vine.lua

--The state of the vine is represented by x, y and a.
--x,y is the center of the base of the vine.
--a is the angle the vine is facing. It will always be a multiple of 1/4.
--(PICO-8 angles are 0..1)
--Each part of the vine is 10x10 pixels.
x=0
y=60
a=0

--We only need to clear the screen once.
cls(7)

--Alias to save chars.
f=circfill

--Begin render loop (this is just a goto label).
::_::

--Save chars. We no longer need the current value of a.
c=cos(a)
s=sin(a)

--Shift a by -1/4 or +1/4. You can consider this line the expansion of:
--  (flr(rnd(2))*2-1)/4
a+=flr(rnd(2))/2-1/4

--Save chars.
n=cos(a)
m=sin(a)

--clip() sets a rectangular clipping region. We use it to draw a single
--quadrant of a circle.
--clip() uses x,y,w,h so we need the top left position. First find the center
--of the current cell with (cos(a)*5, sin(a)*5), then subtract 5 from each.
clip(x+c*5-5,y+s*5-5,10,10)

--u,v is the center of the circle. It should be in a corner of the cell.
--We can find it by moving 5 units in the new direction.
--We need to shift u,v to make sute the center sits *inside* the current cell,
--rather than one pixel adjacent to it.
--First we use 4.5 instead of 5:
--  flr(5 + 5) == 10  ❌
--  flr(5 + 4.5) == 9 ⭕
--  flr(5 - 4.5) == 0 ⭕
--Next we account for when x,y is outside of the cell:
--  a==1/4 -> y is below cell    -> sin(a) == -1 -> dy = min(s,0) == min(s)
--  a==2/4 -> x is right of cell -> cos(a) == -1 -> dx = min(c,0) == min(c)
--We can omit the 0 from min() since the paramters will default to 0.
u=x+n*4.5+min(c)
v=y+m*4.5+min(s)

--Now we can draw the circles!
--1. is the outer outline.
--2. is the highlight. "a and b or c" is Lua's version of the ternary operator.
--   I just guessed the "s-1==m" condition, don't know why it works lol.
--3. is the green fill.
--4. is the inner outline.
f(u,v,8,1)
f(u,v,7,11)
f(u,v,1,1)
circ(u,v,7,s-1==m and 3 or 10)

--To get to the new position, move 5 units in the old direction (to the center
--of the cell) and then 5 units in the new direction.
x=(x+n*5+c*5)%140
y=(y+m*5+s*5)%140

--Swap the buffer and jump to the start of the loop.
flip()goto _

--🦝