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SirRoboGarage/common_libs/gun_harness/virtual_bullets.nim
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SirStone 57b2ac3849 feat(guns): scale-aware power selection (+52% damage); TM classifier gun built, measured, DISABLED
TASK 2 - power selection, a clear win. bestPower used an ABSOLUTE
MinHitRate = 0.40 bar. Measured per-bin virtual rates (rolling-100 fraction)
show no bin ever clears 40%, so 11 of 14 guns were stuck at bin 0 (power 1.0)
even where higher bins were comparable:
  Linear  p1.0 44% p1.5 39% p2.0 30% p3.0 29%   old bin 0 -> new bin 3
  Accel   p1.0 44% p1.5 40% p2.0 26% p3.0 29%   old bin 1 -> new bin 3
  Pattern p1.0 50% p1.5 40% p2.0 27% p3.0 12%   old bin 1 -> new bin 2
Replaced with a scale-aware PowerBarFrac = 0.50 (a dimensionless FRACTION of
the gun's own best bin rate). 13 of 14 selections now pick heavier bullets.
Real effect vs DrussGT (8 rounds x 3 runs): hit rate unchanged (7.56% ->
7.47%) but damage dealt +52% (157 -> 239 per run) and rounds end faster.
Same accuracy, half the shots, half again more damage.

TASK 1 - the TM pattern-classifier gun does NOT earn its slot. It was built as
a mixture of experts with a corrected-Granmo TM as a multi-class gate over
HeadOn/Linear/Circular/WallBounce/Accel, labelled by which expert's prediction
was closest to the actual enemy position (an exact, supervised, per-shot
label - no delayed credit). Offline it loses to the best of its OWN experts on
essentially every fixture, and against DrussGT it cost real performance:
  baseline (path+relative)  7.56% real hit rate, damage 157
  + power fix               7.47%,                 damage 239
  + power fix + TM gun      5.59%,                 damage 133
The gun was selected on 806 ticks and fired 24 real shots at 4.2%.
So the tree ships with EnableTmSelector = false: code and wiring kept intact
for re-enabling, but it is not in the active rack.

Worth recording from the clause dump: the gate DOES latch onto meaningful
structure. On energy-threshold-turner, HeadOn's clauses key on the energy bits
(the rule's own driving variable) while Circular keys on distance/velocity. So
the TM is learning something real and interpretable - it simply cannot beat
'always pick the best expert'. Root cause (INFERRED): the closest-expert label
is noisy because several experts are near-tied, and under the path metric the
winner varies by power bin while the gate sees one shared per-tick input, so a
one-vs-rest gate over a saturated 870-bit clause space has no margin to exploit.
(Zero-padding the 2-frame window was tried first and saturated every clause at
256-755 included literals; alternating the two real frames fixed that.)

Also factors the corrected feedback into an exported tmLearnDir and exports the
encoding/TM primitives; the Tsetlin tests still reproduce the documented
mean=13.8 included literals, so the refactor is behaviour-preserving.

Verified: 33/33 guard checks, tsetlin tests green, metric checks green, new
power-selection guard green (13/14 selections change; relative bar still picks
bin 1 and not bin 3 for a [30,25,12,5]% profile), 12/12 offline==online
acceptance under the shipped default.
2026-09-21 05:19:07 +02:00

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## Virtual bullet tracker.
## Spawns virtual bullets per gun×power bin every tick (no real firing).
## Resolves by travel distance. Rolling window fitness per gun×power.
## Calls onResult() on the owning gun when a bullet resolves.
import std/math
import std/tables
import std/random
import std/algorithm
import std/os
import std/strutils
import gun_interface
const
PowerBins* = [1.0, 1.5, 2.0, 3.0] ## 4 bins; ponytail: fixed array, add runtime config if needed
