# Open CAD Studio — File Open & Render Speed Roadmap

This document lists the planned improvements for cutting **file open time**
and **on-screen draw (render) time**. It builds on the already-landed
**Rendering Optimization** work (Phase 1-4); what is left now sits on the
open-time, allocation, and draw-call sides.

Source-scan summary (references):

- File open flow: [`src/io/mod.rs`](src/io/mod.rs)
  (`open_path_with_phase`, `load_file`, `purge_corrupt_entities`).
- Post-open UI work: [`src/app/update.rs:90-192`](src/app/update.rs#L90-L192)
  (`FileOpened` handler — xref resolve, second purge, linetype populate, etc.).
- Derived-cache build: [`src/scene/mod.rs:90-212`](src/scene/mod.rs#L90-L212)
  (`build_derived_caches` — rayon-parallel for hatch / image / mesh).
- Wire tessellation: [`src/scene/mod.rs:1328-1402`](src/scene/mod.rs#L1328-L1402)
  (rayon, zoom-adaptive curve tol).
- Block defn cache: [`src/scene/block_cache.rs:238`](src/scene/block_cache.rs#L238)
  (`build_defn` — single-threaded today; nested expansion is topological).
- Pipeline: [`src/scene/pipeline/mod.rs`](src/scene/pipeline/mod.rs)
  (batched hatch, frustum cull, LOD — Phase 1-4 done).

---

## Phase 1 — File Open Time

**Goal:** measurably halve the wall time between "Open" click and "first
frame" for a 50 MB DWG.

### 1.1 Drop the second `purge_corrupt_entities`

Today `purge_corrupt_entities` runs once on the
[background thread](src/io/mod.rs#L62-L74) and again in the
[`FileOpened` handler after xref resolve](src/app/update.rs#L138-L145).
XREF content already comes from a separate document — fold the purge
**inline** into xref resolve and delete the outer one. On large files
walking `doc.entities()` again is a measurable cost.

**Work:** make `resolve_xrefs` call purge as it merges each xref; remove
[`update.rs:138`](src/app/update.rs#L138).

### 1.2 Move XREF resolution to the background thread

[`resolve_xrefs`](src/app/update.rs#L132-L166) runs on the UI thread today —
large external references freeze the UI. Move it into the
`open_path_with_phase` worker; have `DerivedCaches` carry the resolved-xref
list back. The UI thread only emits log lines.

**New phase tag:** `PHASE_XREFS` (we already have 3 phases; this is the 4th).

### 1.3 Single-pass entity walk (parse + purge + cache planning)

`load_file` → `purge` → `build_derived_caches` does three separate
`entities()` walks. A single pass can produce:

- corrupt-entity detection,
- hatch / image / mesh handle lists,
- AABB accumulation for `world_offset` (currently a separate pass inside
  `compute_world_offset`).

Target: three `O(N)` passes → one.

### 1.4 Memory-mapped file reads (DWG / DXF)

`DwgReader::from_file` / `DxfReader::from_file` likely load the whole file
into RAM with `std::fs::read`. Switching to `memmap2`:

- eliminates the cold-cache read syscall on large files,
- lets the DWG section index be walked on disk (if the acadrust API
  supports it).

**Dependency:** acadrust upstream may need a `from_reader` / `from_slice`
API; add it in our patched fork (`hakanaktt/acadrust`).

### 1.5 Parallelize the acadrust parser (long-term)

acadrust's DWG parser is single-threaded. Section-based parallelism
(header / classes / objects / blocks / entities — independent offsets) is
the biggest unrealized win. Lives in the upstream fork.

**Order:** profile first — is this really the largest slice? Measure with
`puffin`.

### 1.6 Defer raster image decode

[`build_derived_caches`](src/scene/mod.rs#L177-L186) calls
`ImageModel::from_raster_image` for every `RasterImage` entity — pixel
decode happens up front. Wasted if the entity is off-screen. Defer the
decode until **first render** (per-handle lazy `OnceCell`).

