Courier maze

hard 60 min Python Meta AI-enabled coding-style

1 Through the wall
2 Build a feature
3 Make it fast

repo

courier/
tests/
visible/

read-only

# courier

Routes a delivery robot through a warehouse map. Maps are text, one row per
line, all rows the same width:

    #  wall               .  floor
    S  the robot's start  X  the delivery point
    a-f  keys             A-F  doors (door A needs key a)

The robot moves one cell up, down, left or right per step. It can't leave the
map or walk through walls, and it can only pass a door once it holds that
door's key (keys are picked up by stepping on them and kept).

    courier/grid.py     Grid: parses a map, finds S and X, neighbours of a cell
    courier/courier.py  delivery_steps(text): fewest steps from S to X, or -1
    courier/keys.py     route_with_keys(grid): the search for maps with keys

Run the tests with `python -m pytest` (or press Run in the browser).

Phase 1 of 3 0:00

Through the wall

Ticket #5310 · priority: high · from: warehouse ops

At the Dock site the app told the robot its delivery was 1 step away. The robot then drove into the outer wall. Its route says "go up" from the start cell, which is in the top row. The real route is 18 steps. Other sites look fine, but we're not sure.

The map is DOCK in tests/maps.py; the router is delivery_steps() in courier/courier.py.

tests/visible/test_routes.py reproduces it. Make it pass.
On Submit, hidden tests try every edge and corner, narrow maps, maps with keys, and hundreds of random maps checked against an independent router.

Bring your own AI: paste the copied repo into Claude, ChatGPT or Gemini. In a real interview the assistant usually reads the files itself. Or download the zip, work in your own editor, and upload the files you changed.