WindowSize* = 100 ## rolling window ticks for fitness
MaxBullets* = 8192 ## hard cap; ring buffer. 52 spawns/tick and a
## full-map long shot (~90 ticks) need ~4700 slots;
## 8192 wraps only after ~157 ticks. Each VirtualBullet
## is ~120 bytes, so this array costs ~960 KiB.
MinHitRate* = 0.40 ## LEGACY absolute bar; no longer used by bestPower
## (no bin on the live path-metric scale cleared it, so
## once every bin had data bestPower fell to power 1.0).
PowerBarFrac* = 0.50 ## RELATIVE power bar (dimensionless): a bin is
## acceptable when its virtual hit rate is at least this
## FRACTION of the same gun's best bin rate. Scales with
## the metric instead of assuming a ~40% hit rate.
MinObsBeforeCompete* = 50 ## min observations before a gun×bin enters competition
TieMargin* = 0.02 ## ABSOLUTE mode: guns within this hit-rate margin of best are tied
MinHitRateFloor* = 0.10 ## ABSOLUTE mode: if best gun < this, fall back to gun 0 (HeadOn)
RelTieMargin* = 0.20 ## RELATIVE mode: tied if rate >= bestRate*(1-this). Dimensionless
## fraction of the best rate, so it scales with the metric.
FloorPeakFrac* = 0.25 ## RELATIVE mode: floor fires if bestRate < this*peakRateRef.
## Dimensionless: only if the field collapsed vs its own recent best.
SelectorWindow* = 256 ## ticks of per-tick bestRate kept for the RELATIVE floor reference
MetricEnvVar* = "GUN_VBULLET_METRIC"
## RUNTIME switch selecting how a virtual bullet is scored. Read once per
## process at module init, so the SAME compiled binary can be A/B'd by
## exporting it — no rebuild needed. Both the live ModularBot tracker and
## the offline range replay call `initTracker`, so they always agree.
type
GunId* = int ## index into the guns seq
BulletMetric* = enum
bmPoint ## A bullet is scored at the single point it reaches at the
## fire-time aim distance. HIT iff that point is within BotRadius of
## the target on that tick. Measures prediction accuracy (does the
## bullet arrive at the predicted point at the right time).
bmPath ## DEFAULT. The bullet flies along its straight ray until it leaves
## the arena. Each tick the swept segment (previous -> new position)
## is tested against the target's radius; HIT iff ANY segment came
## within BotRadius. Measures hypothetical hit chance against the
## target's real path. Chosen by the DrussGT A/B: 7.2-7.6% real hit
## rate vs 3.8% for point (p<0.0001).
const DefaultMetric* = bmPath
## Shipped virtual-bullet scoring model. `GUN_VBULLET_METRIC` overrides it at
## runtime; an unset OR empty value means this default.
proc parseMetric*(value: string): BulletMetric =
## Parse a `GUN_VBULLET_METRIC` value. Empty / unknown values fall back to
## the shipped `DefaultMetric` and emit a one-line warning on stderr, so a
## typo can never silently change the metric and a bad value can never take
## the bot down.
case value.strip().toLowerAscii()
of "", "default": DefaultMetric
of "point", "points", "bmpoint": bmPoint
of "path", "paths", "bmpath": bmPath
else:
stderr.writeLine("[gun_harness] unknown " & MetricEnvVar & "='" & value &
"'; falling back to '" & $DefaultMetric & "' (valid: point|path)")
DefaultMetric
let ActiveMetric* = parseMetric(getEnv(MetricEnvVar, ""))
## The metric every tracker uses unless a caller overrides it explicitly in
## `initTracker`. Frozen at process start from the environment.
const SelectorModeEnvVar* = "GUN_SELECTOR_MODE"
## RUNTIME switch selecting the selection-threshold model. Read once per
## process, so one binary can A/B both (§ virtual_bullets).
type
SelectorMode* = enum
smAbsolute ## legacy: fixed 2pp tie band + 10% absolute floor. Correct only
## if the virtual hit-rate scale happens to land near 10%.
smRelative ## scale-aware: tie band is a fraction of the best rate; the floor
## fires only when the field has collapsed vs its own recent peak.
proc parseSelectorMode*(value: string): SelectorMode =
## Empty / unknown values fall back to the shipped `relative` model and warn.
case value.strip().toLowerAscii()
of "", "relative", "rel": smRelative
of "absolute", "abs", "legacy": smAbsolute
else:
stderr.writeLine("[gun_harness] unknown " & SelectorModeEnvVar & "='" & value &
"'; falling back to 'relative' (valid: absolute|relative)")
smRelative
let ActiveSelectorMode* = parseSelectorMode(getEnv(SelectorModeEnvVar, "relative"))
type
VirtualBullet* = object
gunId*: GunId
powerBin*: int ## index into PowerBins
targetId*: int ## enemy bot ID this bullet was aimed at
fireTick*: int ## tick this bullet was spawned; lets a gun pair its
## predict() trace with the exact resolution event
fireX*, fireY*: float
aimX*, aimY*: float ## predicted target (absolute)
