Using Java/Slick 2D, I'm using the mouse to rotate a sprite on the screen and the directional keys (in this case, WASD) to move the spite. Forwards and backwards is easy, just position += cos(ang)*speed or position -= cos(ang)*speed. But how do I get the sprite to move left or right? I'm thinking it has something to do with adding 90 degrees to the angle or something. Any ideas?

Rotation code:

int mX = Mouse.getX();
        int mY = HEIGHT - Mouse.getY();
        int pX = sprite.x+sprite.image.getWidth()/2;
        int pY = sprite.y+sprite.image.getHeight()/2;
        double mAng;

        if(mX!=pX){
            mAng = Math.toDegrees(Math.atan2(mY - pY, mX - pX));
            if(mAng==0 && mX<=pX)
                mAng=180;
        }
        else{
            if(mY>pY)
                mAng=90;
            else
                mAng=270;
        }

        sprite.angle = mAng;
        sprite.image.setRotation((float) mAng);         

And the movement code (delta is change in time):

Input input = gc.getInput();

                Vector2f direction = new Vector2f();
                Vector2f velocity = new Vector2f();

                direction.x = (float) Math.cos(Math.toRadians(sprite.angle));
                direction.y = (float) Math.sin(Math.toRadians(sprite.angle));
                if(direction.length()>0)
                    direction = direction.normalise(); //On a separate note, what does this line of code do?

                velocity.x = (float) (direction.x * sprite.moveSpeed);
                velocity.y = (float) (direction.y * sprite.moveSpeed);          

                if(input.isKeyDown(sprite.up)){
                    sprite.x += velocity.x*delta;
                    sprite.y += velocity.y*delta;
                }if (input.isKeyDown(sprite.down)){
                    sprite.x -= velocity.x*delta;
                    sprite.y -= velocity.y*delta;
                }if (input.isKeyDown(sprite.left)){
                    //???
                }if (input.isKeyDown(sprite.right)){
                    //???
                }