diff --git a/engine.lua b/engine.lua
deleted file mode 100644
index 1a91c80..0000000
--- a/engine.lua
+++ /dev/null
@@ -1,191 +0,0 @@
-function klass()
- local k={}
- k.__index=k
- function k:new(...)
-  local n={}
-  setmetatable(n,k)
-  n:init(...)
-  return n
- end
- return k
-end
-
-song=klass()
-function song:init()
- self.frames={}
- self.ix_to_frame={}
- self.next_frame_start=0
-end
-function song:add(len)
- for i=0,len-1 do
-  self.ix_to_frame[self.next_frame_start+i]={#self.frames+1,i}
- end
- add(self.frames,{
-  pattern:new({len=len}),
-  pattern:new({len=len}),
-  pattern:new({len=len}),
-  pattern:new({len=len}),
- })
- self.next_frame_start+=len
-end
-function song:pattern(channel,offset)
- offset = offset or -1
- channel &= 0xffff
- offset &= 0xffff
- assert(channel>=0 and channel<4, "channel must be [0,4)")
- local n_frames_long=#self.frames
- if offset<0 then
-  assert(offset>=-n_frames_long, "offset must not exceed -"..n_frames_long)
-  offset+=n_frames_long+1
- else
-  assert(offset>=0 and offset<n_frames_long)
-  offset+=1
- end
- return self.frames[offset][channel+1]
-end
-function song:plot(channel,offset,instant)
- assert(channel>=0 and channel<4, "channel must be [0,4)")
- local tup=self.ix_to_frame[offset]
- assert(tup, "invalid offset for current length: "..offset) 
- local f,offset=unpack(tup)
- self.frames[f][channel+1]:plot(offset, instant)
-end
-
-function song:build(
- free_patterns,
- frame_a,
- frame_z
-)
- local n_frames_long = #self.frames
- if (not frame_z) frame_z = frame_a + n_frames_long
- assert(frame_z-frame_a == n_frames_long, "wrong number of frames (must be "..frame_a.." to "..frame_a+n_frames_long..")")
-
- -- dump patterns and frames
- mapped_patterns={}
- function map_to_real_pattern(pat)
-  if (pat:silent()) return 0 | (1<<6)
-
-  local key = pat:key()
-  mapped_patterns[key] = mapped_patterns[key] or {}
-  for other in all(mapped_patterns[key]) do
-   if (pat:eq(other)) return other.map_ix
-  end
-  assert(#free_patterns>0, "out of free patterns")
-  pat:map_to(deli(free_patterns,1))
-  add(mapped_patterns[key],pat)
-  return pat.map_ix 
- end
-
- local fmaddr=0x3100+(frame_a)*4
- for frame=1,n_frames_long do
-  for i=1,4 do
-   poke(fmaddr+i-1,map_to_real_pattern(self.frames[frame][i]))
-  end
-  fmaddr+=1
- end
-end
-
-pattern=klass()
-function pattern:init(o)
- speed = o.speed or 15
- len = o.len or 32
- noiz = o.noiz or 0
- buzz = o.buzz or 0
- detune = o.detune or 0
- reverb = o.reverb or 0
- dampen = o.dampen or 0
- editormode = true
-
- assert(speed >= 1 and speed <255, "speed must be [1,255)")
- assert(len >= 1 and len < 33, "len must be [1,33)")
- assert(noiz >= 0 and noiz < 2, "noiz must be [0,2)")
- assert(buzz >= 0 and buzz < 2, "buzz must be [0,2)")
- assert(detune >= 0 and detune < 3, "detune must be [0,3)")
- assert(reverb >= 0 and reverb < 3, "reverb must be [0,3)")
- assert(dampen >= 0 and dampen < 3, "dampen must be [0,3)")
-
- self.instants={}
- self.len=len 
- -- https://pico-8.fandom.com/wiki/Memory#Music
- self.speed=speed
- self.pattern_flags=(
-  (
-   tonum(editormode) |
-   noiz<<1 |
-   buzz<<2
-  ) + 
-  detune*8 + 
-  reverb*24 + 
-  dampen*72
- )
-
- for i=0,self.len-1 do
-  self.instants[i]=0
- end
-end
-function pattern:plot(ix, iat)
- assert(ix>=0 and ix<self.len, "index invalid")
- self.instants[ix]=encode_instant(iat)
-end
-function pattern:silent()
- for i=0,self.len-1 do
-  if (self.instants[i]&0x0e00!=0) return
- end
- return true
-end
-function pattern:key()
- local key=0
- for i=0,self.len-1 do
-  key ^= self.instants[i]<<(i%16)
- end
- return key
-end
-function pattern:eq(other)
- if (self.len!=other.len) return 
- for i=0,self.len-1 do
-  if (self.instants[i]!=other.instants[i]) return 
- end
- return true
-end
-function pattern:map_to(ix)
- self.map_ix=ix
- local at=0x3200+ix*68
- for i=0,31 do