bulletSpeed*: float
travelDist*: float ## accumulated px so far
fireDist*: float ## distance to target at fire time
active*: bool
# --- path-metric bookkeeping (unused by the point metric) ---
hitSeen*: bool ## a swept segment already touched the target
bestMissDist*: float ## closest segment->target distance seen so far
bestMissX*: float ## target position at that closest approach
bestMissY*: float
FitnessWindow* = object
## Ring buffer of hit booleans.
hits*: array[WindowSize, bool]
count*: int ## total samples so far (capped at WindowSize for rate)
head*: int
GunFitness* = object
bins*: array[len(PowerBins), FitnessWindow]
SelectorDiag* = object
## Optional observability for `chooseFromFit`/`bestGun`. Never needed by the
## bot; lets the offline range report WHY a gun was selected (floor vs tie).
bestRate*: float ## max hit rate over eligible guns (the floor comparison value)
floorFired*: bool ## bestRate below the active floor -> returned gun 0
tiedCount*: int ## eligible guns within the tie band of bestRate (0 if floor fired)
anyQualifies*: bool ## at least one gun reached MinObsBeforeCompete
floorRate*: float ## the floor actually applied this tick
VirtualTracker* = object
bullets*: array[MaxBullets, VirtualBullet]
head*: int ## ring buffer head
numGuns*: int
metric*: BulletMetric ## scoring model (defaults to ActiveMetric)
fitness*: Table[int, seq[GunFitness]] ## keyed by enemy bot ID, indexed by GunId
droppedBullets*: int ## unresolved bullets clobbered by the ring buffer (should stay 0)
# RELATIVE-mode floor reference: per-tick bestRate history + its running max.
rateHist*: array[SelectorWindow, float]
rateHistHead*: int
rateHistCount*: int
peakRateRef*: float ## max bestRate in rateHist; 0.0 = not enough history yet
proc initTracker*(numGuns: int, metric = ActiveMetric): VirtualTracker =
## `metric` defaults to the process-wide `GUN_VBULLET_METRIC` switch; pass it
## explicitly only from tests that need both models in one process.
result.numGuns = numGuns
result.metric = metric
proc hitRate*(fw: FitnessWindow): float =
## Returns fraction of hits in the rolling window. 0.0 when no data.
if fw.count == 0: return 0.0
let n = min(fw.count, WindowSize)
var h = 0
for i in 0..<n: h += (if fw.hits[i]: 1 else: 0)
result = h.float / n.float
proc record(fw: var FitnessWindow, hit: bool) =
fw.hits[fw.head] = hit
fw.head = (fw.head + 1) mod WindowSize
inc fw.count
proc spawnBullets*(t: var VirtualTracker, gunId: GunId,
predictions: array[len(PowerBins), GunPrediction],
state: WorldState, targetId: int) =
## Call once per gun per tick with predictions for all power bins.
## Lazily creates fitness entry for targetId on first spawn.
if targetId notin t.fitness:
t.fitness[targetId] = newSeq[GunFitness](t.numGuns)
for binIdx in 0..<len(PowerBins):
let power = PowerBins[binIdx]
let speed = bulletSpeed(power)
let pred = predictions[binIdx]
let fireDist = hypot(pred.x - state.selfX, pred.y - state.selfY)
let slot = t.head mod MaxBullets
# Measurement integrity: if the slot we are about to overwrite still holds an
# unresolved bullet, that bullet will never be scored. Count it instead of
# silently dropping it (non-zero after a battle means MaxBullets is too small).
if t.bullets[slot].active:
inc t.droppedBullets
t.bullets[slot] = VirtualBullet(
gunId: gunId,
powerBin: binIdx,
targetId: targetId,
fireTick: state.tick,
fireX: state.selfX,
fireY: state.selfY,
aimX: pred.x,
aimY: pred.y,
bulletSpeed: speed,
travelDist: 0.0,
fireDist: fireDist,
active: true,
hitSeen: false,
bestMissDist: Inf,
bestMissX: 0.0,
bestMissY: 0.0,
)
t.head = (t.head + 1) mod MaxBullets
const StaleTicks* = 20 ## discard bullet if target not seen within this many ticks
