nescript/docs/future-work.md

# Future Work

This document catalogs known gaps, incomplete features, and planned improvements
in the NEScript compiler. Items are organized by priority and area.

---

## 1. IR-Based Code Generation

**Status**: The IR pipeline (lowering + optimization) runs during compilation but
its output is discarded. Code generation still works from the AST directly
(`src/codegen/mod.rs`). This means IR-level optimizations have no effect on the
final ROM.

**What exists**:
- `src/ir/mod.rs`: Complete IR type definitions (`IrProgram`, `IrFunction`,
  `IrBasicBlock`, `IrOp`, `IrTerminator`)
- `src/ir/lowering.rs`: AST → IR translation for all statement and expression types
- `src/optimizer/mod.rs`: Constant folding, dead code elimination, strength
  reduction, ZP promotion analysis, function inlining — all operating on IR

**What's needed**:
- A new `src/codegen/ir_codegen.rs` that walks `IrProgram` and emits 6502
  `Instruction` sequences from `IrOp`/`IrTerminator` instead of from AST nodes
- Register allocation strategy for IR temps → A/X/Y/zero-page spill slots
- Replace the `CodeGen::generate(&program)` call in `main.rs` with
  `ir_codegen::generate(&ir_program, &analysis)`
- Once working, delete the AST-based codegen entirely

**IR lowering issues to fix first** (found during code review):
- `ButtonRead` emits `ReadInput` with no destination temp, then uses uninitialized
  temp in the `And` mask operation (`src/ir/lowering.rs:534`). The fix: `ReadInput`
  should store the input byte into a temp, or the lowering should emit
  `LoadVar(t, input_var_id)` after `ReadInput`.
- Logical AND/OR use raw `VarId(self.next_var_id)` for temp storage without
  registering it (`src/ir/lowering.rs:603,637`). Should use `IrTemp` instead.
- Break/continue create unreachable blocks that contain subsequent dead statements
  (`src/ir/lowering.rs:259`). Should either skip lowering after a terminator, or
  the dead code elimination pass should handle it.

**Impact**: Enables all optimizer passes to actually affect output quality.
Currently the optimizer is validated by tests but its results are thrown away.

---

## 2. Codegen Gaps (AST-Based)

These features are parsed and analyzed but produce no 6502 output:

| Feature | Location | Status |
|---------|----------|--------|
| Function calls | `codegen:148` | `Statement::Call` is a no-op |
| Return values | `codegen:148` | `Statement::Return` is a no-op |
| State transitions | `codegen:148` | `Statement::Transition` is a no-op |
| Array indexing | `codegen:199-200` | `LValue::ArrayIndex` assignment is a no-op |
| Array expressions | `codegen:417` | `Expr::ArrayIndex`, `Expr::ArrayLiteral` are no-ops |
| Function call expressions | `codegen:417` | `Expr::Call` returns nothing |
| Scroll | `codegen:151-152` | `Statement::Scroll` is a no-op |
| Load background | `codegen:154-155` | `Statement::LoadBackground` is a no-op |
| Set palette | `codegen:154-155` | `Statement::SetPalette` is a no-op |
| Multiply/divide/modulo | `codegen:450-452` | `BinOp::Mul/Div/Mod` only emit left operand |
| Dynamic shifts | `ir/lowering:575` | Shift amount is hardcoded to 1 |

**Priority fixes for a working multi-state game**:
1. **Function calls**: JSR to function label, pass args via zero-page, return via A
2. **State transitions**: Write state ID to a zero-page variable, jump to dispatcher
3. **Array indexing**: Use X register for index, LDA absolute,X for loads

---

## 3. Sprite Name Resolution

**Status**: `draw SpriteName at: (x, y)` parses the sprite name but codegen
ignores it. All draws use OAM slot 0 with CHR tile index 0 (the built-in smiley).

