than `pythonic`, which means that
library maintainers must be fluent in both languages,
and the old development environment must be kept around.

With less than five thousands lines of python code
``SMOP`` does not pretend to compete with such polished
products as matlab or octave. Yet, it is not a toy.
There is an attempt to follow the original matlab
semantics as close as possible. Matlab language
definition (never published afaik) is full of dark
corners, and ``SMOP`` tries to follow matlab as
precisely as possible.

Should the generated program be `pythonic` or `matlabic`?
For example should array indexing start with zero
(`pythonic`) or with one (`matlabic`)?

I beleive now that some matlabic accent is unavoidable
in the generated python sources. Imagine matlab program
is using regular expressions, matlab style. We are not
going to translate them to python style, and that code
will remain forever as a reminder of the program's
matlab origin.

Another example. Matlab code opens a file; fopen
returns -1 on error. Pythonic code would raise
exception, but we are not going to do `that`. Instead,
we will live with the accent, and smop takes this to the
extreme --- the matlab program remains mostly unchanged.

It turns out that generating `matlabic`` allows for
moving much of the project complexity out of the
compiler (which is already complicated enough) and into
the runtime library, where there is almost no
interaction between the library parts.

.. missing standard library and toolboxes
.. missing grapphics library

Working example: ``solver.m``
We will translate ``solver.m`` to present a sample of
smop features. The program was borrowed from the
matlab programming competition in 2004 (Moving
Furniture). For the impatient, it is possible to compile
and run the example without installing smop::

$ tar zxvf smop-0.25.4.tar.gz
$ cd smop-0.25.4/smop
$ python main.py solver.m
$ python go.py

To the left is ``solver.m``. To the right is ``a.py`` --- its
translation to python. Though only 30 lines long, this
example shows many of the complexities of converting matlab code
to python.

.. code:: matlab

01 function mv = solver(ai,af,w) 01 def solver_(ai,af,w,nargout=1):
02 nBlocks = max(ai(:)); 02 nBlocks=max_(ai[:])
03 [m,n] = size(ai); 03 m,n=size_(ai,nargout=2)

==== ========================================================
02 Matlab uses round brackets both for array indexing and
for function calls. To figure out which is which,
SMOP computes local use-def information, and then
applies the following rule: undefined names are
functions, while defined are arrays.
---- --------------------------------------------------------
03 Matlab function ``size`` returns variable number of
return values, which corresponds to returning a tuple
in python. Since python functions are unaware of the
expected number of return values, their number must be
explicitly passed in ``nargout``.
==== ========================================================

.. code:: matlab

04 I = [0 1 0 -1]; 04 I=matlabarray([0,1,0,- 1])
05 J = [1 0 -1 0]; 05 J=matlabarray([1,0,- 1,0])
06 a = ai; 06 a=copy_(ai)
07 mv = []; 07 mv=matlabarray([])

==== ========================================================
04 Matlab array indexing starts with one; python indexing
starts with zero. New class ``matlabarray`` derives from
``ndarray``, but exposes matlab array behaviour. For
example, ``matlabarray`` instances always have at least
two dimensions -- the shape of ``I`` and ``J`` is [1 4].
---- --------------------------------------------------------
06 Matlab array assignment implies copying; python
assignment implies data sharing. We use explicit copy
here.
---- --------------------------------------------------------
07 Empty ``matlabarray`` object is created, and then
extended at line 28. Extending arrays by
out-of-bounds assignment is deprecated in matlab, but
is widely used never the less. Python ``ndarray``
can't be resized except in some special cases.
Instances of ``matlabarray`` can be resized except
where it is too expensive.
==== ========================================================

.. code:: matlab

08 while ~isequal(af,a) 08 while not isequal_(af,a):
09 bid = ceil(rand*nBlocks); 09 bid=ceil_(rand_() * nBlocks)
10 [i,j] = find(a==bid); 10 i,j=find_(a == bid,nargout=2)
11 r = ceil(rand*4); 11 r=ceil_(rand_() * 4)
12 ni = i + I(r); 12 ni=i + I[r]
13 nj = j + J(r); 13 nj=j + J[r]

==== ========================================================
09 Matlab functions of zero arguments, such as
``rand``, can be used without parentheses. In python,
parentheses are required. To detect such cases, used
but undefined variables are assumed to be functions.
---- --------------------------------------------------------
10 The expected number of return values from the matlab
function ``find`` is explicitly passed in ``nargout``.
---- --------------------------------------------------------
12 Variables I and J contain instances of the new class
``matlabarray``, which among other features uses one
based array indexing.
==== ========================================================

.. code:: matlab

14 if (ni<1) || (ni>m) || 14 if (ni < 1) or (ni > m) or
(nj<1) || (nj>n) (nj < 1) or (nj > n):
15 continue 15 continue
16 end 16
17 if a(ni,nj)>0 17 if a[ni,nj] > 0:
18 continue 18 continue
19 end 19
20 [ti,tj] = find(af==bid); 20 ti,tj=find_(af == bid,nargout=2)
21 d = (ti-i)^2 + (tj-j)^2; 21 d=(ti - i) ** 2 + (tj - j) ** 2
22 dn = (ti-ni)^2 + (tj-nj)^2; 22 dn=(ti - ni) ** 2 + (tj - nj) ** 2
23 if (d<dn) && (rand>0.05) 23 if (d < dn) and (rand_() > 0.05):
24 continue 24 continue
25 end 25
26 a(ni,nj) = bid; 26 a[ni,nj]=bid
27 a(i,j) = 0; 27 a[i,j]=0
28 mv(end+1,[1 2]) = [bid r]; 28 mv[mv.shape[0] + 1,[1,2]]=[bid,r]
29 end 29
30 30 return mv

Which one is faster --- python or octave? I don't know.
Doing reliable performance measurements is notoriously
hard, and is of low priority for me now. Instead, I wrote
a simple driver ``go.m`` and ``go.py`` and rewrote `rand`
so that python and octave versions run the same code.
Then I ran the above example on my laptop. The results
are twice as fast for the python version. What does it
mean? Probably nothing. YMMV.

