Caterpillar Challenge — Solution

My solution to the Sandbox Caterpillar take-home challenge written in Typescript. It pulls down carpool data, rebuilds the groups, computes the averages and then POSTs result back.

Running it

Need Node 18 or later

npm install
npm test                          # runs against the sample data from the spec
npm start -- "<unique-url>"         # GET, solve, POST

Project layout

src/
  types.ts         Type definitions for input, output, and intermediate shapes
  solve.ts         Pure functions: grid lookup, group building, averaging,
                   Manhattan distance, sorting
  solve.test.ts    Lightweight test runner 
  index.ts         I/O entry point: GET → solve → POST

Design decisions I made

Math.floor vs Math.round. The spec says decimals get "rounded down" which is Math.floor and not Math.round. Using the wrong one would lead to silently disagree on certain inputs.

Grid coordinates. The spec defines x/y with origin at top-left, so a 2D array indexed as grid[y][x] (row first, then column).

Using a Set inside buildGroups. The spec promises each rider only shows up once, but I used Map<driverId, Set<riderId>> instead of Map<driverId, riderId[]> anyway. If a duplicate ever did sneak in, an array would double-count the rider in the average. The Set just doesn't.

Throw for missing ids findUserPosition throws if it can't find an id, instead of returning null or (0, 0) instead of returning null or (0, 0). Spec says every non--1 id is a real user, so if it's missing, the input's broken and I'd rather crash than quietly average in a fake position.

Testing

npm test runs against the exact sample data from the spec and checks:

• Grid lookups return proper (x, y)
• Manhattan distance is computed correctly
• buildGroups excludes rejected requests and groups by driver correctly
• averageCoord floors 5.25 to 5
• Full solve output matches the expected output
• The result is sorted ascending by Manhattan distance between the average pickup and average dropoff.

All 11 assertions pass.