Wishful Coding

My first project at Hacker School was to write a compiler for Forth for the LEGO NXT. There are a few real programming languages for the NXT, but before I found Mirah, I never really enjoyed them.

Initially, I wanted to write a Lisp, but the NXT bytecode is so static that even the concept of a cons cell is not viable.

Forth is a very simple stack based language that uses just that, a stack and subroutines(called words). Surpisingly, the only dynamic feature in the NXT are resizable arrays. I think it’s a good fit.

This is the complete stack implementation in NBC.

#define push(stack, val) \
  replace stack.data stack.data stack.offset val \
  add stack.offset stack.offset 1

#define pop(stack, val) \
  sub stack.offset stack.offset 1 \
  index val stack.data stack.offset

My implementation follows the Boostrapping a Forth in 40 lines of Lua code approach of defining a Forth word that evals the host language.

The only difference is that this Forth is compiled, so there is a word for Clojure and one for NBC.

The main Clojure file just defines an atom to store Forth words, and defines the clj word, which evaluates a Clojure expression.

As soon as that is in place, you’re in Forth land.

First thing we do in Forth is some more Clojure defining “:*” to mean “define clojure word”, and then using “:*” to define 3 more words.

• ”:” start of a Forth word
• ”;” end of a Forth word
• “nbc” write a line of assembly

What follows are a few forth words defined in assembly. At this point, the following works

: square
  dup *;

2 square dot

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I did this project together with Sarah, the goal is to suggest good chord progressions to musicians.

We both know some music theory, so we started out by reading about what makes a good chord progression. The longer you look at it, the more complicated it gets. I quickly concluded that music is art, not science, so you can do whatever you want.

Instead, we wrote a scraper for guitar tabs. Just download a ton of them and look for anything that looks roughly like

([A-Ga-g][b#]?)(m)?((?:maj)?[0-9])?(sus[0-9]|add[0-9])?(/[A-Ga-g][b#]?)

We then try to figure out which key the original song was in, and convert the chords to intervals, called universal notation.

Now that we have these sequences of legit chord progressions, we need to persist them in a smart way.

We went through markov chains, tries and some other crazyness, but fnally settled on just a flat list. It turns out just scanning the whole list for the given pattern takes an acceptable amount of time.

The final step was to write a gui that could suggest chords for you. It’s written in Tkinter and uses Mingus for some playing and parsing.

Get my new pop hit

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First post from the US!

I can’t really do nice photography, video and building instructions here, but I did make a robot.

This thing sits under your laptop, and can drive it around and tilt the screen. The idea is that it will always face you, and keep a fixed distance too.

It uses the webcam of my laptop to track my face using OpenCV from Python.

It could also keep a fixed distance using the ultrasonic sensor, but this is not implemented.