**What's needed**:
- Track a mapping from sprite name → CHR tile index in the linker
- When a `sprite` declaration provides inline CHR data or `@chr("file.png")`,
  write that data to the CHR ROM at a known tile index
- In codegen, look up the sprite name to get the tile index and write it to
  OAM byte 1 (tile number)
- Support multiple OAM slots: track a `next_oam_slot` counter per frame,
  allocate slots 0..63 as draws are emitted, warn if >64

---

## 4. Multi-OAM Sprite Support

**Status**: Every `draw` writes to the same OAM bytes ($0200-$0203). Only one
sprite is visible at a time.

**What's needed**:
- Frame-level OAM slot allocator: each `draw` gets the next available 4-byte slot
- Counter reset at the start of each frame handler
- Warn at compile time if a frame handler has >64 static draws
- Runtime: clear unused OAM slots to Y=$FE (off-screen) at frame start

---

## 5. State Machine Dispatch

**Status**: The codegen only generates code for the `start` state. Other states
are parsed and analyzed but their frame handlers are never called.

**What's needed**:
- A `current_state` zero-page variable holding the active state index
- A dispatch table at the start of the main loop: load `current_state`, branch to
  the corresponding state's frame handler
- `transition StateName` writes the new state index to `current_state`
- Generate `on_enter` call on transition, `on_exit` call before leaving

---

## 6. Include Directive

**Status**: The `include` keyword is lexed (`KwInclude`) but not parsed or
implemented.

**What's needed**:
- Parser: `include "path.ne"` at top level
- File resolution: relative to the including file's directory
- Circular include detection (track include stack)
- Merge included declarations into the main `Program` AST
- Span tracking: included files need their own `file_id` for error messages

---

## 7. Debug Mode

**Status**: `--debug` flag is accepted by the CLI but has no effect.

**What's needed**:
- `debug.log(expr, ...)`: write values to emulator debug port ($4800)
- `debug.assert(expr)`: emit runtime check, halt on failure
- `debug.overlay(x, y, text)`: render text to a reserved nametable region
- Array bounds checking in debug mode (compare index against array size)
- Frame overrun detection: count cycles per frame, warn if approaching vblank
- All debug code stripped in release mode (already designed: `DebugLog`/`DebugAssert`
  in the `Statement` enum are defined in the spec but not yet in the AST)

---

## 8. Scroll Hardware Writes

**Status**: `scroll(x, y)` is parsed but produces no output.

**What's needed**:
- Write X scroll value to PPU register $2005
- Write Y scroll value to PPU register $2005 (second write)
- Must happen during vblank (inside NMI handler or after `wait_frame`)
- Split-screen scroll requires MMC3 scanline IRQ (`on scanline`)

---

## 9. Asset Pipeline Completion

**Status**: PNG → CHR conversion exists (`src/assets/chr.rs`) but is never called
from the compilation pipeline.

### 9a. Wire `@chr("file.png")` to actual PNG loading
- When a sprite/background declares `chr: @chr("path.png")`, call `png_to_chr()`
  during compilation
- Resolve the path relative to the source file
- Store resulting CHR data in the ROM's CHR section at a known tile index

### 9b. Wire `@binary("file.bin")` to raw file inclusion
- Read the file as raw bytes and include in CHR or PRG ROM

### 9c. Palette extraction from PNG
- `@palette("file.png")`: analyze image colors, map to nearest NES palette entries
- Already have `nearest_nes_color()` in `src/assets/palette.rs`

### 9d. Nametable conversion
- Full 256×240 PNG → 960-byte nametable + 64-byte attribute table
- Tile deduplication (max 256 unique tiles per pattern table)
- Error if >256 unique tiles

---

## 10. Error Message Polish

**Status**: Errors work and render with ariadne, but many error paths use generic
messages.

### Unused error codes
These are defined in `ErrorCode` but never emitted:
- `E0202` — invalid cast
- `E0203` — invalid operation for type
- `E0301` — zero-page overflow
- `E0403` — unreachable state
- `E0505` — multiple start declarations
- `W0101` — expensive multiply/divide operation
- `W0102` — loop without break or wait_frame
- `W0103` — unused variable
- `W0104` — unreachable code after return/break/transition

### Missing validations
- No error for assigning to a `const`
- No error for `break`/`continue` outside a loop
- No warning for variables declared but never read
- No error for `return` with wrong type vs function signature
- No error for calling a function with wrong argument count/types

---

## 11. Scanline IRQ (MMC3)

**Status**: `on scanline(N)` is in the spec and `on_scanline` field exists in
`StateDecl`, but parsing and codegen are not implemented.

