Three taps at the gate, and a price the phone isn't allowed to decide.

A parking contractor's day is a stream of vehicles in and out, each needing a time, a type, a price and a slip, with a queue forming behind whoever is at the gate. On paper that's a register book and a calculator. In a generic app it's a form with too many fields. The owner's questions at night are simple: how many vehicles, how much money, and does it match the drawer.

The brief was a phone app where each step at the gate is as few taps as it can be, where the attendant never does sums, and where the owner's total comes from the same records the attendants made.

Money in a parking lot leaks at the point where an amount is decided, so the phone isn't allowed to decide it. Each category has a price, a number of included hours and an extra charge. When a vehicle is marked out, the server reads the entry time and the category's rates, takes the current time from its own clock, and works out the amount: the base price, plus the extra charge for every started hour beyond the included ones. One hour and ten minutes over is two extra hours, by design.

The vehicles list uses the same function, so the amount shown on a parked vehicle is a live figure that grows with the clock, and what the attendant sees before tapping Mark Out is what will be charged. The function takes plain numbers and returns plain numbers, which makes it the easiest part of the whole system to test, and I'd add a test suite there first.

Typing a plate with a queue behind you is where mistakes happen, so the entry form has a Scan Number Plate button. It opens the camera, takes a photo and runs Google's on-device text recognition, so nothing leaves the phone. The result is then cleaned up and matched against a plate-shaped pattern (two letters, one or two digits, up to three letters, three or four digits) and put into the form for the attendant to confirm.

The clean-up has a flaw I found reading the code back for this page. To fix common recognition mistakes it replaces O with 0, I with 1, Z with 2, S with 5 and B with 8 across the whole string, before matching. That helps in the digits and hurts in the state code: a plate that begins with letters like BR, OD or TS has its letters turned into digits and no longer matches. The fix is to apply those swaps only to the positions that must be digits.

For this page I wanted real screenshots. The Android toolchain isn't installed on my computer at the moment, so the app can't be built or run. Instead I read the screens and drew them: the colours, the labels, the tabs for each role, the buttons, the empty states and what each list row shows all come from the source. The data is invented and exact spacing is approximate, and the page says so next to every picture.

Reading the code that carefully is also how I found the plate-scanner flaw and noticed that the receipt numbers are chosen as "the last number plus one" without a database lock. Two entries at the same instant could collide. That's a rare case at a single gate and worth fixing before a lot with several.

The app does what a parking gate needs: log, price, print and total, with a login per attendant and a way for the owner to turn parts of it off. The pricing is the part I'd trust most, because it lives in one small function on the server.

What it isn't: tested by a suite, offline-capable, or finished. There's a plate-scanner clean-up to fix, a receipt-number lock to add, tests to write around the pricing function, and an Android build to set up again so I can take real screenshots.