10-liner: 1nvas0r

This is a little game for the 2022 NOMAM's BASIC 10-liners Contest. This program fits in the EXTREM-256 category, and it was written using FastBasic 4.5.2 for the 8-bits ATARI XL/XE. Development started on 2021-09-12, and it took 3+1+2 days. The final version's date is 2022-02-22.

Description

One invader, one button, one shoot, one life. Keep the Earth safe from a lot of single invaders.

Instructions

Development of the game

I was connected live to the ZeroPage Homebrew show some months ago during the presentation of some new A8 homebrew games. One of the games was 1nvader, a 2021 port for the Atari XL/XE by Ken Jennings of a game for the C64 by Darren Foulds in 2019.

During the show, interacting through the chat, I said that the game concept could be implemented as a BASIC tenliner, and some other people agreed, and joked about me releasing it in a couple of days, and that it could be released for 2022 contest. I took the challenge and wrote a prototype using FastBasic. After posting about this proto in Ken's thread at AtariAge and getting some replies, I decided to go for the full game. At that moment, the abbreviated source code for the proto had 5 lines for PUR-120 category of the contest. Extending the code to support two players using a different method to speed up things, the code increased to 12 lines.

As it was not possible to reduce the size of the source code, I decided to move it to the next category: EXTREM-256. That gave me almost 5 extra lines, where I could add code for the mountains and the starts. At the third day, the game was functional and very similar to the C64 version (not too colorful like the A8 one), but I added some gameplay improvements to make it more challenging, and it used only 8 lines of code. Then I forgot this project, waiting for the call for the contest.

When the 2020 rules were published about 5 month later, it was time to check what was done. I didn't remembered a line of code. I opened the latest version in a text editor and found that only half of the code had comments. That was better than no comments at all. In order to add the remaining comments, I had to check and test what was each undocumented variable for. During this stage I found some bugs, which I could fix in a couple of minutes. I also did some fine tunning to the playfield, to make the game more enjoyable.

The final game, including a simple title screen, uses only 8 lines of FastBasic code. I could use the 2 remaining lines of code to improve the title screen, but I prefered to keep it simple.

The name 1NVAS0R was selected because it is the Spanish word for "invader", but written with a "10" for tenliner.

Download and try

Get the INVASOR.ATR file and set it as drive 1 in a real Atari (or emulator). Turn the computer on and the game should start after the loading completes. A joystick in port 1 is required.

NOTE: This game is for PAL computers. The ATR includes "NTSC.XEX" to be played on NTSC computers. I will run a bit faster than normal, but you will get the right colors.

