NumFlow, in the open.

A 2-minute daily number-pattern puzzle. One grid, one path, one set of checkpoints to hit in order. Same puzzle for every player today. Resets midnight UTC.

Play today's NumFlow · iOS app pending Apple review.

The mechanic

NumFlow puts a small grid in front of you (5×5 by default) with numbered checkpoints. Your job: trace a continuous path that visits every checkpoint in numeric order, never crossing itself. The path can twist, double back through unused cells, take detours. What it can't do is skip a checkpoint or land twice on the same cell.

It's a one-rule puzzle. There's no learning curve past the first solve. The texture comes from grid size + checkpoint count: a 5×5 with 4 checkpoints is a coffee-break solve; a 7×7 with 6 is a 5-minute think.

Why this mechanic

Three things made the path-tracing rule the right wedge for a daily puzzle:

Touch-native input. Drag a finger across cells. No keyboard, no menu picking. Works equally well on iPhone and on the web.
Deterministic generation. A single seed (today's date) produces today's puzzle. Every player solves the same one. The share-string at the end is meaningful because we're all looking at the same grid.
One success state. You either visit every checkpoint in order or you don't. No partial credit, no stat-padding. The puzzle has a clear stopping point.

How the puzzle is generated

NumFlow uses Warnsdorff's heuristic — a path-construction rule originally designed for the knight's tour problem. The algorithm:

1. Pick a random start cell (seeded by today's date).
2. Look at the unvisited 4-neighbors.
3. Sort by the count of THEIR unvisited neighbors.
4. Move to the lowest-degree neighbor.
   Tie-break with seeded RNG.
5. Stop when no unvisited neighbors exist.
6. Run 30 attempts; keep the longest path.

The result: a path that covers 90%+ of the grid. The checkpoints are then sampled along the path at equal intervals. The puzzle is solvable by construction (the generator already found a valid solution), but the path you trace doesn't have to match the generator's path — there are usually multiple valid solutions.

Why we publish this: open mechanics build trust. The puzzle isn't a black box; you can verify it's solvable, and if you find a generation bug, file an issue.

The daily seed

The seed is the number of days since 2026-01-01 UTC. So 2026-04-28 = day 118. Mulberry32 (a small, fast PRNG) takes that integer and produces a deterministic stream of random numbers used by the path generator.

Time-zone implication: the puzzle resets at 00:00 UTC. If you're in IDT, that's 03:00 local. If you're in California, that's 17:00 the day before. We picked UTC so the share-string day numbers match across all of Twitter, regardless of which time zone the poster is in.

The share-string

After you solve, NumFlow shows an emoji grid:

NumFlow #047
3:12 · 18 moves · ⭐⭐⭐
🟧🟧🟧⬜⬜
⬜🟧🟧🟧⬜
⬜⬜🟧🟧🟧
numflow.buildinthesun.com

🟧 cells are part of your path; ⬜ cells are unused. The grid leaks no answers — only the shape of how you solved. This is the Wordle pattern, deliberately. Sharing the result is the entire growth loop, so the share has to be spoiler-safe and easy to paste into iMessage, Twitter, Discord.

Stars, time, moves

The completion screen scores you on path efficiency (moves) and time. Stars reflect efficiency relative to grid size: 3 stars if your path is at most 1.2× as long as the cell count, 2 stars if 1.8×, 1 otherwise. Time is informational, not graded.

We don't show a percentile by default in v1. NumFlow v2 (in development) adds "you beat 73% of today's players" once enough daily-challenge results land in the database to make the comparison meaningful.

What NumFlow doesn't do

No accounts. Streaks live in localStorage on web and the keychain on iOS. Hub email reminders are optional and separate from NumFlow progress.
No ads on the daily-challenge screen during launch month.
No microtransactions. There's a free hint counter; if you run out, watch a single rewarded video to refill (only on the iOS app, only outside daily-challenge mode).
No multiplayer, no leaderboard, no friends-list.

Where this fits in Puzzle Hub

NumFlow is the wedge. Puzzle Hub is the home base — a small family of daily puzzles (today: NumFlow, Daily Cipher, Word Chain) sharing one streak across them all. As more games ship, the streak compounds.

The thesis: a small, indie-built family of daily puzzles can outlast hyper-casual mobile games because the daily-puzzle structure is genuinely retentive without dark patterns. NumFlow is the proof point. Today's roster is the reading list.