-  poke2(at+i*2,self.instants[i] or 0)
- end
- poke(at+64,self.pattern_flags)
- poke(at+65,self.speed)
- -- start, end
- poke(at+66,self.len)
- poke(at+67,0)
-end
-
-function encode_instant(o)
- custom = o.c or false
- effect = o.e or 0
- volume = o.v or 0
- waveform = o.w or 0
- pitch = o.p or 0
-
- assert(custom == false or custom == true, "custom must be true or false")
- assert(effect >= 0 and effect < 8, "effect must be [0,8)")
- assert(volume >= 0 and volume < 8, "volume must be [0,8)")
- assert(waveform >= 0 and waveform < 8, "waveform must be [0,8)")
- assert(pitch >= 0 and pitch < 64, "pitch must be [0,64)")
-
- custom = custom
- effect = effect & 0xffff
- volume = volume & 0xffff
- waveform = waveform & 0xffff
- pitch = pitch & 0xffff
-
- -- not a method: handle the nil instant
- -- https://pico-8.fandom.com/wiki/Memory#Music
- return (
-  (tonum(custom) << 15) |
-  (effect << 12) |
-  (volume << 9) |
-  (waveform << 6) |
-  (pitch)
- )
-end
\ No newline at end of file
diff --git a/pulsar.p8 b/pulsar.p8
index fa960bb..b5a69fe 100644
--- a/pulsar.p8
+++ b/pulsar.p8
@@ -1,8 +1,12 @@
 pico-8 cartridge // http://www.pico-8.com
 version 42
 __lua__
-#include engine.lua
-#include song.lua
+#include shared/_meta.lua
+#include shared/nflags.lua
+#include shared/pattern.lua
+#include shared/pflags.lua
+#include shared/track.lua
+#include unique.lua
 __gfx__
 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
diff --git a/shared/_meta.lua b/shared/_meta.lua
new file mode 100644
index 0000000..f8e3881
--- /dev/null
+++ b/shared/_meta.lua
@@ -0,0 +1,15 @@
+function klass()
+ local k={}
+ k.__index=k
+ function k:new(...)
+  local n={}
+  setmetatable(n,k)
+  n:init(...)
+  return n
+ end
+ return k
+end
+
+function assert_range(i,mn,mx,name)
+ assert(i >= mn and i < mx, name.." must be ["..mn..","..mx..")")
+end
\ No newline at end of file
diff --git a/shared/nflags.lua b/shared/nflags.lua
new file mode 100644
index 0000000..588cd7b
--- /dev/null
+++ b/shared/nflags.lua
@@ -0,0 +1,41 @@
+nflags=klass()
+function nflags:init(o)
+ self.c = o.c or 0  -- custom
+ self.e = o.e or 0 -- effect
+ self.v = o.v or 0 -- volume
+ self.w = o.w or 0 -- waveform
+ self.p = o.p or 0 -- pitch
+
+ self:validate()
+end
+
+function nflags:validate()
+ local c,e,v,w,p
+  =self.c,self.e,self.v,self.w,self.p
+
+ assert_range(c,0,2,"custom")
+ assert_range(e,0,8,"effect")
+ assert_range(v,0,8,"volume")
+ assert_range(w,0,8,"waveform")
+ assert_range(p,0,64,"pitch")
+end
+
+function nflags:encode()
+ self:validate()
+ local c,e,v,w,p
+  =self.c,self.e,self.v,self.w,self.p
+
+ c &= 0xffff
+ e &= 0xffff
+ v &= 0xffff
+ w &= 0xffff
+ p &= 0xffff
+
+ return (
+  (c << 15) |
+  (e << 12) |
+  (v << 9) |
+  (w << 6) |
+  (p)
+ )
+end
\ No newline at end of file
diff --git a/shared/pattern.lua b/shared/pattern.lua
new file mode 100644
index 0000000..fd11841
--- /dev/null
+++ b/shared/pattern.lua
@@ -0,0 +1,45 @@
+
+pattern=klass()
+function pattern:init(p)
+ self.pflags=pflags:new(p)
+
+ self.nflags={}
+ for i=0,31 do  -- because the user could change the length
+  self.nflags[i]=nflags:new{}
+ end
+end
+function pattern:plot(ix, n)
+ assert_range(ix,0,self.pflags.len,"ix")
+ self.nflags[ix]=nflags:new(n)
+end
+function pattern:silent()
+ for i=0,31 do
+  if (self.nflags[i].v!=0) return
+ end
+ return true
+end
+function pattern:key()
+ local key=0
+ for i=0,self.pflags.len-1 do
+  key ^= self.nflags[i]:encode()>>(i%16)
+ end
+ return key
+end
+function pattern:eq(other)
+ if (self.pflags:encode() != other.pflags:encode()) return
+
+ for i=0,self.pflags.len-1 do
+  if (self.nflags[i]:encode()!= other.nflags[i]:encode()) return
+ end
+ return true
+end
+function pattern:map_to(ix)
+ self.map_ix=ix
+
+ local at=0x3200+ix*68
+ poke4(at+64,self.pflags:encode())
+
+ for i=0,31 do
+  poke2(at+i*2,self.nflags[i]:encode())
+ end
+end
\ No newline at end of file
