Scatter

A maze chase. Eat every dot. Four things want to stop you, and a power pellet turns them round and makes them edible for seven seconds.

Open index.html. No build step, no dependencies, no images.

scatter/
  index.html      markup
  style.css       the frame around the canvas
  js/engine.js    copied unchanged from Overdrive - the eleventh game on it
  js/maze.js      the maze and the four pursuers - no pixels, no engine, no DOM
  js/scatter.js   the playable layer
  test-maze.js    node test-maze.js

Four animals, one function

All four pursuers run exactly the same code. At every tile each one looks at its legal exits, throws away the one it came from, and takes whichever leaves it closest — as the crow flies — to a tile it is aiming at.

That is the whole engine. Everything that makes them feel like four different creatures lives in one line each:

The Flanker is the interesting one: its target depends on where a different ghost is standing, so the two of them close like pincers without either knowing the other exists. The test works one case out by hand — player at (8,5) facing right, Chaser at (2,3), so two-ahead is (8,7), the vector is (+6,+4), and doubling lands on (14,11) — then moves only the Chaser and asserts the Flanker's aim moves with it.

And it works. Two minutes of play, measuring how far each one stays from the player:

Same code, same maze, same player. The Drifter hovers because it is forever changing its mind at the eight-tile line; the other three commit.

$ node test-maze.js
68 passed, 0 failed

Straight-line distance, on purpose

A ghost cannot see round corners. It compares the four tiles next to it and picks the one nearest its target in a straight line, with no regard for walls in between. That is why they can be led into the wrong branch of a junction, and it is the reason the game is beatable at all. Ties break in a fixed order — up, left, down, right — so the same situation always produces the same answer and a run can be reasoned about rather than merely survived.

The test walks every walkable tile in the maze, from every approach direction, against five different targets — over 500 cases — and checks the choice is always the one that gets closest.

None of them ever turns round except on the tick the mode changes from scatter to chase or a pellet is eaten. Those forced reversals are the game's rhythm; they are the only moment a ghost backtracks, so you learn to hear them coming. A 4,000-step simulation asserts zero unauthorised reversals, zero walls entered, and zero ghosts off the grid.

The one exception is a genuine dead end, where turning round is the only legal move — which is tested too, though the shipped maze has none.

The maze has to be winnable

If one dot sits in a pocket the player cannot get to, the game can never be finished, and the only symptom is somebody wandering an apparently empty maze wondering what they missed.

So the test floods out from the player's starting tile and asserts it reaches every single piece of food — and that all four ghosts start in the same connected space.

It also asserts there are no dead ends, and that caught two: a pair of one-tile pockets on the bottom row where a player would be cornered with no escape at all. The maze row above them is now open.

Two bugs worth recording

Everything walked into walls. Both the player and the ghosts advanced to the next tile and only then checked whether the move was legal. When it was not, the code set the progress counter to zero and stopped — but left the blocked direction in place, so the next frame walked straight in. Nearly 300 frames in a wall over forty seconds. Both now refuse to move at all until the heading is legal.

Ghosts started facing walls. They were handed a direction from a fixed list by index, and for some starting tiles that pointed at solid maze. Their opening heading is now taken from the exits that actually exist.

Controls

A direction pressed slightly before a junction is taken the moment it becomes legal. Without that queued turn, cornering demands frame-perfect timing and the game feels unresponsive rather than difficult.

Scoring

10 a dot, 50 a pellet, and ghosts eaten under one pellet are worth 200, 400, 800, 1600. Clearing a maze is 1000 and deals it again. Three lives, best kept in localStorage.

Things deliberately left undone

• No sound.
• One maze. MAZE is seventeen lines of ASCII in maze.js, and the reachability test applies to whatever you put there.
• No ghost house. The four start loose in the maze rather than filing out of a box, which skips the door mechanic entirely.
• The schedule does not tighten with the level. In the arcade the scatter periods shrink to nothing by the later boards; PHASES is a list and would take a multiplier.