### 1.7 File-hash cache (warm re-open)

When re-opening the same file (`(path, mtime, size)` key) keep a disk
snapshot of `CadDocument` + `DerivedCaches` (e.g. `~/.cache/OpenCADStudio/`). Skip
DWG parse entirely. **Win:** most-recently-opened file goes from 1-2 s to
sub-100 ms.

**Risk:** cache invalidation. Stay conservative — load only on exact
`mtime + size` match, otherwise normal parse.

---

## Phase 2 — First-Frame Wire Tessellation

After `FileOpened`, `bump_geometry()` fires; the first frame tessellates
**every** model-space wire. Measurable hitch at ~100 k entities.

### 2.1 Parallelize block-definition build

[`block_cache::build`](src/scene/block_cache.rs#L238) is single-threaded —
the comment says "nested expansion is fiddly". Fix: stratify blocks in
**topological order** (leaf blocks → callers → …) and build each layer in
parallel via rayon. Dependency order is preserved.

### 2.2 Incremental wire cache (delta tessellation)

`bump_geometry()` invalidates the whole wire cache today
([`scene/mod.rs:650`](src/scene/mod.rs#L650)). Edits usually touch 1-2
entities — re-tessellating the whole doc is waste.

**Fix:** wire cache becomes `HashMap<Handle, (entity_version,
Vec<WireModel>)>`. The editing command bumps the version of the affected
handles; the render path re-tessellates only those, reusing the rest.

Also useful on open: any partial cache (e.g. from block defns) can be
re-used.

### 2.3 Progressive first render

On the first frame emit a **coarse**-tol wire pass (e.g. 4× the normal
tol); refine to full tol on the second frame. The user sees *something*
within 16 ms; detail snaps in smoothly afterwards.

### 2.4 Merge the world-offset scan into the single-pass walk

[`compute_world_offset`](src/scene/mod.rs#L128) walks the whole MSPACE
AABB when the header is unreliable. That scan should join the single-pass
walk from 1.3 (we are already iterating `entities()`).

---

## Phase 3 — Per-Frame Render Cost

After Phase 1-4 culling/LOD, what's left is **upload bytes** and **draw
call count**.

### 3.1 Camera-only invalidation: don't re-tessellate

The wire cache key today is `(geometry_epoch, camera_generation)`
([`scene/mod.rs:414`](src/scene/mod.rs#L414)). A camera change should not
force re-tessellation — only zoom-adaptive curve-tol changes need
resampling, and only for curve entities (Arc / Spline / Ellipse). Straight
geometry is camera-invariant.

**Practical:** split the wire cache in two:

- `tess_cache[handle] → WireModel` (rebuild only if tol-invariant content
  changed),
- `frame_visible[handle] → bool` (recomputed per `camera_generation`).

### 3.2 Persistent GPU buffer pool — diff upload

Today every wire GPU buffer is re-uploaded when
[`cached_epoch`](src/scene/pipeline/mod.rs#L101) changes. A persistent
pool — `HashMap<Handle, GpuSlot>` — uploads only the slots that actually
changed. Big win in CAD-edit scenarios.

### 3.3 Single-draw batched wire pipeline (Phase 4-B-style)

Every `WireModel` today costs one draw call plus a bind-group swap. Port
the batched hatch pipeline (`hatch_batched_gpu.rs`) to wires:

- pack all wire vertices into one storage buffer,
- per-instance `(color, pattern_id, lw_px, visibility)` in a side buffer,
- vertex shader pulls instance data via `instance_index`,
- a single `pass.draw(0..V, 0..N)` covers everything.

At 100 k wires that collapses thousands of draw calls into one. If iced
0.14's widget-pipeline limits allow, immediate win.

