perf(router): make fewer calls per part when building the tree (#195)

<!-- claude-projects-attribution: {"github_login":"dinwwwh"} -->
_Requested by **dinwwwh** · [project
thread](https://claude.ai/code/project/chan_01UcKMz91tMarPV1kWLVgLYE?thread=cmsg_01UcKMz91tMarPV1kWLVgLYE6swgSUM9SyULmBVdRjY2t7)_

## Summary

A cold start builds the tree in V8's interpreter and baseline code,
where each call and each property load goes through a builtin. In the
profile of the first request for a method (PATCH in the REST set, 100
routes), loads and calls are most of the time outside `split`. Two
changes in `expand`'s part loop:

- `decode(part)` became `part.includes('%') ? decode(part) : part`, so a
part without `%` (nearly all of them) skips the call.
- `literals = Math.min(literals, index)` became `if (index < literals)
literals = index`, which drops a global load, a property load and a call
per waiting route and per param.

Stryker showed a gap the second change exposed: with the comparison
mutated to `true`, `literals` moves to the last param, and a late-added
route sharing a param part with the route before it skipped that param.
A case in `late-adds.test.ts` now covers it (`/s/{x}/t/{y}` then
`/s/{x}/t/u`, added after a request).

## Measurements

Callgrind instructions, `--single-threaded`, hash seeds 1 and 2
(averaged), main vs this PR:

| Stage | main | PR | Change |
| --- | --- | --- | --- |
| REST, 3 requests | 3.892M | 3.757M | -3.5% |
| REST, 3 requests on 3 methods | 5.697M | 5.487M | -3.7% |
| REST, 10 requests | 6.615M | 6.452M | -2.5% |
| 100 real nouns, 10 requests | 6.588M | 6.426M | -2.5% |
| RPC (statics only), 10 requests | 420k | 420k | same |
| Router import | | | +6k (+0.4% of the router's share) |

- Size (esbuild `--minify`, gzip -9): 5068 B to 5076 B minified, 2112 B
to 2114 B gzip.
- Steady state: unchanged, since `expand` only runs while building the
tree.

## Checks

- `pnpm run check` green; router tests 73/73 with 100% coverage.
- All fuzzers at 3 seeds: 0 diffs.
- Stryker: 54 survivors (61 on main). The new one, `part.includes("")`,
only makes every part go through `decode`, which checks for `%` itself.

---
_Generated by [Claude
Code](https://claude.ai/code/session_01VVaw7Sutbw6xMTMyDFeNfw)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: dinwwwh

packages/router/src/index.ts

@@ -384,15 +384,17 @@ function expand<T>(node: Node<T>, pending: unknown[], start: number, whole: bool
 
     for (; index < parts.length; index++) {
       const part = parts[index]!
-      let segment: Segment = decode(part)
+      let segment: Segment = part.includes('%') ? decode(part) : part
 
       // Only a part starting with `{` once decoded can be a param. Past its
       // first part, a route waits before one, and before literal text like it,
       // which keeps the first of two that read the same. A route added whole
       // goes after the routes waiting there.
       if (segment.charCodeAt(0) === 123) {
         if (!whole && index > start) {
-          literals = Math.min(literals, index)
+          if (index < literals) {
+            literals = index
+          }
           ;(child.pending ??= []).push(parts, data, params)
           continue next
         }
@@ -408,7 +410,9 @@ function expand<T>(node: Node<T>, pending: unknown[], start: number, whole: bool
         continue
       }
 
-      literals = Math.min(literals, index)
+      if (index < literals) {
+        literals = index
+      }
       // An array literal holds just its items, where a push to `[]` reserves 17.
       if (params === undefined) {
         params = [segment]

packages/router/tests/late-adds.test.ts

@@ -15,6 +15,9 @@ describe('routes added after requests', () => {
     router.add('GET', '/d/{+r}/e', 'duplicate')
     router.add('GET', '/d/f', '/d/f')
     router.add('GET', '/d/{x}', '/d/{x}')
+    // shares its parts up to `t` with the route before it
+    router.add('GET', '/s/{x}/t/{y}', '/s/{x}/t/{y}')
+    router.add('GET', '/s/{x}/t/u', '/s/{x}/t/u')
 
     expectMatches(router, {
       '/a/1/b/2': ['/a/{x}/b/{y}', { x: '1', y: '2' }],
@@ -26,6 +29,8 @@ describe('routes added after requests', () => {
       '/d/f/e': ['/d/{+p}/e', { p: 'f' }],
       '/%64/f': ['/d/f'],
       '/a/1/c': ['/a/{x}/c', { x: '1' }],
+      '/s/1/t/2': ['/s/{x}/t/{y}', { x: '1', y: '2' }],
+      '/s/1/t/u': ['/s/{x}/t/u', { x: '1' }],
     })
   })