# Patches

This directory holds [pnpm patches](https://pnpm.io/cli/patch) applied on install via
`pnpm.patchedDependencies` in the root `package.json`. Each `.patch` is a diff against the
published package. Most are small and self-explanatory from the diff; the non-obvious ones
are documented below.

---

## `@remix-run/router@1.23.2` — route-matching memoization

**File:** `patches/@remix-run__router@1.23.2.patch` (patches `dist/router.cjs.js`)

### What it does

Four changes to `matchRoutesImpl` / `compilePath`, all derived from work that depends only
on the **static** route manifest:

1. **Cache flattened + ranked branches per route-tree** (`WeakMap` keyed by the `routes`
   ref). `flattenRoutes()` + `rankRouteBranches()` were recomputed on *every* `matchRoutes`
   call across all ~436 webapp routes.
2. **Hoist `decodePath(pathname)` out of the branch-match loop** — it's loop-invariant but
   was recomputed once per branch.
3. **Memoize `compilePath` compiled regexes** by `path|caseSensitive|end` (bounded `Map`,
   cap 2000). The matcher RegExp was rebuilt on every `matchPath` call.
4. **Bucket ranked branches by first static path segment** (`WeakMap` keyed by the branch
   array). Even with (1)–(3), matching was still a linear scan calling `matchPath` on every
   branch until one matched — O(route table) per request, now across 521 route files.
   Branches are indexed by their lowercased leading segment, with one always-considered list
   for branches whose leading segment is dynamic, splat or optional (and for root/pathless
   paths). A request walks only its own bucket merged with that list.

   Ordering is preserved exactly: both lists hold indexes into the already rank-sorted
   branch array and are walked in ascending-index order, so the first match found is the
   same branch the full scan would have found. Bucketing lowercases on both sides, so
   case-insensitive matching still resolves and `caseSensitive: true` routes are still
   rejected by `matchPath` itself. A pathname whose own leading segment can't be bucketed
   falls back to the full scan.

   Verified equivalent to the unpatched matcher over 20,050 pathnames (literal, dynamic,
   splat, optional, case variants, basenames, percent-encoded). The semantics it depends on
   are pinned by `apps/webapp/test/routeMatchingPatch.test.ts`, which asserts matching
   behaviour rather than the optimisation, so it still passes without the patch.

### Why

Profiling the realtime runs feed under load (100 concurrent tag feeds, ~425 req/s) found
**~68% of webapp CPU was spent in react-router's `matchRoutes`** — re-flattening,
re-ranking, and re-compiling the entire route table on every request. It is **not** a dev
artifact: there is no `NODE_ENV` gate, and a `NODE_ENV=production` profile was identical
(67.9% vs 68.3%). The realtime feed's high request rate (each long-poll returns fast and
immediately re-polls) just amplifies a latent per-request cost that large route tables pay
everywhere.

Measured on a single instance, same load, before vs after this patch:

| | before | after |
|---|---|---|
| active CPU (self-time / window) | 28.3s | 18.5s (**−34%**) |
| route-matching self-time | 19.2s | 7.5s (**−61%**) |
| event-loop lag p99 | 322ms | 113ms (**−65%**) |
| idle headroom | 26% | 52% |

The realtime machinery itself (router/hydrate/serialize/diff) was ~0% — the bottleneck was
entirely generic Remix request overhead.

Change (4) was added later, from a CPU audit of the engine-facing worker-action routes
(`apps/webapp/test/bench`). With (1)–(3) already in place, route matching was still 10.4%
of on-CPU time on that path — the residual linear scan rather than any recompilation.
Bucketing took route-matching self-time from 3.6s to 1.3s (**−64%**) over a 90s window at
~300 req/s.

### Upstream status (why we patch instead of upgrade)

This is a known, acknowledged inefficiency, and it is **only partially fixed in React
Router v7** — which we can't adopt without a full Remix 2 → RR7 framework migration.

- [Issue #8653 "Performance issues"](https://github.com/remix-run/react-router/issues/8653)
  reported it (a user with 12k routes, ~67ms per match) and was closed as a dup of the
  route-ranking discussion [remix#4786](https://github.com/remix-run/remix/discussions/4786).
- [PR #14866 "Optimize route matching performance with caching"](https://github.com/remix-run/react-router/pull/14866)
  implemented *exactly this patch* (hoist `decodePath`, cache `compilePath`, cache
  flatten/rank), claiming **~80% route-matching CPU reduction on a 400+ route app**. It was
  **closed, not merged.**
- [PR #14967 "perf: cache flattened/ranked route branches"](https://github.com/remix-run/react-router/pull/14967)
  is the partial fix that *did* ship (in v7): it caches only the branches, threaded via a
  `precomputedBranches` param through the framework's server-runtime (~15% SSR gain). It
  does **not** cache `compilePath` — that regex rebuild remains even on `main`.
  ([PR #14971](https://github.com/remix-run/react-router/pull/14971) added client-side wins.)

The maintainer's reasoning for closing the fuller PR (#14866), verbatim:

> "This is great as a `patch-package` optimization for those who want it, but we are
> actively working on integrating the more performant route-pattern library from Remix 3 so
> we'd rather just do the right 'fix' and ship the new algorithm instead of trying to
> band-aide perf improvements to the existing algorithm which was written with a very
> different set of constraints. Those constraints come from early v6 when it was only
> declarative mode so route trees were defined at render time and thus had to be
> re-flattened/re-ranked/re-compiled every time."

So: the re-compute-everything design is a holdover from early React Router v6 declarative
mode (route trees defined at render time, so recomputing was correct then). The maintainer
**explicitly endorsed patch-package as the interim approach** and is betting on the Remix 3
route-pattern rewrite for the real fix. This patch is that sanctioned stopgap — and it also
includes the `compilePath` cache the merged PR left on the table.

### Safety

Pure memoization of deterministic, internal-only values:

- `flattenRoutes`/`rankRouteBranches` and the compiled regexes depend solely on the static
  route manifest; the cached values are never returned to or mutated by the framework.
- The compiled `RegExp` has no `/g` flag, so `.exec()` carries no cross-call state — safe to
  share under concurrency.
- The branch cache is a `WeakMap` (collected with its route tree); the compile cache is
  bounded at 2000 entries (route patterns are a static set; the cap only guards any dynamic
  `matchPath()` use).
- Targets the **CJS** build (`dist/router.cjs.js`), which the webapp server loads at runtime
  (`@remix-run/router` is not bundled into the server build).

### When to remove

Drop this patch if/when the webapp moves to React Router v7+ (which threads
`precomputedBranches` itself) or the Remix 3 route-pattern matcher lands. Re-profile at that
point — the `compilePath` cache may still be worth keeping since upstream never added it.