$options +optimize

' degrees to radians coefficient
D2R=0.3927

'use graphics 8 to clear out RAM and switch to graphics 1 for characters
GR.8:GR.1

'make a display list
DIM A$(96):A$="\70\70\70\46\60\BF\00\00\00\00\00\00\56\80\BD\70\56\A8\BD\70\56\D0\BD\70\56\F8\BD\70\56\20\BE\70\56\48\BE\70\56\70\BE\70\56\98\BE\70\70\70\70\70\70\70\41\50\A0"
A=ADR(A$)
MOVE A,$A050,53
DL=$A050
DPOKE 560,DL

'copy the Asteroids spaceships 
A$="\04\04\0E\0E\1F\01\03\07\0F\03\01\06\1E\0C\04\01\3E\1C\18\00\20\38\3F\38\20\18\1C\3E\01\00\04\0C\1E\06\01\03\0F\07\03\01\1F\0E\0E\04\04\0C\0F\0E\0C\08\04\06\0F\0C\10\06\0E\1F\20\00\01\07\3F\07\01\20\1F\0E\06\00\10\0C\0F\06\04\08\0C\0E\0F\0C"
PM=$AC00
MOVE A,PM,80
POKE 54279,PM/256:REM              Tell ANTIC where PM RAM is
POKE 559,46:REM                     Enable PM DMA with 2-line res
POKE 53277,3:REM                      Enable PM display
POKE 704,15

'copy the space invaders characters into the a new character set
CHSET=$A200
SCRN=$BD80
SCR2=SCRN+2
MOVE $E000,CHSET,$200:POKE 756,$A2
MOVE CHSET,CHSET+$200,$600
A$="\00\00\0F\0F\07\04\06\03\00\00\F0\F0\E0\20\30\18\30\0F\03\03\03\03\04\00\30\C0\00\00\00\00\80\00\06\02\01\01\01\06\04\00\10\30\C0\40\C0\20\30\00\00\0C\03\06\05\04\00\00\00\30\C0\60\A0\20\00\00\20\10\08\04\13\0D\03\04\04\08\10\20\C0\B0\C8\20\01\03\07\0C\1F\18\18\18\80\C0\E0\F0\F8\C0\C0\C0"
MOVE A,CHSET+$108,96
A$="\00\01\0F\1F\07\04\06\06\00\80\F0\F8\E0\20\30\30\30\0C\03\03\03\03\04\08\30\D0\00\00\00\00\C0\00\03\01\01\01\07\04\00\00\00\10\F0\40\C0\40\60\00\00\0E\01\06\05\08\00\00\00\70\C0\60\A0\10\00\00\10\08\04\03\1D\03\04\08\08\10\20\C0\B8\C0\20\10\00\03\07\0E\1F\1B\18\00\00\C0\E0\70\F8\D8\C0\00"
MOVE A,CHSET+$308,96
A$="\02\01\2F\1F\27\04\06\0C\40\80\F4\F8\E4\20\30\60\40\30\0F\03\03\04\08\10\00\30\D0\10\00\C0\00\00\01\01\0B\0E\03\01\03\00\80\00\80\E0\A0\00\00\00\08\06\01\06\05\08\10\00\10\60\80\60\A0\10\08\00\08\04\03\0D\13\04\08\10\10\20\C8\B0\C0\20\10\08\00\00\07\0F\1F\1B\03\00\00\00\E0\30\F8\D8\18\00"
MOVE A,CHSET+$508,96
A$="\06\21\2F\1F\27\24\0C\18\60\84\F4\F8\E4\24\60\C0\00\00\3F\03\05\08\10\20\00\00\F0\10\10\E0\00\00\00\08\0F\02\03\02\06\00\C0\80\80\80\E0\40\00\00\0C\02\01\06\05\08\10\20\30\40\80\60\A0\10\08\04\04\03\0D\33\04\08\10\20\20\CC\B0\C0\20\10\08\04\00\00\07\0F\1F\03\03\03\00\00\E0\90\F8\18\18\18"
MOVE A,CHSET+$708,96

FOR ROW=0TO5
  FOR COL=0TO7
	DP.SCR2+40*(ROW+2*(ROW>2))+COL*2,(2*RAND(6)+$21+RAND(4)*$40)*$101+$100
  N. COL
N. ROW

NS=ROW*COL

'MIN X=48  $30
'MAX X=200 $C8
'MIN Y=16  $10
'MAX Y=100 $64

'make a look up table for direction sines and cosines
DIM TRIGTABLE(1,15)
FOR I=0TO15:TRIGTABLE(0,I)=SIN(D2R*I):TRIGTABLE(1,I)=COS(D2R*I):N.I

REM ARMADA POSITION COARSE AND FIND, DIRECTION, OLD DIRECTION:APC=0:APF=0:AD=0:ADD=0
REM ARMADA VERTICAL AND DIRECTION: YA=0:YD=0
REM FRAME NUMBER AND DIRECTION: FP=0:FD=0