proc distPointToSegment*(px, py, ax, ay, bx, by: float): float =
## Shortest distance from point P to the segment A-B (A/B are bullet
## positions on consecutive ticks).
let abx = bx - ax
let aby = by - ay
let abLen2 = abx*abx + aby*aby
var s = 0.0
if abLen2 > 1e-12:
s = clamp(((px - ax)*abx + (py - ay)*aby) / abLen2, 0.0, 1.0)
hypot(px - (ax + s*abx), py - (ay + s*aby))
# ── rate helpers (shared by the selector and the reference tracker) ──────────
proc gunEligible*(fit: GunFitness, requireMin: bool): bool =
## True if a gun has >= MinObsBeforeCompete samples in at least one bin (or
## when `requireMin` is false, every gun is eligible).
if not requireMin: return true
for binIdx in 0..<len(PowerBins):
if fit.bins[binIdx].count >= MinObsBeforeCompete: return true
false
proc gunRate*(fit: GunFitness, pooled: bool): float =
## A gun's hit rate. `pooled` sums hits/shots across all power bins (more
## samples, immune to one lucky bin); otherwise the max single-bin rate.
if pooled:
var h, n = 0
for binIdx in 0..<len(PowerBins):
let fw = fit.bins[binIdx]
let m = min(fw.count, WindowSize)
n += m
for k in 0..<m:
if fw.hits[k]: inc h
result = if n > 0: h.float / n.float else: 0.0
else:
result = 0.0
for binIdx in 0..<len(PowerBins):
result = max(result, fit.bins[binIdx].hitRate())
proc tableBestRate(t: VirtualTracker, pooled: bool): float =
## Best eligible gun rate across every target's fitness (no merge/allocation).
## Only guns with >= MinObsBeforeCompete samples count: under-sampled bins
## produce 100%/-looking spikes that would inflate the floor reference and
## force HeadOn for the whole window. Zero when nothing is warmed up yet.
result = 0.0
for _, perEnemy in t.fitness:
for gunId in 0..<perEnemy.len:
if not gunEligible(perEnemy[gunId], true): continue
result = max(result, gunRate(perEnemy[gunId], pooled))
proc noteBestRate*(t: var VirtualTracker) =
## Record this tick's field-best rate and refresh the RELATIVE-mode floor
## reference: the max bestRate over the last SelectorWindow ticks. Call once
## per selection tick; the live loop does this from `tickBullets`.
let best = tableBestRate(t, pooled = true)
t.rateHist[t.rateHistHead] = best
t.rateHistHead = (t.rateHistHead + 1) mod SelectorWindow
if t.rateHistCount < SelectorWindow: inc t.rateHistCount
var peak = 0.0
for i in 0..<t.rateHistCount:
if t.rateHist[i] > peak: peak = t.rateHist[i]
t.peakRateRef = peak
proc tickBullets*(t: var VirtualTracker, state: WorldState,
enemies: Table[int, tuple[x, y: float, lastSeenTick: int, alive: bool]],
onResolved: proc(gunId: GunId, binIdx: int, e: FeedbackEvent)) =
## Advance all active bullets one tick.
##
## The `bmPoint` branch (default) is unchanged: resolve when the bullet
## reaches the fire-time aim distance and score the single point it lands on.
##
## The `bmPath` branch flies the bullet along its ray until it leaves the
## arena and tests each tick's swept segment against the target's radius. It
## records exactly one outcome per bullet (at the wall), so every resolved
## bullet contributes exactly one fitness sample. A bullet that goes dead or
## stale is discarded without scoring, mirroring the point metric at
## resolution time.
for i in 0..<MaxBullets:
var b = addr t.bullets[i]
if not b.active: continue
b.travelDist += b.bulletSpeed
case t.metric
of bmPoint:
if b.travelDist < b.fireDist: continue
# Resolved: look up the correct enemy position
var ex, ey: float
if b.targetId in enemies:
let e = enemies[b.targetId]
if not e.alive or (state.tick - e.lastSeenTick) > StaleTicks:
b.active = false
continue
ex = e.x; ey = e.y
else:
# No data for this target — fall back to selected enemy in state
ex = state.enemyX; ey = state.enemyY
let dx = b.aimX - b.fireX
let dy = b.aimY - b.fireY
let dist = hypot(dx, dy)
let (bx, by) =
if dist < 1e-6: (b.aimX, b.aimY)
else: (b.fireX + dx / dist * b.travelDist,
b.fireY + dy / dist * b.travelDist)
let missDist = hypot(bx - ex, by - ey)
let hit = missDist < BotRadius
if b.targetId in t.fitness:
t.fitness[b.targetId][b.gunId].bins[b.powerBin].record(hit)
let fe = FeedbackEvent(
prediction: GunPrediction(x: b.aimX, y: b.aimY),
actualX: ex,
actualY: ey,
bulletPower: PowerBins[b.powerBin],
fireTick: b.fireTick,
powerBin: b.powerBin,
missDistance: missDist,
hit: hit,
)
onResolved(b.gunId, b.powerBin, fe)
b.active = false
of bmPath:
