Show HN for October 13, 2025

Hello HN, I just released this music theory and ear training mobile app for guitar which I've been working on for a bit more than a year on the side.

The idea was to make something for the eternally "intermediate" guitarist (myself included). There are a lot of beginner apps which rely on learning songs, toolkits which give you a bunch of stuff with no explanation but not many in-between apps to actually learn and practice more generic and somewhat advanced stuff.

The app contains short lessons, recaps and most importantly challenges (visual, audio and pure theory) along with a very complete library.

The challenges are made for practicing, they will get increasingly harder and getting to the max score is supposed to be quite hard. The idea being that you have to repeat them regularly until your brain has integrated the info and it flows naturally rather than being a one time quick dopamine shot. This is partly inspired by how language learning apps work.

It has no ads, a lifetime purchase option and you can use it without an account if you don't care about multi-device sync or backing up your progress.

Android: https://play.google.com/store/apps/details?id=com.apizon.cad...

iOS: https://apps.apple.com/us/app/cadence-guitar-theory/id674701...

(This is my second and last post about this sorry for spam. My first post a few weeks ago didn't get any views and posting on a saturday might not have helped...)

Using the new Prompt API to build a free open source privacy-first chat extension that runs on Chrome or Edge on Linux, Mac, Windows and ChromeOS

I am playing around with using arrays of arbitrary dimension as framework for designing FFT implementations, as opposed to the more classical approach of tensor products and butterflies (too complicated in my opinion).

It turns out, that with a modern compiler, you do not need much complexity to make a really fast implementation. This implementation is for powers of 2, and optimized for arrays that do not fit in cache. I do think it would be better to use a higher-level language to implement other cases (e.g. n = 2^a * 3^b * 5^c, multiple small FFTs, higher-dimensional), so I am currently working on getting the SaC-compiler to generate this code.

I've been studying interval arithmetic for the past few weeks and it's a really interesting field because while there is a ton of super interesting research published over the past decades, it has never really gotten the recognition that it deserves, IMO.

One reason for this is that standard interval arithmetic has really poor handling of division by intervals containing zero. If you compute 1 / [-1, 2] in regular interval arithmetic, you get either [-∞, +∞], or you have to say that the operation is undefined. Both solutions are virtually useless. The real answer of course is [-∞, -1] U [0.5, +∞]: i.e. a union of two disjoint intervals.

This is useful because you can confidently exclude a non empty set of the real numbers ([-1, 0.5]) from the set of possible values that you can get by dividing 1 by a number between -1 and 2.

But this definition of interval division yields a value that is not an interval. This is a problem if you want to define a closed arithmetic system, where you can build and evaluate arbitrary expression over interval values.

(This behavior extends to any non continuous function like tan() for example, which is implemented in my project - not without difficulties!)

Well the obvious solution is to define your arithmetic over disjoint unions of intervals. This is the subject of a 2017 paper called "Interval Unions" by by Schichl, H., Domes, F., Montanher, T. and Kofler, K..

This open-source project I made implements interval union arithmetic in TypeScript in the form of a simple interactive calculator, so you can try it out for yourself! The underlying TypeScript library is dependency free and implements interval union arithmetic over IEEE 754 double precision floats (JS native number type) with outward rounding. This guarantees accuracy of interval results in the presence of rounding issue inherent to floating point.

Transform ChatGPT, Claude, and Gemini text into natural, human-like writing with a single click.

I'm pleased to introduce a new Ruby Gem: YPS: YAML Positioning System. https://github.com/taichi-ishitani/yps

YAML is a popular data serialization format in the Ruby ecosystem. We can easily parse YAML into Ruby objects using the built-in `YAML` standard library. However, it becomes difficult to pinpoint invalid or unexpected values within a large YAML document because the parsed Ruby objects lack position information (i.e., filename, line, and column).

YPS was developed to resolve this exact problem.

