CONST SHOWGR    = $C050
CONST SHOWFULL  = $C052
CONST SHOWPG1   = $C054
CONST SHOWPG2   = $C055
CONST SHOWLORES = $C056

DEF TEXTMODE
  DROP ^SHOWPG1
  DROP ROMCALL(0, 0, 0, 0, $FB39)
END

DEF CPYBUF
;  MEMCPY($0400, $0800, 1016)
END

DEF GRMODE
  DROP ROMCALL(0, 0, 0, 0, $F832)
  DROP ^SHOWGR
  DROP ^SHOWFULL
  CPYBUF
;  DROP ^SHOWPG2
  DROP ^SHOWLORES
END

DEF COLOR(CLR)
  DROP ROMCALL(CLR, 0, 0, 0, $F864)
END

DEF PLOT(X, Y)
  DROP ROMCALL(Y, 0, X, 0, $F800)
END

DEF HLIN(LEFT, RIGHT, VERT)
  ^$2C = RIGHT
  DROP ROMCALL(VERT, 0, LEFT, 0, $F819)
END

DEF VLIN(TOP, BOTTOM, HORZ)
  ^$2D = BOTTOM
  DROP ROMCALL(TOP, 0, HORZ, 0, $F828)
END

DEF LINE(X1, Y1, X2, Y2)
  BYTE DX, DY, DX2, DY2, PP, S
  WORD SX, SY, ERR, DXDY2

  IF X1 < X2
    SX = 1
    DX = X2 - X1
  ELSE
    SX = -1
    DX = X1 - X2
  FIN
  IF Y1 < Y2
    SY = 1
    DY = Y2 - Y1
  ELSE
    SY = -1
    DY = Y1 - Y2
  FIN
  DX2   = DX << 1
  DY2   = DY << 1
  DXDY2 = DX2 - DY2
  IF DX > DY
    ERR = DX
    PP  = X1
    IF SX < 0
      FOR S = PP DOWNTO X2
        IF ERR < 0
          HLIN(S + 1, PP, Y1)
          PP  = S
          ERR = ERR + DXDY2
          Y1  = Y1 + SY
        ELSE
          ERR = ERR - DY2
        FIN
      NEXT
      HLIN(X2, PP, Y2)
    ELSE
      FOR S = PP TO X2
        IF ERR < 0
          HLIN(PP, S - 1, Y1)
          PP  = S
          ERR = ERR + DXDY2
          Y1  = Y1 + SY
        ELSE
          ERR = ERR - DY2
        FIN
      NEXT
      HLIN(PP, X2, Y2)
    FIN
  ELSE
    ERR = -DY
    PP = Y1
    IF SY < 0
      FOR S = PP DOWNTO Y2
        IF ERR >= 0
          VLIN(S + 1, PP, X1)
          PP  = S
          ERR = ERR + DXDY2
          X1  = X1 + SX
        ELSE
          ERR = ERR + DX2
       FIN
      NEXT
      VLIN(Y2, PP, X2)
    ELSE
      FOR S = PP TO Y2
        IF ERR >= 0
          VLIN(PP, S - 1, X1)
          PP  = S
          ERR = ERR + DXDY2
          X1  = X1 + SX
        ELSE
          ERR = ERR + DX2
        FIN
      NEXT
      VLIN(PP, Y2, X2)
    FIN
  FIN
END

DEF RADAR(C)
  BYTE I

  FOR I = 0 TO 38
    COLOR(C)
    LINE(20, 24, I, 0)
    CPYBUF
    COLOR(0)
    LINE(20, 24, I, 0)
  NEXT
  FOR I = 0 TO 46
    COLOR(C)
    LINE(20, 24, 39, I)
    CPYBUF
    COLOR(0)
    LINE(20, 24, 39, I)
  NEXT
  FOR I = 39 DOWNTO 1
    COLOR(C)
    LINE(20, 24, I, 47)
    CPYBUF
    COLOR(0)
    LINE(20, 24, I, 47)
  NEXT
  FOR I = 47 DOWNTO 1
    COLOR(C)
    LINE(20, 24, 0, I)
    CPYBUF
    COLOR(0)
    LINE(20, 24, 0, I)
  NEXT
END

GRMODE
REPEAT
  RADAR(15)
UNTIL ^$C000 > 127
DROP ^$C010
TEXTMODE
DONE