SirRoboGarage/common_libs/guns/decay_gf.nim

## Recency-weighted GF gun: exponential decay on histogram bins.
## decay=0.998/tick gives ~350-tick half-life — adapts to mid-battle strategy shifts.
## Everything else identical to guess_factor.nim, including exact (fireTick,
## powerBin)-keyed wave pairing (see guess_factor.nim for the measured FIFO defect).

import std/math
import gun_harness/gun_interface
import gun_harness/virtual_bullets as vb  # PowerBins
import guns/lead_forecast

const
  GFBins     = 31
  GFPrior    = 0.1
  DecayRate  = 0.998  # ponytail: single global decay, tune if adaptation too slow/fast
  DecayWaveRingSlots = 1024  # (fireTick, powerBin) ring; see guess_factor.nim

type
  DWave = object
    fireX, fireY: float
    fireBearing:  float
    fireTick:     int
    bin:          int
    alive:        bool

  DecayGFGun* = object
    bins:             array[GFBins, float]
    # Exact (fireTick, powerBin)-keyed ring; a resolved bullet is matched to the
    # wave it actually fired, no matter how many other shots resolved first.
    # Heap-backed (seq): see guess_factor.nim.
    waves:            seq[DWave]
    waveStoredTick:   array[len(vb.PowerBins), int]   # last tick a wave was queued for this bin
    vt:               VelocityTracker  # enemy velocity history (base selection)
    cachedTick:       int   # last tick bins were decayed
    wavePushes*:      int
    waveStarved*:     int
    waveResolved*:      int
    waveMispaired*:     int  # ring-slot collision (impossible by design)
    debugGraphics*:   bool

proc initDecayGFGun*(): DecayGFGun =
  result.cachedTick = -1
  result.debugGraphics = false
  result.waves = newSeq[DWave](DecayWaveRingSlots)
  for b in 0..<len(vb.PowerBins):
    result.waveStoredTick[b] = -1
  let center = (GFBins - 1) div 2
  for i in 0..<GFBins:
    let d = abs(i - center)
    result.bins[i] = GFPrior + 0.5 / float(1 + d)

proc gfToIndex(gf: float): int {.inline.} =
  clamp(int(round((gf + 1.0) * 0.5 * float(GFBins - 1))), 0, GFBins - 1)

proc indexToGF(idx: int): float {.inline.} =
  float(idx) / float(GFBins - 1) * 2.0 - 1.0

proc peakBin*(g: DecayGFGun): int =
  var best = 0
  for i in 1..<GFBins:
    if g.bins[i] > g.bins[best]:
      best = i
  best

proc binForSpeed(spd: float): int {.inline.} =
  for i in 0..<len(vb.PowerBins):
    if abs(spd - bulletSpeed(vb.PowerBins[i])) < 1e-6:
      return i
  -1

proc binForPower(power: float): int {.inline.} =
  for i in 0..<len(vb.PowerBins):
    if abs(power - vb.PowerBins[i]) < 1e-6:
      return i
  -1

proc waveSlot(fireTick, binIdx: int): int {.inline.} =
  ((fireTick * len(vb.PowerBins)) + binIdx) mod DecayWaveRingSlots

proc predict*(g: var DecayGFGun, state: WorldState, bulletSpeed: float): GunPrediction =
  if bulletSpeed <= 0.0:
    return GunPrediction(x: state.enemyX, y: state.enemyY)

  let mea = arcsin(clamp(8.0 / bulletSpeed, -1.0, 1.0))

  if state.tick != g.cachedTick:
    g.cachedTick = state.tick
    g.vt.observe(state)
    # Decay all bins once per tick
    for i in 0..<GFBins:
      g.bins[i] *= DecayRate

  # Base forecast: the GF learns the residual against a self-consistent base
  # prediction (see lead_forecast.nim).
  let f = forecastRadialBlend(state, bulletSpeed, g.vt)

  # Queue at most one wave per (tick, power bin); the fire site's extra predict()
  # call for the selected bin lands on the same tick and reuses the queued wave.
  let binIdx = binForSpeed(bulletSpeed)
  if binIdx >= 0 and g.waveStoredTick[binIdx] != state.tick:
    let slot = waveSlot(state.tick, binIdx)
    g.waves[slot] = DWave(fireX: state.selfX, fireY: state.selfY,
                          fireBearing: f.bearing, fireTick: state.tick,
                          bin: binIdx, alive: true)
    g.waveStoredTick[binIdx] = state.tick
    inc g.wavePushes

  let peak    = g.peakBin()
  let peakGF  = indexToGF(peak)
  let gfAngle = f.bearing + peakGF * mea
  let px = state.selfX + cos(gfAngle) * f.dist
  let py = state.selfY + sin(gfAngle) * f.dist

  GunPrediction(
    x: clamp(px, BotRadius, state.arenaWidth  - BotRadius),
    y: clamp(py, BotRadius, state.arenaHeight - BotRadius),
  )

proc onResult*(g: var DecayGFGun, e: FeedbackEvent) =
  let binIdx =
    if e.powerBin >= 0 and e.powerBin < len(vb.PowerBins): e.powerBin
    else: binForPower(e.bulletPower)
  if binIdx < 0: return

  let slot = waveSlot(e.fireTick, binIdx)
  var w = addr g.waves[slot]
  if not w.alive:
    inc g.waveStarved
    return
  if w.fireTick != e.fireTick:
    inc g.waveMispaired
    inc g.waveStarved
    return
  inc g.waveResolved

  let speed  = bulletSpeed(e.bulletPower)
  let mea    = arcsin(clamp(8.0 / speed, -1.0, 1.0))
  let actualDx      = e.actualX - w.fireX
  let actualDy      = e.actualY - w.fireY
  let actualBearing = arctan2(actualDy, actualDx)
  var bearingDelta  = actualBearing - w.fireBearing
  while bearingDelta >  PI: bearingDelta -= 2.0*PI
  while bearingDelta < -PI: bearingDelta += 2.0*PI

  let gf        = if mea > 1e-10: clamp(bearingDelta / mea, -1.0, 1.0) else: 0.0
  let centerIdx = gfToIndex(gf)

  for i in 0..<GFBins:
    let d = abs(i - centerIdx)
    g.bins[i] += 1.0 / float(1 + d)

  w.alive = false