# Enemy pose for this tick. A dead/stale target abandons the bullet
# without scoring, exactly as the point metric does at resolution time.
var ex, ey: float
if b.targetId in enemies:
let e = enemies[b.targetId]
if not e.alive or (state.tick - e.lastSeenTick) > StaleTicks:
b.active = false
continue
ex = e.x; ey = e.y
else:
ex = state.enemyX; ey = state.enemyY
let dx = b.aimX - b.fireX
let dy = b.aimY - b.fireY
let dist = hypot(dx, dy)
var ux, uy: float
if dist < 1e-6: ux = 0.0; uy = 0.0
else: ux = dx / dist; uy = dy / dist
let prevD = max(0.0, b.travelDist - b.bulletSpeed)
let ax = b.fireX + ux * prevD
let ay = b.fireY + uy * prevD
let bx = b.fireX + ux * b.travelDist
let by = b.fireY + uy * b.travelDist
let segMiss = distPointToSegment(ex, ey, ax, ay, bx, by)
if not b.hitSeen:
if segMiss < BotRadius:
# First physical contact — freeze it so a later closer approach
# cannot overwrite the contact position the guns learn from.
b.hitSeen = true
b.bestMissDist = segMiss
b.bestMissX = ex
b.bestMissY = ey
elif segMiss < b.bestMissDist:
b.bestMissDist = segMiss
b.bestMissX = ex
b.bestMissY = ey
# Despawn only at a wall (a degenerate zero-length ray also ends here).
let outside =
dist < 1e-6 or
bx < 0.0 or bx > state.arenaWidth or
by < 0.0 or by > state.arenaHeight
if outside:
let missDist = if b.bestMissDist == Inf: segMiss else: b.bestMissDist
let rx = if b.bestMissDist == Inf: ex else: b.bestMissX
let ry = if b.bestMissDist == Inf: ey else: b.bestMissY
if b.targetId in t.fitness:
t.fitness[b.targetId][b.gunId].bins[b.powerBin].record(b.hitSeen)
let fe = FeedbackEvent(
prediction: GunPrediction(x: b.aimX, y: b.aimY),
actualX: rx,
actualY: ry,
bulletPower: PowerBins[b.powerBin],
fireTick: b.fireTick,
powerBin: b.powerBin,
missDistance: missDist,
hit: b.hitSeen,
)
onResolved(b.gunId, b.powerBin, fe)
b.active = false
if ActiveSelectorMode == smRelative:
noteBestRate(t)
proc fitnessFor*(t: VirtualTracker, targetId: int): seq[GunFitness] =
## Returns fitness seq for targetId, or merges all enemies as fallback.
##
## The fallback is a RECENCY-WEIGHTED AGGREGATE over the last WindowSize
## samples, NOT a pooled rate: each per-enemy window is replayed into one fresh
## window, so once the total exceeds WindowSize the earliest samples are
## overwritten by later ones. Enemies are visited in ascending target-id order
## so the result is identical on every run (std/tables iteration order is hash
## order and therefore nondeterministic).
## ponytail: merge is O(enemies*guns*bins*WindowSize), fine for small counts
if targetId >= 0 and targetId in t.fitness:
return t.fitness[targetId]
# Aggregate across all enemies, deterministically ordered.
result = newSeq[GunFitness](t.numGuns)
var enemyIds: seq[int]
for id in t.fitness.keys: enemyIds.add id
enemyIds.sort()
for id in enemyIds:
let perEnemy = t.fitness[id]
for gunId in 0..<t.numGuns:
for binIdx in 0..<len(PowerBins):
let src = perEnemy[gunId].bins[binIdx]
for k in 0..<min(src.count, WindowSize):
result[gunId].bins[binIdx].record(src.hits[k])
proc bestPower*(t: VirtualTracker, gunId: GunId, targetId: int = -1): (int, float) =
