Building the Flak Cannon Game Loop - The Essential Guide to Flash Games - page 191

if (chance <= enemySideChance) {
var leftOrRight:int = Math.floor(Math.random() * 2);
startY is the starting y position of the Enemy plane. We have to incorporate another difficulty
setting to calculate this value, enemySideFloor . enemySideFloor is the lowest point at which an
Enemy plane will arrive on the screen from the side. The idea here is that the lower an Enemy
enters the screen, the harder it will be to shoot down. The ending y value ( endY ) is always
gameHeight , which means it will exit at the bottom of the screen. We don't need to find an endX
value, because Enemy tests only the endY value to see if the Enemy has finished flying.
Now, depending on if the Enemy is flying to the left or right, we will create some slightly different
values. If the plane is flying left, it will start at the extreme right of the screen. For that reason,
startX equals gameWidth . If the plane is flying to the right, it will start at the extreme left (0) minus
the width of the Enemy (-32). If you recall from Chapter 4, the Enemy class will select an angle for
the Enemy to travel based on the direction ( dir ). Flying left to the left, the angle is 135 degrees
(see Figure 5-6); from the right, it is 45 degrees.
startY = Math.floor(Math.random() * enemySideFloor)+1;
endY = gameHeight;
switch(leftOrRight) {
case 0: //left
startX = gameWidth;
dir = Enemy.DIR_LEFT;
break;
case 1: //right
startX = -32;
dir = Enemy.DIR_RIGHT;
break;
}
Figure 5-6. Enemy coming in from the left at a 135-degree angle
For Enemy planes that fly down ( Enemy.DIR_DOWN ), the values are quite easy to calculate. The
starting x value ( startX ) is a random number generated that is the width of the screen
( gameWidth ) minus the width of the Enemy plane (32). This will keep the Enemy planes within the