Blitz Basic Tutes - Jigaw Program - 1st attempt

Graphics 640,480
SetBuffer BackBuffer()
SeedRnd MilliSecs()
Dim piece_imgs(12)
pointer_img=LoadImage("pointer.bmp")
img=LoadImage("hill_castle.jpg")
DrawBlock img,0,0
init()
shuffle()
last_n=0
Repeat
  display()
  DrawImage pointer_img,MouseX(),MouseY()
  Flip
  If MouseHit(1)
    n=which_one()
    If last_n=0 Then
      last_n=n
    Else
      t=piece_imgs(n):piece_imgs(n)=piece_imgs(last_n):piece_imgs(last_n)=t
      last_n=0
    End If
  End If
Until KeyHit(1)
End

Function init()
  n=1
  y=0
  For row=1 To 3
    x=0
    For col=1 To 4
      piece_imgs(n)=CreateImage(160,160)
      GrabImage piece_imgs(n),x,y
      n=n+1
      x=x+160
    Next
    y=y+160
  Next
End Function

Function display()
  n=1
  y=0
  For row=1 To 3
    x=0
    For col=1 To 4
      DrawBlock piece_imgs(n),x,y
      n=n+1
      x=x+160
    Next
    y=y+160
  Next
End Function

Function shuffle()
  For i=1 To 200
    r1=Rand(12)
    r2=Rand(12)
    t=piece_imgs(r1):piece_imgs(r1)=piece_imgs(r2):piece_imgs(r2)=t
  Next
End Function

Function which_one()
  n=1
  y=0
  For row=1 To 3
    x=0
    For col=1 To 4
      If mouse_in(x,y,x+160,y+160) Then piece=n
      n=n+1
      x=x+160
    Next
    y=y+160
  Next
  Return piece
End Function

Function mouse_in(x1,y1,x2,y2)
  x=MouseX(): y=MouseY()
  m=True
  If x<x1 Then m=False
  If x>x2 Then m=False
  If y<y1 Then m=False
  If y>y2 Then m=False
  Return m
End Function