improve legibility

lots of things rely on exactly one primary ship now, so this was just
overhead and wasted tokens / cspace.

collisiontest.p8

@@ -0,0 +1,141 @@
+pico-8 cartridge // http://www.pico-8.com
+version 42
+__lua__
+bx0=0
+by0=0
+bx1=127
+by1=127
+lx0=0
+ly0=0
+lx1=127
+ly1=127
+
+-->8
+
+function collides()
+	local tmin=0
+	local tmax=1
+	
+	local ldx=lx1-lx0
+	local ldy=ly1-ly0
+	
+	
+	-- x
+	if ldx==0 then
+		if (lx0<bx0 or lx0>bx1) return
+	else
+ 	local tx0=(bx0-lx0)/ldx
+ 	local tx1=(bx1-lx0)/ldx
+ 	if (tx0 > tx1) tx0,tx1=tx1,tx0
+ 	if (tmin < tx0) tmin=tx0
+ 	if (tmax > tx1) tmax=tx1
+ end
+ 
+	if ldy==0 then
+		if (ly0<by0 or ly0>by1) return
+	else
+ 	local ty0=(by0-ly0)/ldy
+ 	local ty1=(by1-ly0)/ldy
+ 	if (ty0 > ty1) ty0,ty1=ty1,ty0
+ 	if (tmin < ty0) tmin=ty0
+ 	if (tmax > ty1) tmax=ty1
+ end 
+ 
+ if (tmax < tmin) return
+ return tmin,tmax
+end
+
+
+-->8
+function _init()
+	poke(0x5f2d,1) -- enable mouse
+end
+
+function _bounce_screen(x)
+ return _bounce(x*128,128)
+end
+function _bounce(x,mx)
+	x=x%(mx * 2)
+	if (x>=mx)return mx-(x-mx)
+	return x
+end
+
+function _to_halfopen(x0,x1)
+ -- turn two numbers into a
+ -- half-open integer range
+ x0=flr(x0)
+ x1=flr(x1)
+ local lo=min(x0,x1)
+ local hi=max(x0,x1)
+ if (hi==lo) return lo,hi
+ return lo,hi-1
+end
+
+function _update60()
+ local t=time()/16
+ 
+	local bx0_=_bounce_screen(t*1)
+	local by0_=_bounce_screen(t*2)
+	local bx1_=_bounce_screen(t*3)
+	local by1_=_bounce_screen(t*4)
+	
+	bx0,bx1=_to_halfopen(bx0_,bx1_)
+	by0,by1=_to_halfopen(by0_,by1_)
+	
+	update_line()
+	
+	--[[
+ local lx0_=_bounce_screen(t*1.5)
+ local ly0_=_bounce_screen(t*2.5)
+ local lx1_=_bounce_screen(t*3.5)
+ local ly1_=_bounce_screen(t*4.5)
+	
+	lx0,lx1=lx0_,lx1_
+	ly0,ly1=ly0_,ly1_
+	]]--
+end
+
+was_down=false
+last_mx,last_my=nil,nil
+
+function update_line()
+ local mx,my=stat(32),stat(33)
+ local is_down=stat(34)!=0
+
+ if is_down then
+  if was_down then
+   lx1,ly1=mx,my
+  else
+  	lx0,ly0=mx,my
+  end
+ end
+ was_down=is_down
+ last_mx,last_my=mx,my
+end
+-->8
+function _draw()
+ cls(0)
+	rect(bx0,by0,bx1,by1,6)
+	line(lx0,ly0,lx1,ly1,2)
+	local cmin, cmax = collides()
+	if cmin then
+	 local dx,dy=lx1-lx0,ly1-ly0
+	 line(lx0 + dx*cmin,
+	      ly0 + dy*cmin,
+	      lx0 + dx*cmax,
+	      ly0 + dy*cmax,
+	      8)
+	 pset(lx0 + dx*cmin,
+	      ly0 + dy*cmin,
+	      12)
+	end
+	
+	pset(last_mx,last_my,7)
+end
+__gfx__
+00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+00700700000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+00077000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+00077000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+00700700000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000