more efficient bullet spawning

Then don't check for nil spark colors in sparks itself.

Also uses a more verbose but more efficient exit check, and fixes a bug.

vacuum_gambit.p8

@@ -539,7 +539,7 @@ function ship_m:move()
  if (self.dead) return;
  self:refresh_shield()
  local dx, dy, shoot_spec1, shoot_spec2 = self:act()
- local sg, xm, ym = self.special_guns, self.xmomentum, self.ymomentum
+ local sg, xm, ym, sp = self.special_guns, self.xmomentum, self.ymomentum, self.sparks
  dx = self:constrain(self.x, xm, self.xmin, self.xmax, dx)
  dy = self:constrain(self.y, ym, self.ymin, self.ymax, dy)
  self:maybe_shoot(self.main_gun)
@@ -547,7 +547,7 @@ function ship_m:move()
   if (shoot_spec1) self:maybe_shoot(sg[1])
   if (shoot_spec2) self:maybe_shoot(sg[2])
  end
- spark(self.sparks, self.x + 4*self.size, self.y + 4*self.size, dx*2.5, dy*2.5, self.sparkodds)
+ if(sp) spark(sp, self.x + 4*self.size, self.y + 4*self.size, dx*2.5, dy*2.5, self.sparkodds)
  xm = self:calc_velocity(xm, dx)
  ym = self:calc_velocity(ym, dy)
 
@@ -618,14 +618,6 @@ end
 -->8
 -- bullet and gun behaviors
 
-function player_blt_cat()
- return pbullets
-end
-
-function enemy_blt_cat()
- return ebullets
-end
-
 -- x, y: position
 -- dx, dy: movement (linear)
 -- f: frames remaining; nil for no limit
@@ -638,9 +630,9 @@ end
 --   details, check impl
 -- damage -- damage to do to
 --   a ship that gets hit
--- category -- function that
+-- category -- string naming
---   returns which bullet list
+--   which bullet list to spawn
---   to spawn onto
+--   onto, from _ENV
 -- hitship -- event handler,
 --   takes ship as argument.
 --   default: die, return true.
@@ -775,7 +767,7 @@ end
 function bullet_base:move()
  local x,y = self.x + self.dx, self.y+self.dy
  self.x,self.y=x,y
- return (y<=128) and (y >= -(self.height<<3)) and (x <= 128) and (x >= -(self.width<<3))
+ return (y<=128) and (y >= -16) and (x <= 128) and (x >= -16)
 end
 
 function bullet_base:draw()
@@ -785,7 +777,7 @@ end
 function bullet_base:spawn_at(x, y)
  self.x = x - self.x_off
  self.y = y - self.y_off
- add(self.category(), self)
+ add(_ENV[self.category], self)
 end
 
 function gun_base:shoot(x, y)
@@ -855,7 +847,7 @@ zap_p = mknew(bullet_base.new{
 
  hitship = const_fxn(true),
 
- category = player_blt_cat,
+ category = "pbullets",
 })
 
 zap_gun_p = mknew(gun_base.new{
@@ -914,7 +906,7 @@ blast = mknew(bullet_base.new{
    end)
   end
  end,
- category=player_blt_cat
+ category="pbullets"
 })
 
 blast_gun = mknew(gun_base.new{
@@ -953,14 +945,14 @@ protron_e = mknew(bullet_base.new{
  y_off = 4,
  
  damage = 1,
- category = enemy_blt_cat,
+ category = "ebullets",
 })
 
 protron_p = mknew(protron_e.new{
  sprite=23,
  dym = -1,
  y_off = 0,
- category=player_blt_cat,
+ category="pbullets",
 })
 
 protron_gun_e = mknew(gun_base.new{
@@ -1007,13 +999,13 @@ vulcan_e = mknew(bullet_base.new{
  y_off = 0,
  
  damage = 0.5,
- category=enemy_blt_cat
+ category="ebullets"
 })
 
 vulcan_p = mknew(vulcan_e.new{
  sprite=22,
  y_off = 4,
- category=player_blt_cat
+ category="pbullets"
 })
 
 vulcan_gun_e = mknew(gun_base.new{
@@ -1352,10 +1344,10 @@ end
 -- box: x1, y1, x2, y2
 function collides(b1, b2)
 	return
-	  b1.x1<=b2.x2
+   b1.y1<=b2.y2
-	  and b1.y1<=b2.y2
-	  and b1.x2>=b2.x1
 	  and b1.y2>=b2.y1
+	  and b1.x1<=b2.x2
+	  and b1.x2>=b2.x1
 end
 
 collider = mknew{
@@ -1637,7 +1629,7 @@ function spark_particle:draw()
 end
 
 function spark(sprs, x, y, dx, dy, odds, fg)
- if (sprs==nil or flr(rnd(odds) or (abs(dx) < 0.5 and abs(dy))) ~= 0) return
+ if ((dx > -0.5 and dx < 0.5 and dy > -0.5 and dy < 0.5) or rnd(odds) >= 1) return
  local target = fg and intangibles_fg or intangibles_bg
  target[#target+1] = spark_particle.new{
   x = x + rnd(4) - 2,
@@ -1656,7 +1648,7 @@ xp_gem = mknew(bullet_base.new{
  dy = 0.75,
  width=1, -- not used for spr but
  height=1,-- bullet_base uses it
- category = enemy_blt_cat,
+ category = "ebullets",
  damage = 0,
  hurt = {
   x_off = -2,