### 3.4 Hardware instancing for repeated block inserts

When the same block defn is `INSERT`-ed N times (every door / window in
an architectural drawing) each instance currently renders as its own wire
set. Hardware instancing:

- upload the block defn vertex buffer once,
- one 4×4 transform row per Insert in an instance buffer,
- `pass.draw_indexed(0..V, 0..N_instances)`.

Typical architectural DWGs: 10-100× faster.

### 3.5 Glyph-stroke batching

`tessellate.rs` produces one `WireModel` per glyph stroke today — one text
entity = dozens of models. Cache stroke geometry per font once
(`HashMap<(font, glyph), Vec<Point2>>`), then per-text only a transform
matters.

---

## Phase 4 — Allocation & Memory

### 4.1 Swap `HashMap` for `rustc-hash::FxHashMap`

`Handle` is an integer wrapper; the default `SipHash` is overkill.
`FxHashMap` gives 20-40 % in hash-heavy sites (block_cache, hatches /
images / meshes, viewport_wire_cache).

### 4.2 Arena (`bumpalo`) for transient wire vertices

Tessellation allocates millions of small `Vec<Vec3>`s. A bump arena —
single allocation, frame-end reset — kills the per-vertex malloc cost.
`bumpalo` plays well with rayon (per-thread arenas).

### 4.3 `SmallVec` for small collections

`Polyline.vertices`, `Hatch.boundary_paths`, glyph-stroke lists are
typically < 8-16 entries. `SmallVec<[T; 8]>` skips the heap on the common
case.

### 4.4 Compact entity-ID representation

`Handle` is 8 bytes. 100 k entities → 800 KB just in keys. Hot handle
HashSet / HashMap usage can be flattened to `Vec<u32>` indices plus a
single `FxHashMap<Handle, u32>` translation table — cache-friendlier.

---

## Phase 5 — Profiling Infrastructure (prerequisite)

Don't start any of the above **without measuring first**.

### 5.1 Add `puffin` or `tracy` spans

- `io::open_path_with_phase` → `parse`, `purge`, `caches` spans.
- `Scene::wires_for_block` → `block_cache`, `tess`, `sort` spans.
- `Pipeline::prepare` → `upload`, `cull`, `draw` spans.

Gate behind `debug_assertions` or a `--features profile` flag.

### 5.2 Open-time breakdown log

When open completes, push to the command line:

```
Opened "x.dwg" — 84321 entities — parse 1.2s, purge 80ms, caches 340ms, xref 60ms, first frame 210ms
```

Regressions are visible immediately.

### 5.3 Frame-budget HUD

Add a CLI `PERF` toggle (or F12) that shows per-frame breakdown: tess ms,
upload ms, draw ms, GPU wait ms. Makes PR-to-PR comparison trivial.

---

## Priority Order

**Phase 5 first** (profiling) — avoids speculation.

Then, measurement-guided:

1. **Phase 1.1 + 1.2** (cheap, low-risk, certain win).
2. **Phase 1.3 + 1.6** (single-pass + lazy image).
3. **Phase 2.2** (incremental wire cache — wins on both edit and open).
4. **Phase 3.1** (camera-only invalidation — users pan/zoom constantly).
5. **Phase 3.3** (batched wire pipeline) and **Phase 3.4** (instancing) —
   biggest render win, highest complexity.
6. **Phase 1.7** (warm cache) — dramatic UX, but invalidation must be
   correct or it creates nasty bugs.
7. **Phase 1.5** (acadrust parallel parse) — hardest, longest-term; only
   worth it if profiling confirms it is the dominant slice.

## Deliberate non-goals (for now)

- **GPU compute culling:** for orthographic 2D CAD the CPU quadtree is
  enough. Already covered by Phase 1-4.
- **Out-of-core entity streaming:** meaningful for 100 MB+ single files;
  typical Open CAD Studio files are not there yet.
- **Multi-frame async tessellation pipeline:** if 2.3 progressive render
  works cleanly, this isn't needed.