'ship position
X=124:Y=52
'number of lives
NL=3
'clear screen and print score and lives
?CHR$(125);SC,NL
'CLS:?SC,NL

' game loop
REP.
'clear the collision register
POKE 53278,1
' read the fire button
T0=T:T=STRIG(0)
' read the joystick
S0=S:S=STICK(0)
LR=(S&4=4)-(S&8=8):UP=S&1=0:DN=(S0&2=2)&(S&2=0)
F=T0&(T=0)
'rotate the ship
ANGLE=(ANGLE + 16 + LR + 8*DN) MOD 16

'thrust the ship
IF UP
  SOUND 0,99,2,15
  VANG=ANGLE
  'V=V+VA-CD*V
  VX=VX+0.2*TRIGTABLE(0,VANG)
  VY=VY+0.2*TRIGTABLE(1,VANG)
ENDIF
IF (VX<>0)!(VY<>0)
  'drag
  VX=VX-.02*VX
  VY=VY-.02*VY
  'position update
  X=(X+VX -48 + 152) MOD 152 + 48
  Y=(Y-VY -21 + 84) MOD 84 + 21
  MOVE PM+$200,PM+$201,127
ENDIF

'update the player
MOVE PM+ANGLE*5,PM+$200+(Y),5

'fire a missile
IF F&(FC=0)
  FC=21:MA=ANGLE:MX=X+4+3*TRIGTABLE(0,MA):MY=Y+3-3*TRIGTABLE(1,MA)
ENDIF
'animate missile
IF FC
  SOUND 0,99-FC,10,FC DIV 2
  FC=FC-1
  'update missile location
  MX=(MX+2*TRIGTABLE(0,MA) -48 + 152) MOD 152 + 48
  POKE 53252,MX
  POKE PM+$180+(MY),0
  MY=(MY-2*TRIGTABLE(1,MA) -21 + 84) MOD 84 + 21
  POKE PM+$180+(MY),FC>0
ENDIF

'move player horizontally
POKE 53248,(X)

'vsync
PAUSE1
'collision registers
P0PF=PEEK(53252):M0PF=PEEK(53248)

'missile hit
IF M0PF
  SOUND 0,20,0,15
  MROW=(MY-21-3.5*YA)/8
  MCOL=(MX-48-APF)/16
  'clear invader
  DP.SCR2+40*TRUNC(MROW)+2*TRUNC(MCOL),0
  'clear missile state
  FC=0
  'decrement invader counter
  NS=NS-1
  'increase score
  SC=SC+4
  'update score
  ?CHR$(125);SC,NL
  'CLS:?SC,NL
  'clear missile
  POKE PM+$180+(MY),0
  'if no more invaders, exit loop
  IF NS=0:EXIT:ENDIF
ENDIF

'player hit
IF P0PF
  SOUND 0,250,10,15
  'lose a life
  NL=NL-1
  'update scoreboard
  ?CHR$(125);SC,NL
  'clear missile
  IF FC:FC=0: POKE PM+$180+(MY),0:END.
  'start a new ship
  Y=50+YA*4
  VX=0:VY=0
  MOVE PM+$200,PM+$201,127:PAUSE 15
ENDIF

'rotate invader character sets
FP=FP+2*((FD=0)-FD)
FD=(FD&(FP>0))!(FP=6)
'FP FD
'0  0
'2  0
'4  0
'6  1
'4  1
'2  1
'0  0 --- and so forth
POKE 756,$A2+FP

'coarse and fine horizontal scrolling
ADD=AD
IF AD
  'decrement fine counter
  APF=APF-1
  'if roll counter, then coarse move
  IF APF=-1
    M.SCR2,SCRN,358
	APF=15
	APC=APC-1
  ENDIF
  'update fine scroll register
  POKE $D404,APF
  'switch directions if need to
  AD=(APC>0)!(APF>0)
ELSE   
  'increment fine scroll counter
  APF=APF+1
  'if roll counter, then coarse move
  IF APF=16:-M.SCRN,SCR2,358: APF=0: APC=APC+1: ENDIF
  'update fine scroll register
  POKE $D404,APF
  'switch directions if need to
  AD=(APC=1)&(APF=15)
ENDIF

'make bump sound at edge and shift display list for vertical scroll
IF AD-ADD
  SOUND 0,200-AD*30,10,15
  'move blank screen parts around
  POKE DL+47+YA,$70*(YD=1)
  POKE DL+6+YA,$70*(YD=0)
  'update Y position of armada
  YA=YA-YD+(YD=0)
  'change direction at top or bottom
  YD=(YD&(YA>0))!(YA=5)
ENDIF

'turn off beeps
SOUND: rem 0,0,0,0

'end game when out of lives
U.NL=0