As my dad is giving a guest lecture about the mathematics behind building society savings (Bausparen) at University of Karlsruhe, he wanted to give his students some tools they can play with easily.

Last evening, we then build the calculator shown in the picture below. It’s a portable web application using Ruby and my new Wee framework. I spent most time on generating the diagram (it’s done using gnuplot). The rest was straightforward, once we had the underlying calculations working.

The total code size is 360 lines of code (including HTML), of which 110 are used for the calculation and for generating the diagram. Lots of lines of code are currently spent in conversion and validation routines (german number format).

You can try it out online here: ntecs.de/wee/ehf/