33. Astrosmash

Status: built (games/astrosmash.xml, tests/test_astrosmash.cpp); no new verbs

Why this game

Astrosmash (Intellivision, 1981) is what the two most recent games would make if crossed: Kaboom’s falling pieces, with their <random> speeds and heights, and Space Invaders’ gun. It was picked to see whether a fixed set of pieces that fall, are shot, and fall again is enough for a game whose whole point is shooting them.

The game as described

An 800 by 600 window. Eight rocks fall down fixed columns: four slow boulders and four fast pebbles, each with its own speed and its own starting height above the screen, drawn once when the game loads. A ship slides along the bottom (A and D, or the arrows) and fires one shot at a time straight up with space. A rock that is shot scores a point and goes back to the top of its column to fall again; a rock that reaches the ground, or lands on the ship, costs a life and goes back too. Twenty points win, five lives are lost and it is game over. Enter starts and restarts.

The two groups share the class rocks, so the shot has one rule for every kind of rock and each rock has the same three rules (ground, ship, shot).

What it tests

Question	How the file answers it
Can a rock be “destroyed” and come back?	Yes. The shot’s rule is <die /> and the rock’s rule for the shot is <inc variable="ship.score" /> then <reset />. <die /> is for things that stay dead (Space Invaders’ aliens); <reset /> puts a rock back at the start of its fall.
Do the two rule directions for one pair both run?	Yes: the shot has a rule about rocks and each rock has one about shot, and both take effect in the frame the two touch.
Does an object that is not in flight interfere?	No: until it is fired, the shot is hidden and has its collisions off, so a rock passing where it sits does nothing.

Options considered

• Rocks that split when hit, or that drift sideways. Splitting needs an object created while the game runs, which the language cannot do. Drifting is just an x velocity and would work, but then a rock that leaves a side wall needs a rule, and it made the columns harder to reason about. Left out.
• Differently scored rocks. Needs an amount on inc (21).
• A rock that reaches the ground costs nothing. It makes the game unloseable by neglect; here a miss is what makes shooting matter.
• Draw a new fall on every reset. As for Kaboom: <random> is worked out once at load, so a rock repeats its own fall. The rhythm is different every launch but not within one.

Approximations

Only vertical fall; no UFO, no bonus items, no ground, no sound; rectangles for everything; one shot in flight at a time.