INVASÃO ALIENÍGENA

Free online Alien Invasion

Alien Invasion is a fixed shooter in the mold of late-1970s arcade cabinets: a fleet of 50 invaders marches side to side, steps down every time it reaches an edge and answers with bombs, while you, locked to the bottom of the screen, can only slide left and right and fire upward. What follows are the numbers the fleet actually obeys, and what to do with them.

History

Space Invaders, from the Japanese company Taito, came out in 1978, created by Tomohiro Nishikado, who also designed much of the hardware. It became an arcade phenomenon, was ported to the Atari 2600 in 1980 and opened the era of fixed shooters, quickly followed by Galaxian (1979).

The fleet's acceleration came from the hardware: with fewer invaders to draw, the game ran faster, and the effect was kept. This version recreates that feel with an explicit formula, described below.

How to play

Move the cannon with Left and Right or A and D and fire with Space (holding the key repeats the shot). P pauses and resumes. On a phone, use the four buttons under the field: left, fire, right and pause (⏸). Before you start, choose Easy, Normal or Hard; changing difficulty restarts the match.

Only two of your shots can be on screen at once, at least 160 ms apart, and a missed shot occupies one of those slots for about a second. Clearing the fleet starts a new wave, usually faster, with the shields rebuilt.

Rules

• The fleet is 10 columns × 5 rows = 50 invaders. The top row is worth 30 points, the two middle rows 20 and the two bottom rows 10: 900 points for a full fleet.
• 3 lives, no extra lives. When you are hit, your ship returns to the center and the bombs vanish, but there is no invulnerability period.
• You lose in two ways: running out of lives, or the lowest invader reaching your ship's line. The second is instant, even with lives to spare.
• Four shields wear down from shots on both sides. They do not regenerate within a wave; they are rebuilt at the start of each one.
• A UFO crosses the top every 9 to 16 seconds and is worth 50, 100, 150 or 300 points, each equally likely.
• The high score is shared by all three difficulties and is saved the moment you beat it, even if you restart afterwards.

Strategies

• Open a side column early. It widens the sweep, becomes a safe lane and clears the way to the UFO. The start of a wave, with the fleet slow, is the best moment.
• Clear bottom-up when time runs short. Defeat depends on the lowest invader; one whole row removed buys back 34 units, more than two steps.
• Shoot with rhythm, not haste. You have two shot slots, each held for 0.5 to 1 s: a missed shot leaves you with a single gun.
• Lead the last invaders. A lone invader on Normal, wave 1, runs about 84 per second and your shot takes 0.4 to 0.6 s: aim about 40 ahead.
• Fire through the gaps between shields, then step back under one. That way your own shots never chew up your cover.

The fleet's clock

Fleet speed combines three factors: difficulty, invaders already shot and wave number. On Normal it starts near 25 per second and passes 80 when a single invader is left, about 3.4 times faster. Up to wave 15, each wave adds 18% to the base speed (wave 15 already moves 3.5 times faster than wave 1); from wave 16 on that growth has a ceiling, described below.

The fleet reverses and drops 16 units only when its outermost living invader is 10 from the edge. That is why removing the end columns widens the sweep and delays the descent, while removing the middle speeds things up without buying time.

With the bottom row intact, wave 1 allows 18 steps down before defeat. Up to wave 15, each wave starts 14 units lower: 15 steps on wave 5, 11 on wave 10 and 6 on wave 15, the lowest start. On wave 16 the fleet is born at the wave 1 height again and the cycle of 15 heights starts over, and the pressure breathes too: fleet speed and bombs drop back to the pace of wave 11, climb slowly (one level every four waves) and stop at the pace of wave 13, from wave 24 on. That is the ceiling: nothing goes past it, and from wave 16 on bombs never come less than 297 ms apart on Normal. There is no final wave: a match only ends when your lives run out or the fleet reaches your line.

Bombs: who fires and when

At fixed intervals a bomb drops from the lowest invader of a randomly chosen column; it does not aim at you. The interval shrinks 10% per wave and the bombs fall faster every wave, up to wave 15. On Normal:

• wave 1: one bomb every 1.05 s, falling at 228 per second
• wave 5: every 0.69 s, at 300
• wave 10: every 0.41 s, at 390
• wave 15: every 0.24 s, at 480 (the fiercest pace in the game)
• from wave 16: back to every 0.37 s, at 408, rising to 297 ms, at 444, on wave 24, where it stays

Reading the bomb lanes

Easy and Hard start at 1.5 s and 0.76 s. The ceiling is proportional: the shortest interval in the game is wave 15's (0.34 s on Easy, 0.17 s on Hard). With few columns alive, each one is picked more often; with a single column left, every bomb comes from it.

Your ship is 42 wide and the columns are 44 apart. Centered under a completely empty column, it is out of reach of the neighbors' bombs. The fleet moves slowly in the early waves and the ship moves about 300 per second, so you can follow that lane.

Shields: what they absorb

There are four blocks of 13×9 squares, 4 units each, with the top corners cut off and 86 of gap between them. Each impact, whether a bomb or your own shot, erases a 3×3 cross of squares plus, at random, some of the corners; a column of 9 falls after about 3 hits on the same spot.

Invaders pass through the shields without erasing them: only shots destroy them. Your own shot, fired from below, wears the shield down before leaving it, and cover exists only while a whole block stands above you.

The UFO: how to hit it

The UFO crosses the top at about 130 per second and appears 9 to 16 seconds after the previous one; the first comes 5 to 12 seconds into a wave. Your shot needs about 0.9 seconds to climb up there, and in that time the UFO moves more than 120: aim roughly three ship-widths ahead.

Your shot hits the first invader in its column, so it only reaches the UFO through an open column or through the 16-unit corridor between two columns. A UFO is worth 150 points on average.

Common mistakes

• Shooting from under a shield. You dismantle your own cover.
• Chasing the 30-point row first. The bottom row decides defeat, and every invader must fall anyway.
• Restarting a good run. New Game or changing difficulty resets score, wave and lives. Your high score is already saved, but the wave you were on is lost.
• Leaving a single column. Every bomb then comes from it.

Tips for beginners

• Start on Easy: a fleet at about 18 per second and bombs every 1.5 s.
• Holding Space repeats the shot on a keyboard, but the two-shot limit still applies.
• The match pauses by itself when the tab loses focus; to continue, tap the field (or the ⏸ button) or press P.

Trivia

• The original had 55 invaders (11×5); this version uses 50 (10×5).
• The art here is 11×8-pixel bitmaps in two animation frames, in the style of the classic's squid, crab and octopus.
• The UFO bonus is random in this version: 50, 100, 150 or 300 points, averaging 150.

Mobile version

On a phone there are four buttons: left and right (hold to move), fire (one tap, one shot) and pause (⏸). You can hold one side with one thumb and tap fire with the other. If the tab loses focus the match pauses by itself and shows a notice on screen; a tap on the field (or on any button) picks up where you left off.

Frequently asked questions

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