diff --git a/shared/pflags.lua b/shared/pflags.lua
new file mode 100644
index 0000000..147cd1c
--- /dev/null
+++ b/shared/pflags.lua
@@ -0,0 +1,53 @@
+pflags=klass()
+function pflags:init(o)
+ self.speed = o.speed or 15
+ self.len = o.len or 32
+ self.noiz = o.noiz or 0
+ self.buzz = o.buzz or 0
+ self.detune = o.detune or 0
+ self.reverb = o.reverb or 0
+ self.dampen = o.dampen or 0
+ self.editormode = o.editormode or 1
+end
+
+function pflags:validate()
+ local speed,len,noiz,buzz,detune,reverb,dampen,editormode
+  =self.speed,self.len,self.noiz,self.buzz,
+   self.detune,self.reverb,self.dampen,self.editormode
+ 
+ assert_range(speed,1,255,"speed")
+ assert_range(len,1,33,"len")
+ assert_range(noiz,0,2,"noiz")
+ assert_range(buzz,0,2,"buzz")
+ assert_range(detune,0,3,"detune")
+ assert_range(reverb,0,3,"reverb")
+ assert_range(dampen,0,3,"dampen")
+ assert_range(editormode,0,2,"editormode")
+end
+
+function pflags:encode()
+ local speed,len,noiz,buzz,detune,reverb,dampen,editormode
+  =self.speed,self.len,self.noiz,self.buzz,
+   self.detune,self.reverb,self.dampen,self.editormode
+
+ speed &= 0xffff
+ len &= 0xffff
+ noiz &= 0xffff
+ buzz &= 0xffff
+ detune &= 0xffff
+ reverb &= 0xffff
+ dampen &= 0xffff
+ editormode &= 0xffff
+
+ self:validate()
+ local byte0 = (
+  dampen*72 +
+  reverb*24 +
+  detune*8 +
+  buzz*4 +
+  noiz*2 +
+  editormode  -- "in editor mode" flag
+ )
+ return len | speed>>8 | byte0 >>16
+end
+
diff --git a/shared/track.lua b/shared/track.lua
new file mode 100644
index 0000000..a7a0407
--- /dev/null
+++ b/shared/track.lua
@@ -0,0 +1,78 @@
+track=klass()
+function track:init()
+ self.frames={}
+ self.ix_to_frame={}
+ self.next_frame_start=0
+end
+function track:add(len)
+ for i=0,len-1 do
+  self.ix_to_frame[self.next_frame_start+i]={#self.frames,i}
+ end
+ add(self.frames,{
+  pattern:new({len=len}),
+  pattern:new({len=len}),
+  pattern:new({len=len}),
+  pattern:new({len=len}),
+ })
+ self.next_frame_start+=len
+end
+
+function track:pattern(channel,offset)
+ offset = offset or -1
+ channel &= 0xffff
+ offset &= 0xffff
+
+ assert_range(channel,0,4,"channel")
+ local n_frames_long=#self.frames
+ assert_range(offset,-n_frames_long,n_frames_long,"offset")
+ if offset<0 then
+  offset+=n_frames_long
+ end
+ return self.frames[offset+1][channel+1]
+end
+
+function track:plot(channel,offset,inst)
+ assert_range(channel,0,4,"channel")
+ assert_range(offset,0,self.next_frame_start,"offset")
+ local tup=self.ix_to_frame[offset]
+ assert(tup)  -- should be unable to fail
+ local frame,offset=unpack(tup)
+ self.frames[frame+1][channel+1]:plot(offset,inst)
+end
+
+function track:build(
+ free_patterns,
+ frame_a,
+ frame_z
+)
+ local n_frames_long = #self.frames
+ if (not frame_z) frame_z = frame_a + n_frames_long
+ assert(frame_z-frame_a == n_frames_long, 
+  "wrong number of frames (must be "..
+  frame_a.." to "..frame_a+n_frames_long..")")
+
+ -- dump patterns and frames
+ local mapped_patterns={}
+ local function map_to_real_pattern(pat)
+  if (pat:silent()) return 0 | (1<<6)
+
+  local key = pat:key()
+  mapped_patterns[key] = mapped_patterns[key] or {}
+  local t = mapped_patterns[key]
+  for other in all(t) do
+   if (pat:eq(other)) return other.map_ix
+  end
+  assert(#free_patterns>0, "out of free patterns")
+  pat:map_to(deli(free_patterns,1))
+  add(t,pat)
+  return pat.map_ix 
+ end
+
+ local fmaddr=0x3100+(frame_a)*4
+ for frame=1,n_frames_long do
+  for i=0,3 do
+   poke(fmaddr+i,map_to_real_pattern(self.frames[frame][i+1]))
+  end
+  fmaddr+=1
+ end
+end
\ No newline at end of file
diff --git a/song.lua b/unique.lua
similarity index 95%
rename from song.lua
rename to unique.lua
index cf5df70..34b55a7 100644
--- a/song.lua
+++ b/unique.lua
@@ -16,7 +16,7 @@ function _draw()
 end
 
 function build_bgm()
- local bgm=song:new()
+ local bgm=track:new()
 
  bgm:add(32)