Step 3: Lining them up

What we need to do is make each and every dot follow the one that came before it in a line. That's easy enough. The position of the next position should equal the position of the last.

snakex[i] := snakex[i-1]

So how do we go over all the points of the snake so that this will work? There's two options:

Option 1: Front to back

We start from the front of the snake and go to the end.

repeat i from 1 to snakecount

If we run these commands in order:

snakex[1] = snakex[0]
snakex[2] = snakex[1]
snakex[3] = snakex[2]

This doesn't work. When you simplify it, you end up with this:

snakex[3] = snakex[2] = snakex[1] = snakex[0]

In other words, a single dot. Lame.

Option 2: Back to front

We can also try starting from the back of the snake and going to the front.

repeat i from snakecount to 1

You'll see that running it this way yields the result we want.

snakex[3] = snakex[2]
snakex[2] = snakex[1]
snakex[1] = snakex[0]

In other words, snakex[3] is the old value of snakex[2], snakex[2] is the old value of snakex[1], and snakex[1] is the old value of snakex[0].

Well, gee, that's just what we want!

There is only one value that isn't set this way, and that's snakex[0]. That's okay though, because we already set snakex[0] via player direction.

Here's the code that calculates the locations of the snake's tail. We should place this code after drawing the first snake sprite but before lcd.Draw.

Snake.spin

        if snakedir == DOWN and snakey[0] < constant(64-2)
            snakey[0]++

        gfx.Sprite(@dot_gfx, snakex[0], snakey[0], 0)

        repeat i from snakecount to 1
            snakex[i] := snakex[i-1]
            snakey[i] := snakey[i-1]

        lcd.Draw

DAT
    dot_gfx

Don't forget, though. We also want to see the tail, so make sure to include a call to gfx.Sprite to draw the tail at each position.

Snake.spin

        gfx.Sprite(@dot_gfx, snakex[0], snakey[0], 0)

        repeat i from snakecount to 1
            snakex[i] := snakex[i-1]
            snakey[i] := snakey[i-1]
+++            gfx.Sprite(@dot_gfx, snakex[i], snakey[i], 0)

        lcd.Draw

In essence, we now have a snake, not just a single dot, that can move around the field.