Atom Outer Product
Pat Hickey
24 Oct 2010

I wrote this code[1] to play with Tom Hawkins's[2] Atom[3,4], an embedded Haskell DSL. This is a simple example project which generates C code to calculate the outer product of a vector with itself; that is, a^T * a, which multiplies each element of a by each element of a. Presently the code is implemented for a length 4 input.

I have implemented this simple algorithm in two ways: 
1. 'outerprod', which iterates across the input and performs the calculation using atom expressions
2. 'betterop', which iterates across the input at using Haskell expressions (that is, at compile time), and performs the calculation using atom expressions.
    
I've provided two C wrappers for the compiled Atom code:
1.  funcTestPrePostCode, which performs an outer product of a vector given as an argument to the compiled C program. I used this to verify the code works.
2.  benchTestPrePostCode, which performs a rudimentary execution time test. It demonstrates that betterop is several times faster than outerprod, as you would expect. 

I've also included a third implementation of the algorithm, 'op_triangle', which was an attempt to use an imperative style intermediate variable to store the result of the input multiplication, and assign that value to multiple outputs. This code turns out to be impossible to write with the Atom language, because assignments (<==) are carried out at the end of rule execution, so they cannot cause a side effect during rule execution.

LICENSE
You may do whatever you like with this code.

[1] http://github.com/pchickey/atom-outer-product
[2] http://tomahawkins.org
[3] http://github.com/tomahawkins/atom
[4] http://hackage.haskell.org/package/atom-1.0.7