# Map Loader

Loads level data from a Tiled CSV export, replacing the old hardcoded `build_world` wall loop.
Implemented in `sim/src/map.rs`; wired into the world in `sim/src/lib.rs::load_world`.

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## Format

A bare comma-separated tile grid — Tiled's "CSV" layer export, stripped of the surrounding
TMX/XML. No header, no dimensions. The loader derives them from the content:

- **width** = number of fields in the first row
- **height** = number of rows

The current test map lives at `assets/map_test` (30×30, tile ids 25 / 74 / 75 / 146).
A trailing comma per row is tolerated; ragged rows panic (they indicate a corrupt export).

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## Tile flip flags — the "negative numbers"

Tiled packs per-tile flip/rotation into the **top 3 bits** of each 32-bit GID. A flipped tile
therefore shows up in the CSV as a large *negative* decimal when read as `i32`. Example:

```
-2147483574  as u32 = 0x8000004A
                       │       └── 0x4A = 74   ← the real tile id
                       └────────── 0x80000000  = horizontal flip
```

| Bit | Mask         | Meaning             |
|-----|--------------|---------------------|
| 31  | `0x80000000` | horizontal flip     |
| 30  | `0x40000000` | vertical flip       |
| 29  | `0x20000000` | diagonal flip (rot) |

The loader parses each field as `i64`, reinterprets the low 32 bits as `u32`, and strips the
flip flags with `& 0x1FFF_FFFF`, leaving the bare tile id. **Flip orientation is discarded for
now** — flipped tiles render unflipped. Real flipping would need a flip-aware `blit_tile` plus
the flags carried through the chunk palette; deferred to the camera/sprite pass (09).

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## Tile id ↔ tileset

Tile ids are used directly:

- **Sim** — `tile_flags(id)` (in `map.rs`) maps an id to gameplay `TileFlags`
  (collidable / opaque) via the `sim::tile_collidable` vocabulary. Hand-maintained; test values only.
- **Renderer** — the id indexes straight into `overworld.png` (`game.rs`). The tileset's tile
  order *is* the id space; keep the Tiled tileset and `overworld.png` in lockstep.

No `firstgid` subtraction: the authored ids already line up with `overworld.png`.

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## World assembly

`load_world` slices the map into 32×32 chunks via `Chunk::generate`, which builds each chunk's
6-bit palette automatically. The map's top-left tile sits at world (0, 0).

**Edge border:** a 30×30 map only fills part of chunk (0,0) (which spans tiles 0–31). Tiles
inside a loaded chunk but outside the authored map are emitted as an *invisible solid border*
(id 0, `COLLIDABLE`) so the walkable edge sits flush with the visible map rim rather than two
tiles past it at the chunk boundary. Beyond any loaded chunk, movement is blocked anyway by
`sim.rs`'s `map_or(true)` (no chunk = world edge).

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## Not done yet

- Multiple layers (only a single tile layer is read)
- Tile properties from Tiled (collision comes from the hand-maintained `tile_flags`, not the file)
- Object layers (spawns, triggers)
- Tile flipping (orientation discarded — see above)
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