Browser support and privacy

The solver at /solve/ does all of its work inside your browser. It reads images, edits boards and runs the search on your own device. That keeps your images private, and it means your browser has to support a few modern features.

What your browser needs

WebAssembly. The search engine is Z3, a constraint solver written in C++ and compiled to WebAssembly so it can run in a web page.
Shared memory between threads. Z3 uses several threads, and browsers only allow threads to share memory on pages that are "cross-origin isolated". The solver page asks for this through security headers sent with the page. If the browser opened the page without that isolation, you'll see Solving needs this page to reload with its security headers. You can still check and edit boards. Tap Reload. Until then, Solve puzzle stays disabled with the note Solving needs this page to reload.
DecompressionStream. The solver download is compressed, and the page unpacks it with this browser feature. A browser without it shows This browser cannot unpack the solver. Please update your browser.
IndexedDB for the local library. Without it, the page shows Local storage is unavailable. Use the download buttons instead. Importing and solving still work, and you can keep boards as exported .flow files.

Current versions of Chrome, Edge, Firefox and Safari have all of these, on computers and on phones. As a rough guide, Safari needs version 16.4 or later (iOS and iPadOS 16.4 on an iPhone or iPad), and Firefox needs version 114 or later. If something fails on an older device, updating the browser is the first fix.

Links opened inside other apps, such as a messaging or social app, often open in that app's built-in browser. Some of those don't provide everything above. If the solver won't start there, open the page in Safari or Chrome instead.

Memory and speed on phones

The solver download is about 7.5 MiB, and it unpacks to about 33 MiB. The search then needs working memory of its own, which grows with the size of the board and the number of colors. Compression shrinks the download, not the memory the search uses.

Phones have far less memory to spare than computers. If the browser runs short, it may reload the page, which ends the search and clears a board you haven't saved. To avoid that, close other tabs before solving a large board, save the board first, or solve 15 × 15 and larger boards on a computer. A phone is also slower, so a board that runs out of time there may solve within the limit on a computer.

Images are scaled down before they are read, to 1,600 pixels on the long side. The solver accepts JPEG and PNG files up to 20 MB and 40 megapixels.

What stays on your device

Images. Screenshots and photos are decoded and read in the browser. They are never uploaded. The page's security policy only lets it connect to this site, so it couldn't send them elsewhere even by mistake.
Boards and solutions. The board you edit, and the search for its answer, run in a background worker on your device. The solver download is the same file for every visitor, so fetching it reveals nothing about your puzzle.
Saved puzzles and problem reports. These live in your browser's storage until you delete them or clear the site's data. See Saving and moving puzzles.
Files you export. Exported .flow files and downloaded reports go wherever you save them. They leave your device only if you send them to someone.

You don't need an account, and the solver page doesn't ask for your name or email.

What the hosting provider sees

Cloudflare delivers this site and the solver download. As with any website, serving a page involves network information such as your IP address, details of your browser, the address you requested and the time. Your images and boards aren't part of those requests, because they never leave your device. The solver page also tells your browser not to pass its address on to other sites you go to from it.

Advertising

The solver pages never load advertising. Their security policy only allows scripts from this site itself, so no ad network's code can run there, and nothing on them depends on viewing or clicking an ad. Lesson, help and puzzle pages may show labeled advertising. The privacy page explains how that works and what choices you have.