## Returns (binIdx, power). Prefers the HIGHEST power bin whose virtual hit
## rate is acceptable, where "acceptable" is measured RELATIVE to the same
## gun's best bin (`rate >= PowerBarFrac * bestBinRate`, dimensionless) — not
## against the legacy absolute `MinHitRate`. On the live path-metric scale a
## gun's rates sit around 3-40%, so the absolute 40% bar never fired once every
## bin had data and bestPower silently collapsed to power 1.0; the relative bar
## discriminates between bins at any scale.
##
## An EMPTY bin is still handed out (highest power first) so every bin keeps
## getting sampled, and a fully cold gun (no data anywhere) returns the lowest
## power bin. Uses per-enemy fitness when targetId >= 0 and data exists; else
## the deterministic aggregate.
let fit = t.fitnessFor(targetId)
result = (0, PowerBins[0])
var anyObs = false
var bestRate = 0.0
for binIdx in 0..<len(PowerBins):
if fit[gunId].bins[binIdx].count > 0:
anyObs = true
bestRate = max(bestRate, fit[gunId].bins[binIdx].hitRate())
if not anyObs:
return (0, PowerBins[0])
let bar = PowerBarFrac * bestRate
for binIdx in countdown(len(PowerBins) - 1, 0):
let fw = fit[gunId].bins[binIdx]
if fw.count == 0 or fw.hitRate() >= bar:
return (binIdx, PowerBins[binIdx])
proc chooseFromFit*(fit: seq[GunFitness], diag: ptr SelectorDiag = nil,
mode: SelectorMode = smAbsolute,
referenceRate = -1.0): GunId =
## Core gun ranking over an already-resolved fitness seq. Split out from
## `bestGun` so the offline range can rank without copying a VirtualTracker,
## and so callers can request `diag` for the selection internals.
##
## Guns with fewer than MinObsBeforeCompete observations are skipped unless
## every gun is below threshold (then fall back to best of all).
##
## `mode` chooses the threshold model:
## smAbsolute — legacy fixed TieMargin / MinHitRateFloor.
## smRelative — tie band = bestRate*RelTieMargin; floor = FloorPeakFrac
## * `referenceRate` (the recent field-best rate). Pooled over
## power bins, since one lucky bin is a poor ranker.
## `referenceRate` <= 0 disables the RELATIVE floor (no history yet).
## Ties (within the band) are broken randomly to avoid index-0 bias.
let pooled = mode == smRelative
var anyQualifies = false
for gunId in 0..<fit.len:
for binIdx in 0..<len(PowerBins):
if fit[gunId].bins[binIdx].count >= MinObsBeforeCompete:
anyQualifies = true
break
if anyQualifies: break
let requireMin = anyQualifies
if diag != nil: diag[].anyQualifies = requireMin
var bestRate = 0.0
for gunId in 0..<fit.len:
if requireMin and not gunEligible(fit[gunId], true): continue
bestRate = max(bestRate, gunRate(fit[gunId], pooled))
if diag != nil: diag[].bestRate = bestRate
let floorRate =
if mode == smAbsolute: MinHitRateFloor
elif referenceRate > 0.0: FloorPeakFrac * referenceRate
else: 0.0
if diag != nil: diag[].floorRate = floorRate
# No hit at all, or the field collapsed below its own recent peak: HeadOn.
if bestRate <= 0.0 or bestRate < floorRate:
if diag != nil: diag[].floorFired = true
return 0
let tieBand =
if mode == smAbsolute: TieMargin
else: bestRate * RelTieMargin
var tied: seq[GunId]
for gunId in 0..<fit.len:
if requireMin and not gunEligible(fit[gunId], true): continue
if gunRate(fit[gunId], pooled) >= bestRate - tieBand:
tied.add(gunId)
if diag != nil: diag[].tiedCount = tied.len
if tied.len == 0: return 0
result = tied[rand(tied.len - 1)]
proc bestGun*(t: VirtualTracker, targetId: int = -1,
diag: ptr SelectorDiag = nil): GunId =
## Pick gun with highest hit rate across all power bins.
## Uses per-enemy fitness when targetId >= 0 and data exists; else aggregate.
## `diag`, when non-nil, receives the selection internals (bestRate, floor,
## tie count) exactly as used by the decision.
result = chooseFromFit(t.fitnessFor(targetId), diag,
mode = ActiveSelectorMode,
referenceRate = t.peakRateRef)