* Gameplay details
  - 10 absorbed bullets = 1 homing missile
  - homing missiles are 10x more powerful than regular shot
  - how missiles work:
    - find the closest enemy
    - allocate as many missiles as needed to kill it, or all the missiles
      if there aren't enough to kill it
    - if there are still missiles left over, repeat with the next
      closest enemy
  - when changing polarity, there's about 7-8 frames where your shield
    isn't working and any type of bullet can kill you
  - chain score bonuses:
    - 1 Chain --- 100 points
    - 2 Chain --- 200 points
    - 3 Chain --- 400 points
    - 4 Chain --- 800 points
    - 5 Chain --- 1,600 points
    - 6 Chain --- 3,200 points
    - 7 Chain --- 6,400 points
    - 8 Chain --- 12,800 points
    - Max Chain - 25,600 points
  - 100 points per bullet absorbed

* Ikaruga Stage 1
** Enemy Types
   - Utatsugumi (big popcorn orbs)
     - health: 1
     - points: 20
     - bullets on death: 5
   - Toratsugumi (little popcorns)
     - health: 1
     - points: ?
     - bullets on death: 5
   - Renjyaku (fast bird-like shooters)
     - health: ?
     - points: ?
     - bullets on death: 7
   - Torafuzuku (big laser shooters)
     - health: ?
     - points: ?
     - bullets on death: 32?
   - Ajisashi (train-like thing)
     - points: ?
     - bullets on turret death: 5
     - bullets on death: 32?
   - Eboshidori (boss)
   - Eboshidori's satellites

** Phases
**** intro
ikaruga enters the screen, player controls activate
**** utatsugumi sweeps
6 white from the top-right
6 black from the top-left
6 white
6 black
**** toratsugumi wall
a 3 wide by 6 tall block of white toratsugumi enter from the right and
take up position on the left.  at the same time a 3 wide by 6 tall
block of black toratsugumi enter from the left and take up position on
the right.  each block rests a moment then drifts off screen on the
same side they entered.
**** small toratsugumi sweep
3 white from the right followed by 3 black from the left.
**** interstitial
chapter 1 text appears, ship flies to next section
**** renjyaku attack
3 white renjyaku in triangle formation enter from the top-left corner
and take position on the center-left, about halfway down the
screen. they take a moment to setup, aim at the player, and fire a
direct stream of bullets at them.  3 more white renjyaku do the same,
but from the top-right and take position in center-right. repeat for
black, black, white, black. alternate the side each time. 3 white
renjyaku take position in the top-left, less than one second later 3
black renjyaku take position in the top right.

brief pause, then two triangle formations (so 6 ships total) enter the
top of the screen. from left to right, the color sequence is: black,
black, white, black, white, white.

alternating from top-right/top-left corner, 3 white renjyaku sweep the
screen while shooting straight down. then renjyaku enter in the same
alternating fashion, but instead of sweeping they swap sides
occasionally as they move down the screen. every time they reach a new
side, they shoot straight down.  the color sequence is: white, black,
white, black, white, black.  as soon as all 6 ships have appeared on
screen, 6 black renjyaku enter: 3 on the far-left, 3 on the far-right,
with vertical gaps between forming 2 lines. they fly straight down the
screen and do not fire.

finally, one white triangle formation quickly appears on the left,
shooting straight down. less than a second later, a black formation
does the same on the right.  if not killed quickly, they leave the
screen on their respective sides.
**** small toratsugumi sweep
same as the other
**** utatsugumi wall
the utatsugumi spawn in-place at the top of the screen with a
formation like this:

