The kind of software you would normally treat as a black box — databases, compilers, codecs — written in bite-size pieces. Each lesson is one focused twenty-minute session with a concrete spec to hit, and the pieces add up to something that actually runs.
No streaks. No badges. 100% free.
// evaluate both sides, then the operator l, r := Eval(n.Left), Eval(n.Right) switch n.Op { case "+": return l + r case "*": return l * r }
Crafting Interpreters, by Robert Nystrom.
Build an in-memory search index you add documents to and query, with your own inverted index and ranking.
Take source text all the way to a working interpreter: tokenize, parse into a syntax tree, then evaluate it into real results.
Emulate the CPU, memory map, and pixels of real handheld hardware.
Read a line, run a program, and wire up pipes the way your terminal does.
Speak HTTP over a raw socket: parse requests, serve files, handle many clients.
Grow a regex library from a thirty-line matcher into a Thompson NFA that matches in linear time.