Why I'm building an app for a game of tag

There is a film from 2018 called Tag, about a group of grown men who have kept the same game of tag running since they were nine years old. One month a year, no rules except the ones they agreed on, and being it when the month ends is a whole year of shame.

We tried it. It is exactly as good as it sounds.

It is also, it turns out, very hard to actually play.

The group chat is the problem

Everything falls apart in the same three places.

Nobody agrees on what happened. Someone claims a tag from across a parking lot. Someone else says they were nowhere near. There is no referee, only two people who both remember it their way and a chat full of laughing emojis.

Nobody remembers the rules they agreed on. We said no tag-backs. For how long? An hour? Until you tag someone else? We decided it in a voice message in the middle of August and by September nobody could reconstruct it.

Nothing survives the year. Who was it the longest? Who lost last time? Who has never once been tagged? All of it lives in scroll-back nobody is going to read again.

The game deserves better bookkeeping than a chat thread. That is the whole idea.

What the app is

TAG is an iOS app for playing a month-long game of tag, and it does four things a group chat cannot.

It verifies tags with GPS. A tag is only valid if the two phones were actually close. Not "close enough that you can argue about it" — close, with the accuracy of both readings subtracted before the comparison, and every raw number saved so a dispute can be reviewed afterwards.

It knows the rules. No tag-backs, truces, who is allowed to tag whom right now. The app does not warn you and then let you do it anyway. If a rule says no, the tag does not happen.

It counts down. The first screen is always the countdown. Before the season, to the start. During it, to the end, and it gets visually louder in the final week. Being it on the 31st of August at 23:59 is the entire point of the game, so it is the first thing you see.

It keeps score forever. Every stretch of being it is logged with a start and an end. Total time as it, longest single stretch, who lost which year — all of it derived from that one log, exactly, for every year we play.

The part I care most about

The rules do not live in the app. They live in the database.

That sounds like a boring technical detail and it is the most important decision in the project. If the rules live in the app, then a friend with a modified app can tag from another city. If they live in the server, every tag goes through the same check no matter what is asking. The app is a shell. It cannot cheat, because it is not the one deciding.

The same goes for location. Being it does not give you the right to read anyone's coordinates. The server calculates whether someone is nearby and sends you the conclusion — someone is within a kilometre — never the position. That is enforced by the database, not by me remembering to be careful in the app code.

Where this is going

Season one runs August 2027. That is a year from now, and it is deliberate: I would rather build it right than fast, and I want a full month of it working before anyone's dignity is on the line.

I will write up each stage here as I go. What I decided, what I got wrong, and what it looks like on screen.