Hi there, I created this engine for my new game Anarchy which is similar to Tyrian.
This is a really stripped down version which doesnt talk about buildings or tanks or multiple weapons (gotta keep my secrets) but it does give you a starting point.

It is regulated by groups and alterable values, so you can add new things with minimal coding.
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Scrolling

Always:

Player: Set Y [Y-player -X (however fast you want him to go)]

Scrolling: Set horizontal to [X player] set virtical to [Y top playfield -X (this must be the same as the one you put for the player)]
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Movement
this is basic. It works for all directions but im to lazy to write em all. You get the idea though, if its not hitting a visible playfield edge, it can move.

Repeat while left is pressed
X player > X left playfield:

Set position to X(Player) -1
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Enemies
This is for a enemy on a path movement

Group.Enemies

Y>= Y Top Playfield
Alterable value A>0:

Start

every X:

Shoot enemy bullet at speed X
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Your Shooting
value A is health.
value B is for gun cooling.

Repeat while (fire) is pressed
Alterable value B (Player)<= 0
Alterable value A (Player)>0:

Shoot bullet at speed 100
set alterable vaue B to X (X= time for weapon to cool)

Every 00.01:

Sub 1 from Alterable value B (player)
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Bullet collisions
Value A of a bullet is how much damage it does.

Collision between Group.Enemies and bullet:

Bullet: Destroy
Enemy: Subtract [value A (Bullet)] from value A

(Repeat this for the player, but replace the objects for enemy bullet and player ship.)
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Destroying things

Enemy: Value A<= 0:

Destroy

Player: Value A<=0

Destroy (or run your death sequence as a group of events)
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Well, Thats the most basic but useable engine I want to release (Im using this engine but with a bit of extra stuff for my new game, so im not gonna give away everything I know) but I hope it works for you. Please post comments!

Thanks, Crash

My spoon is too big!