**What's needed**:
- Parser: `on scanline(N) { ... }` event handler in state bodies
- MMC3 IRQ setup: write scanline counter to $C000/$C001/$E000/$E001
- IRQ handler generation: branch to the scanline handler code
- Only valid with `mapper: MMC3`

---

## 12. Audio

**Status**: `play`, `start_music`, `stop_music` keywords are lexed but produce
no output. No audio driver exists.

**What's needed**:
- `@sfx("file.nsf")` / `@music("file.ftm")` asset directives
- Audio driver running in the NMI handler (after OAM DMA)
- `play SfxName` → trigger one-shot sound effect
- `start_music TrackName` / `stop_music` → start/stop background music
- FamiTracker export format parsing (complex — consider using existing tools)

---

## 13. Language Features (Post-v0.1)

From the spec's "Reserved for Future Versions" section:

| Feature | Description |
|---------|-------------|
| **Structs** | `struct Vec2 { x: u8, y: u8 }` — composite types with known layout |
| **Enums** | `enum Direction { Up, Down, Left, Right }` — mapped to u8 values |
| **Fixed-point** | `fixed8.8` type for sub-pixel movement |
| **Text/HUD** | Font sheet declarations, layout system for scores/health/menus |
| **Metasprites** | Multi-tile sprite groups with relative positioning |
| **Tilemaps** | Declarative level data with collision queries |
| **SRAM/saves** | Persistent storage declarations for battery-backed save data |
| **NES 2.0** | Extended iNES header format |

---

## 14. Inline Assembly

**Status**: `asm { }` blocks are lexed (`KwAsm`, `AsmBody`) but not parsed or
compiled. The lexer has raw-capture mode for asm content.

**What's needed**:
- Parser: capture asm body text, parse `{variable_name}` substitutions
- Codegen: emit raw 6502 instructions with variable address substitution
- Labels: local to the asm block scope
- `raw asm { }` variant with no substitution or safety checks

---

## 15. Compiler Performance

**Status**: Compilation is fast (<100ms for all examples) but has no benchmarks.

**What's needed**:
- `cargo bench` benchmarks for each pipeline phase
- Regression test: compilation must stay under 500ms for any reasonable project
- Profile-guided optimization of hot paths (lexer, parser)

---

## 16. WASM Build Target

**What's needed**:
- Factor out all file I/O behind a trait (`FileSystem` / `VFS`)
- Core pipeline takes `&str` source → `Vec<u8>` ROM with no filesystem access
- Compile the compiler to WASM for a browser-based IDE
- In-browser NES emulator integration for instant preview

---

## 17. Open Design Questions

From the engineering plan:

1. **Inline asm label syntax**: `.label:` (ca65 style) vs `label:` (generic)?
2. **Debug port address**: $4800 is conventional but not universal. Support
   multiple debug output methods?
3. **OAM allocation strategy**: Sequential allocation vs priority-based with
   automatic sprite cycling for the 8-per-scanline limit?
4. **Error recovery granularity**: How aggressively should the parser recover?
   More recovery = more errors per compile, but risk of cascading false errors.

---

## 18. Missing Assignment Operators

`<<=` and `>>=` (shift-assign) are lexed as `ShiftLeftAssign`/`ShiftRightAssign`
tokens but have no corresponding variants in the `AssignOp` AST enum. They can
never appear in parsed code. Adding them requires:
- New `AssignOp::ShiftLeftAssign` and `AssignOp::ShiftRightAssign` variants
- Parser handling in `parse_assign_or_call` (alongside the other compound ops)
- Codegen: load value, shift, store back

---

## 19. Player 2 Controller

`Player::P1` and `Player::P2` are defined in the AST but marked `#[allow(dead_code)]`.
The parser always produces `ButtonRead(None, ...)` — it never parses `p1.button.X`
or `p2.button.X` syntax. The runtime only reads controller 1 ($4016).