YPS parses a given YAML string and adds each parsed Ruby object, except for Hash key, with its position information. You can use `#position` method to get the position info of the receiver object like below:

``` require 'yps'

yaml = YPS.load(<<~'YAML') children: - name: kanta age: 8 - name: kaede age: 3 YAML

# output # name: kanta (filename: unknown line 2 column 11) # age: 8 (filename: unknown line 3 column 10) # name: kaede (filename: unknown line 4 column 11) # age: 3 (filename: unknown line 5 column 10) yaml['children'].each do |child| child.each do |key, value| puts "#{key}: #{value} (#{value.position})" end end ```

I’ve always liked fast online tools for jotting notes or organizing thoughts — but most “productivity” sites either save data to the cloud, require sign-ups, or are full of distractions.

So I built an online productivity tools website, a small collection of browser-based tools. It includes an online notepad where you can write and save notes right in your browser, a cornell notes tool available in more than 20 languages, and more coming soon like a pomodoro timer and a simple blank page for focused writing.

Everything works entirely in-browser using localStorage — no accounts, no cloud sync, no tracking. The site uses only minimal ads (anchor and vignette) to keep it free.

I built it mostly for myself to have a clean, private place to write and study, but I figured others might find it useful too. Feedback, bugs, and suggestions are all welcome.

Hi!

Struggled to find something private and easy to use to track my pet project's money flow and control balance. So I built it by myself. Based on the security best practices for crypto wallets and password managers.

Any feedback is much appreciated.

I needed to take a look at some live logs and quickly analyze some old ones, but I couldn't find anything effective to highlight terms, except for esoteric sed and awk commands. So I built ch - Colored Highlighter - a simple terminal tool to highlight specific words in your command output with colors. Perfect for tailing logs, debugging, and making command output more readable.

Hi, let me introduce the document service I've been developing for about a year.

There are many document hosting services, but few prioritize interaction between developers. I think most documentation services target general readers and aren't designed to be used by developers themselves.

dodo-doc is a documentation hosting service built for myself and fellow software engineers. It supports the following functionalities that other documentation services don't have:

Strong CLI Tool I'm aiming for a documentation service that can be interactively manipulated from the terminal. You can not only upload but also search and open docs directly from the terminal. We also have experimental support for MCP.

CI Affinity Don't you want to preview documents on CI? I do. You can easily set up deploy and preview flows on GitHub Actions.

Private Use Other documentation tools may have this feature too. But it's REQUIRED for me because I write journals in git. Therefore, dodo-doc supports private documents out of the box.

It's easy to set up, so I'd love for you to give it a try. I'm looking forward to your feedback!

I am building QueryDeck.io, a no-code way to turn your Postgres into production-ready REST APIs in minutes — and it now also auto-generates LLM tool definitions and MCP servers from your SQL. You can deploy to our cloud or export a Node.js app to run on your own infra.

Would love some feedback from the community!

After seeing a nice built article about how trustMeIAmANinja did a really good job creating a web radio interface, worth to check out solely. Got interested on the python code and made a spin off using the terminal as interface, showing a simple ASCII map with selected station location and allowing to interactively search for stations by searching countries, cities and station names, allowing to favorite then, or to get a completely random one for exploring!

It made no sense to me that QuantLib makes it so hard to do simple loan portfolio modeling, so I've been working on credkit to fill that gap with something that works the way I think it should work.

Check it out on PyPI: https://pypi.org/project/credkit/

Blog post: https://jt-hill.com/introducing-credkit/

I made a Matrix chat client for my 2001 BMW infotainment system using Kotlin, Jetpack Compose, a Rasbperry Pi, and a custom PCB I designed to plug into the factory wiring harness: https://e39.dev/hmi/Matrix/Matrix.html

I used Compose MPP cross-compiled to a Raspberry Pi (running Raspbian) to make my own Infotainment system from scratch. It works on the 2001-vintage factory BMW display with a retractable screen that has a cassette deck behind it.

I then wrote a Matrix Chat Client for the system. The only input device (no touch screen) is a rotary knob that can be pressed. The knob events are sent via the car's IBUS serial bus, which I integrated into the Compose MPP UI state https://e39.dev/hmi/WidgetFramework/KnobListener.html

I've been working on this project since January 2021, when Compose MPP was at the Milestone 2 release.

The Matrix page has a great video of the system in action on the bench-top car-surrogate I made. The homepage, https://e39.dev/, shows an overview of the whole system, and also has writeups on the other features I made, such as the Open Street Map viewer that I also made from scratch.

I'm building an integration for Notion that allows you to automatically sync data from your SQL database into your Notion databases.

This should be useful for power-users and companies using Notion for their knowledge management.

What it does:

- Works with Postgres, MySQL, SQL Server, and other major databases - You control the data with SQL queries (filter, join, transform however you want) - Scheduled syncs keep Notion updated automatically

Looking for early users. There's a lifetime discount for people who join the waitlist!

Hey HN! I’m Salem, 15, and I made CrossRun — a public domain open standard that aims to make software truly cross-platform, regardless of OS, bitness, or architecture. It’s open and public domain (CC0/Unlicense). I’d love feedback, contributions, and especially critique from systems-level devs.

Take or upload a photo of yourself, then a photo of the outfit, start try-on => watch and share the result, no account required.runs entirely in the browser. Made for thrifters, second-hand lovers, and online shoppers who want to see if something fits before they click. Easy.