Explaining Code using ASCII Art

People tend to be visual: we use pictures to understand problems. Mainstream programming languages, on the other hand, operate in an almost completely different kind of abstract space, leaving a big gap between programs and pictures. This piece is about pictures drawn using a text character set and then embedded in source code. I love these! The other day I asked around on Twitter for more examples and the responses far exceeded expectations (thanks everyone!). There are a ton of great examples in the thread; here I’ve categorized a few of them. Click on images go to the repositories.

Data Structures

One of the most common kinds of ASCII art in code is illustrating the shape of a data structure.

The example I started with comes from LLVM:

The layout of a data structure in the Jikes RVM:

A tree rotate in Musl:

Double-ended queue from the Rust library:

Swift compiler internals:

Malloc header layout:

State Machines

JavaScript profiling:

RPCs in Cloud Spanner:

I/O stream states:

Logical Structure in the Problem Domain

Control flow in a JavaScript program being decompiled:

ECC internals:

Formatting numbers:

A quantum circuit:

Balancing memory management objectives in an OS kernel:

Subtyping relations (this is a very cool special case where the ASCII art is also code):

The format of a DBF file:

A lookup table for image processing:

Shape of a color function:

Structure of a URI:

A very quick tutorial on undo systems from emacs:


Attitude control in the Apollo Guidance Computer (!!!):

Image tiling:

Boomerang trajectories in Nethack:

Rendering CSS borders:


Speed control in a milling machine:

Scrolling web pages:

I hope you enjoyed these as much as I did!