TL;DR

• We turned year one of The Coder Cafe into a 260-page book.

• Published on Leanpub: DRM-free EPUB/PDF.
  Works on Kindle, Kobo, iPad: read it anywhere.

• Pay what you want, min $4.90.

• Buying a copy is a way to support The Coder Cafe.

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Behind the Book

Today marks the first anniversary of The Coder Cafe newsletter 🥳. To celebrate, I gathered the core concepts we explored this year into one book.

I just published The Coder Cafe Season 1: Timeless Concepts for Software Engineers on Leanpub. If you’re unfamiliar with Leanpub, it’s a platform for DRM-free EPUB/PDF books with pay-what-you-want pricing and free updates. You can set your price (min $4.90) and read it on Kindle, Kobo, iPad, or e-reader/app.

Drawn from the first year of the newsletter, it’s a single, carefully sequenced journey. Read sequentially, or jump to the concept you need. I’ve also included a special bonus with the book: my personal algorithms & data structures Anki deck, the support I mainly used to prepare for the Google SWE interviews.

Buying the book helps support The Coder Cafe into year two and some even more ambitious projects.

Thank you!

Get the book

Last month, we reached a big milestone: more than 1,000 coders have subscribed to The Coder Cafe. Thanks to every one of you for reading it!

I’m also excited to announce that we now have our own Discord server. This will be a space for deeper discussions, networking, and learning together, so feel free to join!

Also, to celebrate this milestone, I’m launching a coding challenge with prizes (check out at the end of the post) for the winners! Let’s jump in.

Coding Challenge

🎁 Prizes

We're giving away five prizes worth approximately $1,000!

One Keychron K3 MAX Ultra slim

The first prize will be a Keychron K3 MAX Ultra slim. It will come with the standard ANSI layout, but the winner can request the ISO layout if preferred.

Keychron keyboards are fantastic! Thank you very much to them for supporting this contest.

An Annual Subscription to The Pragmatic Engineer Newsletter

, the author of the The Pragmatic Engineer—the #1 technology newsletter on Substack—has generously accepted to offer a one-year paid subscription as a prize!

If you haven’t heard of this newsletter yet, I highly recommend checking it out. It’s full of insightful content for engineers and tech leaders. For example, I really enjoyed reading What is Reliability Engineering? and listening to Observability: the present and future, with Charity Majors.

Big thanks to Gergely!

One IntelliJ IDEA Ultimate License

I discussed with JetBrains, and they generously offered a free 1-year personal subscription for IntelliJ IDEA Ultimate! Outside of Google, this is the IDE that I use, and I still consider it the best on the market.

A huge thank you to JetBrains!

🏷️ JetBrains also provided a 25% discount code on all products pack, including IntelliJ IDEA Ultimate, GoLand, CLion, and more. Use code TheCoderCafe during check-out at jetbrains.com/store.

Two Copies of Designing Data-Intensive Applications

O’Reilly kindly agreed to give away two eBook copies of Designing Data-Intensive Applications by . This is, without a doubt, one of the best technical books I’ve ever read, so I’m thrilled to share it with the community1.

Huge thanks to O’Reilly as well!

🧩 Problem Description

Skylink, a futuristic airborne logistics network, is tasked with optimizing its passenger transit routes between floating cities. Each floating city has a limited landing bay capacity, restricting the number of passengers it can receive per day.

You are given:

• A set of transmissions that describe relays between cities, each with a maximum allowed quota per day (capacity limit).

• A primary hub city where all transmissions originate.

• A set of final arrival points where passengers need to be delivered.

Your task is to compute the maximum number of passengers that can be transported per day from the primary hub to all final arrival points, ensuring that no relay exceeds its quota.

Input Format

The input consists of an unstructured log file, where each transmission is recorded as follows:

[LOG ID] TRANSMISSION: NODE <from> RELAYS <to> UNDER QUOTA <capacity>

Additionally, there will be a hub city alert indicating the main starting point of transmissions:

[LOG ID] ALERT: PRIMARY NODE IS <hub>

And a final arrival points notification listing the required destination cities:

[LOG ID] CRITICAL: FINAL ARRIVAL POINTS ARE <arrival1>, <arrival2>, ...

Example input:

[10001] TRANSMISSION: NODE A0 RELAYS A1 UNDER QUOTA 10  
[10002] TRANSMISSION: NODE A0 RELAYS A2 UNDER QUOTA 5  
[10003] ALERT: PRIMARY NODE IS A0  
[10004] CRITICAL: FINAL ARRIVAL POINTS ARE A1, A2  

In this example:

• There are two transmission routes between cities:

  • A0 → A1 (max 10 passengers)

  • A0 → A2 (max 5 passengers)

• The primary hub is A0.

• The final destinations are A1 and A2.

For this input, the expected output is 15 (10 + 5).

(Hub)    10
  A0 ---------> A1 (Destination)
  |
  | 5
  |
  v
  A2 (Destination)

Constraints

• There will always be at least one valid path from the hub to a destination.

• No duplicate routes between the same two cities.

• Capacities are always positive integers.

Input File

Download the input file here.

🏆 Contest Rules

• You have 7 days to solve the challenge.

• This is not a speed contest—five winners will be randomly selected (one per prize).

• Submit your solution at contact [at] thecoder [dot] cafe with the email you used to subscribe to The Coder Cafe and your country of residence (some prizes can only be sent to some specific countries). While not required, feel free to send me the GitHub repo containing your solution as well if you’d like.

• While I can’t enforce it, please be a gentleman and do not copy and paste the entire problem into an LLM.

💬 Good luck to everyone participating! See you in one week to discuss one approach to solve the problem and announce the winners.