**What's needed**:
- Parser: `p1.button.X` and `p2.button.X` syntax producing `Player::P1`/`P2`
- Runtime: read controller 2 from $4017 into a second zero-page byte
- Codegen: select the correct input byte based on player

---

## 20. Register Allocator

The plan describes `src/codegen/regalloc.rs` for managing A/X/Y allocation, but
no register allocator exists. The current codegen uses A for everything and
spills to zero-page $02 for comparisons. A proper allocator would:
- Track A/X/Y liveness across basic blocks
- Use X for array indexing and loop counters
- Use Y as secondary index
- Spill to zero-page temps only when all three are live

---

## Priority Order

### Recently completed (removed from backlog)

These items were documented as future work but have since been implemented:

- **State machine dispatch** — CMP + BNE + JMP trampoline dispatch table,
  `current_state` in ZP $03, on_enter/on_exit handlers as JSR targets
- **Function call codegen** — JSR to function labels, ZP $04-$07 param passing,
  RTS for returns
- **Break/continue** — loop_stack with JMP to continue/break labels
- **Return** — evaluate expr to A + RTS
- **Transition** — write state index + JMP to main loop
- **Array indexing** — TAX + LDA/STA with ZeroPageX or AbsoluteX
- **Scroll** — PPU $2005 writes (X then Y)
- **Multiply/divide/modulo** — JSR __multiply/__divide with shift-and-add/restoring division
- **Shift left/right** — ASL A / LSR A
- **Multi-OAM sprites** — sequential slot allocation (0-63), reset per frame
- **Const assignment error** — E0203 for assigning to constants
- **Break outside loop error** — E0203 for break/continue without enclosing loop
- **Math routines wired into linker** — gen_multiply/gen_divide included in ROM
- **Sprite name resolution** — sprite declarations map to CHR tile indices,
  draw statements use the correct tile number
- **Inline sprite CHR data** — sprite decls with `chr: [0x..., ...]` work
- **Include directive** — `include "path"` inlines files at parse time,
  with circular include detection
- **Shift-assign operators** — `<<=` and `>>=` work in all contexts
- **Player 2 controller** — `p1.button.X` / `p2.button.X` syntax, P2 input
  read from $4017 into ZP $08
- **Unused variable warning** — W0103 emits for declared-but-never-read
  globals (underscore-prefix silences)
- **Unreachable state warning** — W0104 emits for states not reachable from
  the start state via transitions
- **E0502 "did you mean" suggestions** — undefined variable errors include
  a suggestion for nearby-named symbols
- **debug.log / debug.assert** — parses into `Statement::DebugLog` /
  `Statement::DebugAssert`, codegen emits runtime writes to $4800 when
  `--debug` is set, stripped in release mode
- **--debug CLI flag wired** — threads through `CodeGen::with_debug`
- **IR-based codegen** — `src/codegen/ir_codegen.rs` walks `IrProgram` and
  emits 6502, handling all IR ops (load/store, arithmetic, comparisons,
  arrays, calls, draws, input, control flow). Enabled via `--use-ir` flag;
  all 7 examples compile through it. Default is still AST codegen while
  IR path is experimental.
- **IR lowering bug fixes** — `ReadInput` now has a destination temp,
  `ButtonRead` uses the proper input temp, logical AND/OR use a new
  `emit_move` helper instead of the buggy raw VarId temp storage
- **Asset pipeline @binary / @chr loading** — `resolve_sprites()` reads
  raw binary files and converts PNGs via `png_to_chr()`. Missing files
  are silently skipped (documentation-friendly)

### Remaining priority order

For someone picking up this codebase, the recommended order of work:

1. **Make IR codegen the default** — currently behind `--use-ir`. Once
   it matches AST codegen for all code paths (state dispatch, multi-OAM,
   debug statements), switch over and delete AST codegen.
2. **IR codegen: state dispatch** — currently no main loop / dispatch
   table; IR codegen only generates function bodies. Need to emit the
   state machine dispatch like the AST codegen does.
3. **IR codegen: multi-OAM** — currently all draws use slot 0
4. **Audio** — SFX/music driver
5. **on_scanline for MMC3** — scanline IRQ handlers
6. **Language features** — structs, enums, fixed-point
7. **Register allocator** — proper A/X/Y allocation to replace
   zero-page spills used by the current IR codegen
8. **Inline assembly** — `asm { }` blocks