Avoid copying the match table for every queued then block (O(N^2) fireRules)

[PDepaula]

Problem

In fireRules, the loop that executes then blocks does

for (node, idAttrs) in thenQueue:
  let matches = nodeToMatches[node]

Table assignment is a deep copy, so each queued then copies the whole match table of its rule. When one pass queues N then blocks of the same rule (one match per entity, the usual "many enemies / particles" shape), the rule's N-entry table is copied N times: O(N²) per fireRules, with each copied entry carrying a full MatchT.

Fix

Index nodeToMatches[node] in place (two lines). The snapshot semantics are unchanged: nodeToMatches is still filled once before any then runs, so a then that mutates matches does not affect the same pass (the "non-deterministic behavior" test still passes).

Evidence

Benchmark: one rule with (id, Pos), (id, Speed), (Global, DeltaTime); then updates Pos. Time per fireRules with N entities, -d:release --gc:orc, Nim 2.2.10:

entities	before	after
100	0.57 ms	0.14 ms
300	3.70 ms	0.27 ms
1000	41.31 ms	0.94 ms
3000	494.58 ms	3.50 ms

benchmark source

import pararules, times, strutils
type
  Id = enum Global
  Attr = enum DeltaTime, Pos, Speed
schema Fact(Id, Attr):
  DeltaTime: float
  Pos: float
  Speed: float
let (initSession, rules) = staticRuleset(Fact, FactMatch):
  rule move(Fact):
    what:
      (id, Pos, pos, then = false)
      (id, Speed, speed, then = false)
      (Global, DeltaTime, dt)
    then:
      session.insert(id, Pos, pos + speed * dt)
proc run(n: int): float =
  var s = initSession(autoFire = false)
  for r in rules.fields: s.add(r)
  for i in 0 ..< n:
    s.insert(i + 1, Pos, 0.0)
    s.insert(i + 1, Speed, 1.0)
  let t0 = epochTime()
  for tick in 0 ..< 20:
    s.insert(Global, DeltaTime, 0.016)
    s.fireRules()
  (epochTime() - t0) / 20 * 1000
for n in [100, 300, 1000, 3000]:
  echo n, " entities: ", run(n).formatFloat(ffDecimal, 2), " ms per fireRules"

Found while profiling a Vampire Survivors–style game on pararules (https://github.com/PDepaula/parasurvivors) where 300 enemies each have a per-entity movement rule: fireRules was ~83% of the frame in nimprof, all in this loop. That project currently ships this change via patchFile; with it upstream the workaround goes away.

nimble test passes.

🤖 Generated with Claude Code