# War — Implementation Plan

A production-quality card game example for NEScript. This document is
the living plan: each step is checked off as it's completed, and any
mid-flight design changes are recorded in the "Design revisions" section
at the bottom.

---

## 1. Scope & quality bar

A production-grade NES version of War:

- **Title screen** with a big logo, a 3-option menu (`0 PLAYERS` /
  `1 PLAYER` / `2 PLAYERS`), blinking "PRESS START", and music.
- **Gameplay** with an animated deal, two live decks with running card
  counts, cards that slide between the decks and the center, a readable
  face-up card on each side.
- **War / tie-breaker** flow with a distinct "WAR!" banner, an exciting
  SFX, and buried cards that actually come out of each player's deck.
- **Victory screen** with the winning player highlighted and a fanfare.
- **Sound**: title music, card-flip click, distinct round-win cues for
  A vs B, exciting WAR tie-break sfx, victory fanfare.

---

## 2. File layout

```
examples/war.ne                  # top-level: game decl, palette, background, sprite sheet, includes, start
examples/war/PLAN.md             # this file
examples/war/constants.ne        # gameplay + layout constants
examples/war/audio.ne            # sfx + music declarations
examples/war/state.ne            # global variables (decks, phase, rng, anim timers, ...)
examples/war/rng.ne              # 8-bit LFSR PRNG helpers
examples/war/deck.ne             # queue ops on decks + pot
examples/war/compare.ne          # card rank/suit extraction, round resolver
examples/war/render.ne           # draw_card_front / draw_card_back helpers
examples/war/title_state.ne      # state Title
examples/war/deal_state.ne       # state Deal
examples/war/play_state.ne       # state Playing (phase machine)
examples/war/victory_state.ne    # state Victory
```

The top-level `examples/war.ne` is the file the pre-commit hook and
emulator harness see. It compiles to `examples/war.nes` without any
tooling changes. The split source files all live under `examples/war/`
and are pulled in via `include "war/<name>.ne"` directives.

---

## 3. Hardware budget

### Sprite budget per frame

All sprites use sprite sub-palette 0 (the NEScript codegen hard-wires
the OAM attribute byte to 0). That gives 4 shared colours:
**transparent, red, white, black** — enough for both red and black suits
on a white card face with a black outline.

Cards are **16×24 px (2 cols × 3 rows = 6 sprites)**. Max on-screen
simultaneously in the steady state:

| Entity        | Qty | Sprites | Notes |
|---------------|-----|---------|-------|
| Deck A top    | 1   | 6       | card back |
| Deck B top    | 1   | 6       | card back |
| Face-up A     | 1   | 6       | varies by round |
| Face-up B     | 1   | 6       | varies by round |
| Flying card   | 1   | 6       | only during anim |
| Cursor / UI   | ≤2  | ≤2      | title selector + misc |
|               |     | **32**  | ≤ 64 OAM slots ✓ |

**8-per-scanline check.** Decks are at `y = 80`, face-up cards at
`y = 128`. Those two bands never overlap vertically, so the worst case
is 2 cards side by side within the same band = 4 sprites per scanline.

### Tile budget (max 256)

| Group | Tiles | Purpose |
|-------|-------|---------|
| Card frame | 6 | outline corners + top/bottom blank cells |
| Card back  | 4 | diamond-lattice pattern, symmetric |
| Rank glyphs | 13 | A, 2-9, 10, J, Q, K (8×8 each) |
| Small suit (corner) | 4 | ♠ ♥ ♦ ♣ |
| Big suit left half | 4 | left half of large centre pip |
| Big suit right half | 4 | right half |
| Font A-Z | 26 | title/menu/HUD |
| Font 0-9 | 10 | deck counts + HUD |
| Punctuation + space | ~4 | `:`, `!`, space, `?` |
| Big "WAR" banner | 12 | 2×2 block per letter |
| Felt-table tile | 2 | background fill + subtle pattern |
| Border / divider | 4 | thin framing |
| Cursor / arrow | 2 | menu selection |
| **Total**  | **~95** | leaves ~150 tiles free |

### RAM budget

| Structure | Bytes | Notes |
|-----------|-------|-------|
| `deck_a: u8[52]` | 52 | circular buffer, packed `rank<<4 \| suit` |
| `deck_b: u8[52]` | 52 | |
| `pot: u8[52]`    | 52 | cards currently in play |
| queue cursors, counts, phase, anim timers | ~20 | |
| RNG state | 1 | |
| **Total** | **~180** | well under the ~1700 bytes of general RAM |

---

## 4. Card representation & RNG

### Card encoding

One byte per card, packed as `(rank << 4) | suit`:

- `rank` = 1..13 (`A=1`, `2=2`, …, `10=10`, `J=11`, `Q=12`, `K=13`)
- `suit` = 0..3 (`♠=0`, `♥=1`, `♦=2`, `♣=3`)

### PRNG

8-bit Galois LFSR seeded from the frame counter on title-screen exit.

### Shuffle

Bounded random-swap shuffle: 200 swaps between two 6-bit random
indices, retrying when either index ≥ 52. Uses only `&` and compare,
no multiply or divide.

### Deck as a queue

Each deck is a circular buffer with `front` / `count` cursors. `draw`
bumps `front`, `push_back` writes at `(front + count) % 52`. Modulo is
implemented with `if x >= 52 { x -= 52 }` to avoid the expensive `%`
software routine.

