diff --git a/vacuum_gambit.p8 b/vacuum_gambit.p8
index 6d75805..ddb1cd8 100644
--- a/vacuum_gambit.p8
+++ b/vacuum_gambit.p8
@@ -420,6 +420,13 @@ ship_m = {
  
  xmomentum = 0,
  ymomentum = 0,
+
+ -- xmin, xmax, ymin, ymax:
+ --   movement constraints
+ --   enforced by `constrain`.
+ xmin = 0, xmax = 104,
+ -- ymin, ymax default to nil
+ --   pship needs more constraint 
 }
 mknew(ship_m)
 
@@ -428,31 +435,59 @@ function ship_m:die()
  if (self.hp <= 0) boom(self.x+self.size*4, self.y+self.size*4,12*self.size, self.boss)
 end
 
+function ship_m:calc_velocity(v0, t)
+ v0 = mid(v0 + t, self.maxspd, -self.maxspd)
+ return v0 - mid(self.drag, -self.drag, v0)
+end
+
+function ship_m:brake_dist(v0)
+ local brake_max = self.thrust + self.drag
+ local tri_frames = abs(v0\brake_max)
+ local chunks = tri_frames * (tri_frames - 1) >> 1
+ local chunk_zone = chunks * brake_max
+ local overage = abs(v0) - tri_frames * brake_max
+ return (chunk_zone + overage * (tri_frames + 1)) * sgn(v0), (overage > 0) and tri_frames + 1 or tri_frames
+end
+
+function ship_m:constrain(p, dp, pmin, pmax, want)
+ if (not pmin) return want
+ local v1, bd, bf, bp
+ function calc_targets()
+  -- velocity after move
+  v1 = self:calc_velocity(dp, want)
+  -- brake distance and frames
+  bd, bf = self:brake_dist(v1)
+  -- brake point
+  bp = p + bd + v1
+ end
+ if bp < pmin then
+  -- undershoot. max thrust,
+  -- then treat as overshoot
+  -- targeting minimum bound
+  want, pmax = self.thrust, pmin
+  calc_targets()
+ end
+ if (bp <= pmax) return want
+ -- spread overshoot across frames
+ want -= (bp - pmax)/(bf+1)
+ return max(want, -self.thrust)
+end
+
 function ship_m:move()
  self:refresh_shield()
  self.power = min(self.max_power, self.power + self.generator)
  local dx, dy, shoot_spec, shoot_main = self:act()
+ dx = self:constrain(self.x, self.xmomentum, self.xmin, self.xmax, dx)
+ dy = self:constrain(self.y, self.ymomentum, self.ymin, self.ymax, dy)
  if (shoot_main) self:maybe_shoot(self.main_gun)
  if (shoot_spec) self:maybe_shoot(self.special_gun)
  if (dx ~= 0 or dy ~= 0) spark(self.sparks, self.x + 4*self.size, self.y + 4*self.size, dx*2.5, dy*2.5, self.sparkodds)
- self.xmomentum += dx
- self.ymomentum += dy
- self.xmomentum = mid(-self.maxspd, self.maxspd, self.xmomentum)
- self.ymomentum = mid(-self.maxspd, self.maxspd, self.ymomentum)
- 
+ self.xmomentum = self:calc_velocity(self.xmomentum, dx)
+ self.ymomentum = self:calc_velocity(self.ymomentum, dy)
+
  self.x += self.xmomentum
  self.y += self.ymomentum
  
- if self == primary_ship then
-  self.x = mid(0, 112 - 8 * self.size, self.x)
-  self.y = mid(0, 128 - 8 * self.size, self.y)
- end
- 
- --friction
- local d = self.drag
- self.xmomentum -= mid(d, -d, self.xmomentum)
- self.ymomentum -= mid(d, -d, self.ymomentum)
- 
  -- "scrolling" behavior
  if self.slip then
   self.y += scrollrate