You're correct, that part is not too hard, it was the next part that I was thinking of. Detecting if a bunch of atoms have been joined completely with each other... which I'm lost with.

bmp image -
http://socoder.net/uploads/124/atoms.rar

[code]
Graphics 320,200,0,2
AppTitle "Atomino - Blitz"

atom_sprites = LoadAnimImage("atoms.bmp",24,24,0,9)
electron_sprites = CreateImage(24,24,128)

For t# = 0 To 127
 x = 10.5 + 10.5 * Sin(t# * 2.8125)
 y = 10.5 + 10.5 * Cos(t# * 2.8125)
 SetBuffer ImageBuffer(electron_sprites,127-t#)
 DrawBlock atom_sprites,x,y,8
Next

SetBuffer BackBuffer()
Dim atom(20,12) ; which atom is where (1-4)
Dim peg(20,12) ; which sides are connected
Global space = 24
Global PUP = 1, PLEFT = 2, PDOWN = 4, PRIGTH = 8
Global nextAtom = 1

While Not KeyHit(1)
mx = MouseX()
my = MouseY()

Cls
Color 192,192,192
Rect 63,15,194,170,0
DrawBlock atom_sprites,0,0,nextAtom-1


If mx>63 And mx<253 And my>15 And my<183 Then
 cx = (MX-64)/space
 cy = (MY-16)/space
 DrawBlock atom_sprites,64+Floor(cx)*space,16+Floor(cy)*space,nextAtom-1 ; draw current atom at mouse position (cursor)


If MouseHit(1) And atom(cx,cy)=0 Then 
 atom(cx,cy)=nextAtom ; or whatever atom number
 nextAtom = Rnd(1,4)


 If cy>0 And atom(cx,cy) > peg(cx,cy) Then
  ; check UP
  a$=Bin$(peg(cx ,cy-1))
  total=0
  For temp = 1 To Len(a$)
   If Mid$(a$,temp,1)=1 Then total = total+1
  Next
  If total < atom(cx,cy-1)
   peg(cx,cy-1)=peg(cx,cy-1) Or PDOWN
   peg(cx,cy)=peg(cx,cy) Or PUP
  End If
 End If ; cy>0

 If cx>0 And atom(cx,cy) > peg(cx,cy) Then
  ; check LEFT
  a$=Bin$(peg(cx-1 ,cy))
  total=0
  For temp = 1 To Len(a$)
   If Mid$(a$,temp,1)=1 Then total = total+1
  Next
  If total < atom(cx-1,cy)
   peg(cx-1,cy)=peg(cx-1,cy) Or PRIGTH
   peg(cx,cy)=peg(cx,cy) Or PLEFT
  End If
 End If ; cx>0

 If cy<6 And atom(cx,cy) > peg(cx,cy) Then
  ; check DOWN
  a$=Bin$(peg(cx ,cy+1))
  total=0
  For temp = 1 To Len(a$)
   If Mid$(a$,temp,1)=1 Then total = total+1
  Next
  If total < atom(cx,cy+1)
   peg(cx,cy+1)=peg(cx,cy+1) Or PUP
   peg(cx,cy)=peg(cx,cy) Or PDOWN
  End If
 End If ; cy<6

 If cx<7 And atom(cx,cy) > peg(cx,cy) Then
  ; check RIGHT
  a$=Bin$(peg(cx+1 ,cy))
  total=0
  For temp = 1 To Len(a$)
   If Mid$(a$,temp,1)=1 Then total = total+1
  Next
  If total < atom(cx+1,cy)
   peg(cx+1,cy)=peg(cx+1,cy) Or PLEFT
   peg(cx,cy)=peg(cx,cy) Or PRIGTH
  End If
 End If ; cx>0


End If ; mousedown

End If


For y=0 To 6
 For x = 0 To 7
  If atom(x,y)>0 Then

   a$=Bin$(peg(x,y))
   total=0
   For temp = 1 To Len(a$)
    If Mid$(a$,temp,1)=1 Then total = total+1
   Next
   If atom(x,y)-total > 0 Then 
    For t = 1 To atom(x,y)-total
     DrawImage electron_sprites, 64+x*space,16+y*space, (electron_frame + 128/(atom(x,y)-total)*t)And 127
    Next
   End If

   DrawImage atom_sprites, 64+x*space,16+y*space,atom(x,y)-1
   If peg(x,y) And PUP Then DrawImage atom_sprites, 64+x*space,16+y*space,4
   If peg(x,y) And PLEFT Then DrawImage atom_sprites, 64+x*space,16+y*space,5
   If peg(x,y) And PDOWN Then DrawImage atom_sprites, 64+x*space,16+y*space,6
   If peg(x,y) And PRIGTH Then DrawImage atom_sprites, 64+x*space,16+y*space,7
  End If
 Next
Next
Flip
electron_frame = electron_frame + 3 ; adjust for animation speed
electron_frame = electron_frame And 127
Wend
End[/code]


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