---

## 5. State machine

```
Title → Deal → Playing → Victory → Title
```

`Playing` contains an inner phase machine driven by a `u8` phase var:

| Phase | Notes |
|-------|-------|
| `P_WaitA` | Human: wait for input; CPU: wait for think timer |
| `P_FlyA` | 16-frame lerp of A's card from deck to play position |
| `P_WaitB` | symmetric |
| `P_FlyB` | symmetric |
| `P_Reveal` | Both cards visible; brief beat |
| `P_Resolve` | Compare, branch to `P_WinA` / `P_WinB` / `P_WarBanner` |
| `P_WinA` / `P_WinB` | Cards slide to winner deck |
| `P_WarBanner` | Flash "WAR!" banner, play `WarFlash` |
| `P_WarBury` | Bury 3 face-down cards from each deck |
| `P_Check` | Win-condition check, possibly `transition Victory` |

Game modes (`0`/`1`/`2` players) are captured by two `bool` flags
`a_is_cpu` / `b_is_cpu` chosen on title exit.

---

## 6. Audio

| Name       | Channel | Shape |
|------------|---------|-------|
| `FlipCard` | pulse 1 | short descending click |
| `CheerA`   | pulse 1 | 8-frame rising arpeggio, pitch envelope |
| `CheerB`   | pulse 1 | 8-frame descending arpeggio, pitch envelope |
| `WarFlash` | pulse 1 | 16-frame pitch sweep, loud → soft → loud |
| `ThudDown` | noise | 4-frame noise burst (bury animation) |
| `TitleTheme` | pulse 2 | brisk 4/4 march, looping |
| Victory | builtin `fanfare` | one-shot on win |

---

## 7. Implementation steps

All steps complete. The order they actually shipped in is below
(the original 12-step plan got compressed once the early steps
turned up enough compiler bugs to demand investigation in
parallel).

- [x] Step 1: Skeleton — top-level file, every included file
      filled with a real implementation, compiles cleanly.
- [x] Step 2: Felt background — replaced the builtin-smiley
      grid with a custom `TILE_FELT_BG` cross-hatch tile.
- [x] Step 3: Card art — bold rank glyphs (1 tile each), small
      corner suits, big centre pip halves, card-back lattice,
      card frame helpers (`draw_card_face` / `draw_card_back`).
- [x] Step 4: Deck data structures — circular buffers with
      front/count cursors, packed `(rank << 4) | suit` cards,
      Galois LFSR PRNG, bounded random-swap shuffle, deal/split.
- [x] Step 5: Title state — BIG WAR banner, "CARD GAME"
      subtitle, 3-line menu with cursor, blinking PRESS A,
      title-music + autopilot.
- [x] Step 6: Deal state — animated deal with FlipCard sfx and
      growing deck-back stacks.
- [x] Step 7: Play state phase machine — `match phase` over the
      11 P_* phases, fly animation that doesn't overshoot, win
      cues, debounced human input.
- [x] Step 8: War tie-break — BIG WAR banner reused as a
      strobing flash, ThudDown noise sfx for each buried card,
      pot grows by 6 per side per war.
- [x] Step 9: Victory state — staggered "PLAYER X / WINS"
      banner, top-of-deck showcase card, builtin fanfare,
      auto-return to Title.
- [x] Step 10: Polish pass — bold sprite font, card-fly
      timing/overshoot fix, P_WAR_BURY redraws the previous
      face-ups, draw_word_war removed (was orphaned by the BIG
      WAR helper), title state shares the BIG WAR helper.
- [x] Step 11: Capture goldens, verify all 31 ROMs match, war
      golden lands cleanly on a mid-fly-A frame at frame 180.
- [x] Step 12: Update README.md and examples/README.md.
- [x] Code review pass: read every file end-to-end, fix any
      mistakes found.

Two compiler bugs were discovered along the way and fixed in
the same PR (E0506 too-many-params + the wide_hi map leak in IR
lowering). Three more compiler limitations are documented in
`COMPILER_BUGS.md` with workarounds in the war source for now.

---

## 8. Design revisions

A few things shifted from the approved plan:

- **`arm_fly` is 4 params, not 6.** The 5th and 6th params
  (`fly_card`, `fly_face_up`) are written to globals at every
  call site instead, because the v0.1 ABI silently drops params
  past the 4th. The 4-param limit now produces a clean E0506
  diagnostic so future authors won't trip on it.
- **`card` parameter in `card_rank` / `card_suit` / `draw_card_face`
  was renamed to per-function unique names** to dodge the IR
  lowering's shared-VarId issue documented in
  `COMPILER_BUGS.md` §1b. Same for `wrap52`'s `v` and the
  `push_back_*` helpers. `x` and `y` are still shared because
  they sit at the same parameter position in every helper that
  takes them, so the routed slot mapping is consistent.
- **The `Playing` state's phase machine uses `match`, not a
  flat if-chain.** The if-chain shape allowed two phases to run
  in the same frame after a `set_phase` transition, which made
  the card-fly animation overshoot its endpoint by `FLY_STEP`.
  `match` runs only the first matching arm.
- **Card frame outline tiles (`TILE_FRAME_TL`/`TR`/`BL`/`BR`)
  are still allocated in the Tileset but unused.** The card
  faces use the rank/suit/pip tiles directly with white card
  bodies that visually separate from the dark felt — a card
  frame would have made each tile cramped without much
  readability gain. Constants are kept for layout stability;
  the tiles themselves serve as a 4-tile reserve for future
  art tweaks.
- **The deal animation** is a single bouncing card-back, not a
  full 52-card cascade. Cleaner and cheaper, and the FlipCard
  click rhythm carries the "we're dealing!" feel by itself.