The code

	1NVAS0R Tenliner port of 1NVADER (C64) (C) 2021-2022 Víctor Parada 2022-02-22
graphics 8	Short way to clean 8K at the top of the RAM
graphics 17	Sets graphics mode 1 (ANTIC 6 mode) without text window
pmgraphics 1	Sets P/M graphics as single line resolution
	Data arrays for DLI. 23 bytes required for each one, as there are 23 interrupts. NOTE: Contained data is used for game initialization, then reused. This won't work in a ROM version of the game.
data x() byte="{binary data}"	Invader position
data y() byte="{binary data}"	Left cannon position
data z() byte="{binary data}"	Right cannon position
data w() byte="{binary data}"	Explosion position
dli set l = z into $D002, x into $D000, y into $D001, w into $D003
a=pmadr(0)+48 move adr(x)+9,a+168,8	Sets up the cannon in P0
move adr(x)+1,a,8 move a,a+8,160	Sets up the bullets into P0
move $E3F0,a-16,8	Sets the score pointer in P0
move adr(x)+17,a+512,8 move a+512,a+520,168	Sets up the mothership in P2
move adr(y)+1,a+768,8 move a+768,a+776,168	Sets up the explosion in P3
a=pmadr(-4) move $E000,a,512 move adr(z)+1,a+8,48 poke 756,a/256	Sets up the character set
move adr(y)+9,704,8	Sets the colors
d=dpeek(88) move adr(y)+17,d+166,7 move adr(w)+1,d+240,20	Prints the title screen
mset dpeek(560)+6,23,$86	Enables DLI
dim c(23)	Stores current position of the shining starts
data md() = 1,-1	Direction
o=0	High score
do	Main loop
repeat a=peek(53279) until a<7	Wait for a console key
p=a=5	SELECT for 2 players (P=1), otherwise only 1 player (P=0)
for a=0 to 22	Initializes P/M horizontal position arrays for DLI
z(a)=9	Invader
x(a)=0	Left cannon
y(a)=0	Right cannon
w(a)=0	Explosion
c(a)=d+40 next	Stars
dli l	Activates DLI
mset d,380,0 move adr("{binary data}")+1,d+380,100	Set ups the screen, mountains with a clear sky
if o then exec hs position 11,0 print #6,"{binary data}"	Shows high score
s=1+p	Invader speed
b=10	Current invaders count for current round
bb=b	Invaders max count for the next round
h=0	Invader height
exec si	Invader direction
x(21)=104 u=1	Left cannon setup
x(22)=88	Left score pointer (P/M horizontal position wraps from the bottom of the screen)
if p	2nd player?
move pmadr(0),pmadr(1),256	Right cannon setup
y(21)=144	Sets its position at the right of the left cannon
v=-1	It starts moving to the left
y(22)=200	Enables right score pointer
rr=9	Min height of previous shot for a new shooting is higher than in one player mode
else	No second player, so
mset pmadr(1),256,0	No right cannon
y(21)=200	Sets the limits for the left cannon at the right side
v=0	Right cannon does not move
y(22)=0	No right score pointer
rr=15 endif	Min height of previous shot for a new shooting is lower than in two players mode
t=0	Timer
i=0 q=0 j=0 r=0	Shoot
e=0 g=0	Explosion
m=0 n=0	Score
f=1	Game over flag
poke $D01E,0	Clear collisions
while f or e	Game loop Repeat until one of the players is dead (and the end of the last explosion)
inc t pause
if t&1	Moves the cannons, only one on each iteration
a=x(21)+u if a>55 and a+6<y(21)	Checks if the next position of the left cannon is within the left border and the right cannon position
x(21)=a else	Updates left cannon position
u=-u endif	Changes left cannon direction
elif p	Is the 2nd player enabled?
a=y(21)+v if a>x(21)+6 and a+6<200	Checks if the next position of the right cannon is within the left cannon position and the right border
y(21)=a else	Updates right cannon position
v=-v endif endif	Changes right cannon direction
if e	Explosion?
poke 707,e*17	Modifies explosion color, every time a darker one
dec e	Updates the explosion timer
sound 3,40-e,8,e	Explosion sound
if e=0 then w(g)=9	Removes the explosion image when animation is complete
else a=z(h)+w	Move the ship
if a>217 or a<32	Out of bounds?
z(h+1)=z(h) z(h)=9 inc h	Move to next row
w=-w	Changes direction
else z(h)=a endif endif	Moves horizontally
a=t&7	Twinkle stars
poke c(a),0	Removes a star from its previous position
c(a)=d+40+rand(340)	Chooses a new position for the star
poke c(a),65+64*rand(3)	Displays a star in a random color
if strig(0)	Enables shoot for left player
q=1	Allows next shoot when the trigger is released
elif i<rr and q	Is the current shoot over the limit and the trigger was just pressed?
q=0	Blocks the next shoot until the trigger is released
if i<21 then x(i)=0	Removes previous shoot from screen
i=21	Enables the shoot at the cannon height
u=-u endif	Changes the cannon moving direction
if strig(1)	Enables shoot for right player
r=1	Allows next shoot when the trigger is released
