sdinthrough

Writing code and getting immediate feedback about its correctness is satisfying, but most LeetCode-style programming challenges grade small snippets of code, so you can't practice writing large programs with them. At the other end of the spectrum are the long tutorials that teach you a complex concept, but they don't give you immediate feedback; they expect you to self-evaluate whether you are on the right path.

sdinthrough is a long-form coding challenge with 600+ testcases where you build a large program while receiving instant feedback. Each testcase shows you exactly what your program should be printing next, and your job is to write only the small missing piece that gets you there, a bit like following a LEGO manual, and definitely not like this:

sdinthrough's interface is a simple binary running on your machine, constantly reevaluating your solution. Hit save, get verdict. No cloud, no account, no network once installed.

The challenge is long enough to take weeks to complete, and by the end you'll have built a tokenizer, a parser, and JSON-conversion utilities for @/sdin, a config language I designed for this challenge. You practice data structure design, recursion, AST manipulation, and, most importantly, refactoring. Some requirements have surprising depth to them, and once tested for, they can topple your data structures and algorithms; can you foresee the traps you might be coding yourself into? Each step is small, but the complexity keeps growing, and eventually tech debt will bite you if you tread hastily.

(Can AI easily solve this? Yes, and a forklift can lift your barbell too. If you struggle to avoid AI, then I recommend pair programming with a friend, both committing to avoid AI. Parsing code is bug-heavy, so this is a debugging-heavy challenge. Hunting a hideous bug together with caveman tools is a friendship-building experience.)

Your solution can be in any programming language that reads stdin and writes stdout. Choose your poison carefully.

import sys

for line in sys.stdin:
    parts = line.rstrip().split(' ', 2)
    if len(parts) < 3: break
    sys.stderr.write('==NextNotes==\n')
    sys.stdout.write('==NextAnswer==\n')
    input = bytes.fromhex(parts[2][5:]).decode()
    sys.stdout.write(input)

== unescape.SingleWord (1/635)
auto mode: shows last emitted or first incorrect result, press h for help




-> fail  unescape.SingleWord [start]
   fail  unescape.TwoWords
   fail  unescape.SingleTab
   fail  unescape.Newline1
   ... (631 testcases)

== intro
Welcome to the sdinthrough challenge!
Read https://github.com/ypsu/sdinthrough/blob/main/README.md for instructions.

== input
"hello"
== want.answer
~ hello
== got.answer
~ "hello"
== verdict
###########
## WRONG ##
###########

import sys

for line in sys.stdin:
    parts = line.rstrip().split(' ', 2)
    if len(parts) < 3: break
    sys.stderr.write('==NextNotes==\n')
    sys.stdout.write('==NextAnswer==\n')
    input = bytes.fromhex(parts[2][5:]).decode()
    sys.stdout.write(input[1:len(input)-1])

== unescape.SingleTab (3/635) (+2 since startup)
auto mode: shows last emitted or first incorrect result, press h for help


     ok  unescape.SingleWord [start]
     ok  unescape.TwoWords [best]
-> fail  unescape.SingleTab
   fail  unescape.Newline1
   fail  unescape.Newline2
   fail  unescape.Newline3
   ... (629 testcases)

== input
"hello\tworld"
== want.answer
~ hello	world
== got.answer
~ hello\tworld
== verdict
###########
## WRONG ##
###########
Progressed 2 testcases.

As a quick taste, here's a simplified online demo of the first two tasks. In this demo, you need to solve the tasks in JavaScript. Enter the source code directly into the textbox below and press the judge button. For instance, change return input to return input.slice(1, -1) and hit Shift-Enter. Note that the first task is just a warm-up to get the hang of the exercise; then comes the tokenizer task. Check unescape.md and tokenize.md for the requirements.

Want more challenge, better feedback, or the ability to use a different language? Then run the full challenge from https://github.com/ypsu/sdinthrough (the judge requires Go):

I built this slowly over one and a half years and wrote a small retrospective about it at @/sdinretro. If you give it a go, let me know at https://github.com/ypsu/sdinthrough/discussions how far you got and which parts ended up being too hard or too boring. Also let me know if you end up making a tutorial or a video about solving it, and I may feature it in the project's README for others to learn from.

sdinthrough: build a parser, one failing test at a time