  BWBW
BWBWBWBW
BWBWBWBW
  BWBW

They rotate around each other and I don't how to really describe it so
just watch gameplay footage.
**** small toratsugumi sweep
same as before, but the next phase starts as this happening
**** torafuzuku attack
black torafuzuku on the left, white on the right.  2 utatsugumi of the
respective color circling each.  each begins by firing arcs of bullets
in clusters of 3 with big gaps between each cluster.  if not killed
quickly enough, they start firing things I can only describe as
"whips".  when the first one is killed, another replaces it
immediately, for a total of 4 torafuzuku.
**** big toratsugumi sweep
left sweep formation:  WWB WBB BWW WWB
right sweep formation: WBB WWB BBW WBB
**** small renjyaku attack
3 black renjyaku aligned horizontally followed by an offset group of 2
white renjyaku.  all firing a barrage immediately.
**** small torafuzuku attack
a single white torafuzuku appears in the top center and immediately
charges and shoots a laser that sweeps from left to right.
**** utatsugumi ring
ring of 12 utatsugumi appear in this formation: BBBWWWBBBWWW. the ring
spins and adjusts diameter in a way I don't yet understand.
**** toratsugumi line
as the utatsugumi ring finshes it's sequence, 2 vertical lines of
toratsugumis move down the screen. left line order: BWBWBWBW. right
line order: WBWBWB.
**** ajisashi attack
just as the toratsugumi line appears, 2 ajisashis appear at the top of
the screen. black on the left, white on the right. the black one moves
vertically to the bottom of the screen and both activate their
turrets. then the black one moves horizontally to the right side, and
the white one moves horizontally to the left side. then the white one
moves to the bottom and then black one moves to the top. they continue
this pattern until they are killed or time out. new ajisashi's spawn
if the first is killed quickly enough.
**** toratsugumi lines
about 7-8 seconds after the ajisashis spawn, another 2 vertical lines
of toratsugumi scroll down the screen. each line has the same color in
this pattern: BWBWBWBWBWBWBW
**** toratsugumi wall
this only appears if you defeat the ajisashis and toratsugumi lines
fast enough. 5 horizontal lines of toratsugumi appear. as they scroll
down the screen, the toratsugumi move either to the left or to the
right, alternating with each line. the first line moves right, the
next left, etc.  seems to be in this formation (the | marks the screen
edge).

   WWWBBB|WWWBBB|WWWBBB ->
<- BBBWWW|BBBWWW|BBBWWW
   WWWBBB|WWWBBB|WWWBBB ->
<- BBBWWW|BBBWWW|BBBWWW
   WWWBBB|WWWBBB|WWWBBB ->
**** boss intro
warning message. boss enters stage. assumes position at top of
screen. 100 second timer appears.
**** boss phase 1
white bullets from sword.  black bullets from black satellites.
**** boss phase 2
black bullets from shield thing.  white bullets from white satellites.
**** boss phase 3
alternating white and black bombs.  white and black bullets from white
and black satellites.
**** boss phase 4
white semi-homing missiles from arm (sword was destroyed in phase 1.)
white and black bullets from white and black satellites.
**** end
boss is either killed or flees after 100 second timeout. battle report
is displayed.


* Sprites

List of sprites needed:
- Ikaruga
- Ikaruga shield circle
- Utatsugumi
- Toratsugumi
- Renjyaku
- Torafuzuku
- Ajisashi
- Eboshidori
- Eboshidori's satellites
- Eboshidori's bombs
- explosion
- Ikaruga shot
- small circular enemy bullets
- medium circular enemy bullets
- large circular white bullet (Eboshidori's final attack)
- tapered enemy bullets (renjyaku fire these first)
- enemy laser
- player homing missile
- level background
- bitmap font
- chain UI
- Ikaruga UI icon
- missile charge UI

* Audio

Make sound effects with sfxr.  See if there's free chiptune style
music that is anywhere close to fitting the mood set by the real
Ikaruga music.

* Time management

I think it's most important to get the intro phases, some
approximation of the renjyaku attack, and the boss battle implemented
to be close enough to the original stage 1.  other enemies and phases
can be cut as needed for time.

Hard to choose if chaining and/or missiles has high priority.
Probably missiles.