.. code:: matlab

ai = zeros(10,10);
af = ai;

ai(1,1)=2;
ai(2,2)=3;
ai(3,3)=4;
ai(4,4)=5;
ai(5,5)=1;

af(9,9)=1;
af(8,8)=2;
af(7,7)=3;
af(6,6)=4;
af(10,10)=5;

tic;
mv = solver(ai,af,0);
toc
---------------------------------------------------------------------

Work in progress below this line
================================


Running the test suite::

$ cd smop
$ make check

Command-line options
--------------------

.. code:: sh

lei@dilbert ~/smop-github/smop $ python main.py -h
SMOP compiler version 0.25.1
Usage: smop [options] file-list
Options:
-V --version
-X --exclude=FILES Ignore files listed in comma-separated list FILES
-d --dot=REGEX For functions whose names match REGEX, save debugging
information in "dot" format (see www.graphviz.org).
You need an installation of graphviz to use --dot
option. Use "dot" utility to create a pdf file.
For example:
$ python main.py fastsolver.m -d "solver|cbest"
$ dot -Tpdf -o resolve_solver.pdf resolve_solver.dot
-h --help
-o --output=FILENAME By default create file named a.py
-o- --output=- Use standard output
-s --strict Stop on the first error
-v --verbose

---------------------------------------------------------------------


+-----------------------------------------+-------+-------+-------+
| |matlab |fortran|python |
+=========================================+=======+=======+=======+
| | | | |
| A. Base-one indexing | yes | yes | no |
+-----------------------------------------+-------+-------+-------+
| | | | |
| B. Columns-first data layout | yes | yes | no |
+-----------------------------------------+-------+-------+-------+
| C. Auto-expanding arrays | yes | no * | yes |
+-----------------------------------------+-------+-------+-------+
| D. Update to create | yes | no * | yes |
+-----------------------------------------+-------+-------+-------+
| E. Assignment as copy | yes | yes | no |
+-----------------------------------------+-------+-------+-------+


+-----------------------------------------+-------+-------+-------+
| |matlab |fortran|python |
+=========================================+=======+=======+=======+
| F. Matrices everywhere | yes | no | no |
+-----------------------------------------+-------+-------+-------+
| G. Single subscript implies ravel | yes | | |
+-----------------------------------------+-------+-------+-------+
| H. Broadcast | | | |
+-----------------------------------------+-------+-------+-------+
| I. Boolean indexing | | | |
+-----------------------------------------+-------+-------+-------+
| J. Type and rank must be known | no | yes | no |
| in compile time | | | |
+-----------------------------------------+-------+-------+-------+

+-----------------------------------------+-------+-------+-------+
| |matlab |fortran|python |
+=========================================+=======+=======+=======+
| K. Garbage collection | yes | no * | yes |
+-----------------------------------------+-------+-------+-------+
| L. All uppercase | no | yes | no |
+-----------------------------------------+-------+-------+-------+
| M. Structs | | | |
+-----------------------------------------+-------+-------+-------+
| N. Interpreted | yes | no | yes |
+-----------------------------------------+-------+-------+-------+
| P. Strings are arrays of chars | yes | no | yes |
+-----------------------------------------+-------+-------+-------+


Base-one indexing
Following fortran tradition, matlab starts array indexing with one,
not zero. Correspondingly, the last element of a N-element array is
N, not N-1.

C_CONTIGUOUS and F_CONTIGUOUS data layout
Matlab matrix elements are ordered in columns-first, aka
F_CONTIGUOUS order. Numpy arrays are C_CONTIGUOUS by default, with
some support for F_CONTIGUOUS arrays. Instances of matlabarray are
F_CONTIGUOUS except if created empty, in which case they are
C_CONTIGUOUS.

Auto-expanding arrays
Matlab arrays are auto-magically resized on out-of-bounds update.
Though deprecated, this feature is widely used in legacy code.
Supporting this feature is one of the main reasons behind creation
of the dedicated ``matlabarray`` class. If we chose the `pythonic`
option --- smop arrays directly mapped to ndarrays --- any array
update that could not be proven to be safe, should have been
enclosed in try-except-resize-retry. It would not look any better.

In fortran, the pattern should be somehow (how exactly?) detected in
compile-time. In python ``__setitem__`` hides ``try-catch``, with
``resize`` called inside ``catch``. Is try-catch in fortran?

In numpy out-of-bounds assignment is an error. In smop,
out-of-bounds assignment is supported for row and column matrices
and their generalizations having shape

[1 1 ... N ... 1]

These arrays may be resized along their only non-singular dimension.
For other matrices, new columns can be added to F_CONTIGUOUS arrays,
and new rows can be added to C_CONTIGUOUS arrays.

Finally, scalar array of any dimension, having shape

[1 1 ... 1]

can be resized along any dimension.

Update to create
In matlab, arrays may be created by updating a non existent array,
as in the example::

>>> clear a
>>> a(17)=42

This unique feature is not supported by smop, but can be worked
around by inserting assignments into the original matlab code::

>>> a=[]
>>> a(17_=42

-------------------------------------

SMOP assumes that the input is syntactically correct and
passes some test suite.

.. code:: matlab

01 ok = 0 01 def solver_(c):
02 if c 02 if c:
03 ok = f00 03 ok = f00()

.. code:: matlab



.. vim: tw=60

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