elif j<rr and r and p	Removes previous shoot from screen
r=0	Blocks the next shoot until the trigger is released
if j<21 then y(j)=0	Enables the shoot at the cannon height
j=21	Enables the shoot at the cannon height
v=-v endif	Changes the cannon moving direction
k=peek($D00E)&3 if k	Checks for collision detection between the ship and the bullets or cannons
e=15	Starts an explosion
g=h	Sets the height of the explosion
if h=21	Is the explosion in the bottom of the playfield?
f=0	Sets the end of game flag
if k=1	Checks which cannon exploded
w(h)=x(h) x(h)=0 else	Removes left cannon and puts an explosion there
w(h)=y(h) y(h)=0 endif else	Removes right cannon and puts an explosion there
if k&1	Checks which cannon hit the ship
exec sc m,4	Left cannon scored
m=a	Updates right score
if i<21 then x(i)=0 endif if k&2	Removes the laser
exec sc n,18	Left cannon scored
n=a	Updates right score
if j<21 then y(j)=0 endif	Removes the laser
w(h)=z(h) z(h)=9	Replaces the ship with an explosion
h=h-(h>1)-(h>0)	Sets the new height of the ship two layers far (except in the top ones)
dec b	Decreases the number of remainin ships for the round
if b=0	End of round?
if s<4+p	Checks for maximum speed
inc s else	Not yet, increases speed
s=1+p	Slows down
bb=bb-(bb>1)	Decreases the number of targets for the next round
rr=rr-(rr>2) endif	Raises the height of current bullet for a new shoot
b=bb endif	Sets remaining ships for next round
exec si endif	Sets up the new invader
poke $D01E,0 endif	Clear collisions
poke 77,0	Disables attract mode
if t&1	Move one bullet only on every iteration
if i sound 0,100,12,i/2	Move left bullet
x(i-1)=x(i)	Copies the bullet horizontal position from the previous height or from its cannon
if i<21 then x(i)=0	Removes the bullet if it was already on screen
dec i else	Updates bullet height
sound 0 x(0)=0 endif else	No left bullet
if j sound 1,108,12,j/2	Move right bullet
y(j-1)=y(j)	Copies the bullet horizontal position from the previous height or from its cannon
if j<21 then y(j)=0	Removes the bullet if it was already on screen
dec j else	Updates bullet height
sound 1 y(0)=0 endif endif	No right bullet
wend
sound	Shuts off pending sounds
move adr("{binary data}")+1,d+244,11	Game Over message
loop
proc si	Set invader
a=rand(2)	Chooses side
z(h)=32+a*180	Puts the invader at the chosen side
w=s*md(a) endproc	Assigns the moving direction according to the side
proc sc a aa	Updates a score, returns new score in A
if h	Where was the ship?
a=a+2*(21-h) else	Score depends on the height of the ship
a=a+100 endif	Top row has a bonus
if a>9999 then a=9999	Limits the score to 99990
position aa-len(str$(a)),0 print #6,a;"0"	Displays the score for the current player
if a>o o=a exec hs endif endproc	New record?
proc hs position 11-len(str$(o)),0 print #6,o endproc	Displays high score (without trailing "0H")

	When the title scren appears, press START to begin in single player mode, or SELECT for two players mode.
	In single player mode, you cannon moves from side to side, bouncing on the edges. An spaceship will appear from one side at the top. Each time it reaches the other side, it will descend one step and cross back to the other side. You can change the moving direction by shooting.
	Shoot with your cannon to destroy the ship. If you think your shot won't reach the invader, you can shoot again only if your bullet has already travelled a protion of the space. When you success, another ship will appear two steps higher.
	Each time you hit and destroy an invader, your score will increase. The higher is the ship, the higher are the earned points. If you hit at the top, you will get a bonus.
	When some amount of invaders have been destroyed, the following ones will go faster. After some waves of increasing speed, a slow wave will continue, but the number of invaders for the next increase will be less than the previous, and the delay for a repeated shooting will be increased.
	If the invader reaches the earth and hits you, he won and the game is over. Press START key to protect the earth from another invasion. Press SELECT key to keep the earth safe with a companion.
	In two players mode, both cannons share the same space, and they bounce between them.
	Both can cooperate to avoid the invasion, or compete to see which one scores better.
	The game is over when the spaceship reaches the earth and destroys any of the cannons. Press SELECT to start again in two players mode, or press START for a single player.