// Given difficulty D (big integer)
TWO256M1 = (1n << 256n) - 1n
target   = TWO256M1 / D
valid    = BigIntFromBytes(result) <= target

// Difficulty selection by era
CalcDifficulty(config, anchor, parent, header):
  if height(header) < 17560:
    // BTC-style DAA: 2016-block window with epoch anchors
    return CalcNakamotoDifficulty(config, parent)
  else:
    // ASERT: per-block retarget relative to fixed anchor
    return CalcAsertDifficulty(config, anchor, parent, header)

// ASERT core idea (fixed-point, aserti3-2d)
CalcAsertDifficulty(config, anchor, parent, header):
  Δh = height(header) - anchor.height
  Δt = header.Time - anchor.parentTime   // seconds
  // τ = 600 s target, T_half = 172800 s (2 days)
  e  = ((Δt - Δh * τ) * RADIX) / T_half  // fixed-point exponent
  target = anchor.target * 2^e           // via cubic approximation in integer math
  target = clamp(target, 1, maxTarget)
  return DifficultyFromTarget(target)

MTP(n):
  ts = timestamps(n, upTo=11) // back from n inclusive
  sort(ts)
  return ts[len(ts)//2]

Work(block):
  target = TWO256M1 / Difficulty(block)
  // Use an approximation that preserves ordering; e.g.,
  return TWO256M1 / (target + 1)

SelectBest(tips):
  return argmax(tips, ChainWork[tip])

OnNewTip(candidate):
  if ChainWork[candidate] > ChainWork[currentTip]:
    currentTip = candidate
    ReorgTo(candidate)

ValidHeader(h, parent):
  require(len(h.Extra) <= MaximumExtraDataSize)
  require(h.Time <= now() + 300)
  require(h.Time > MTP(parent))
  require(h.Difficulty > 0)
  // difficulty rules depend on height:
  //   < 17560: BTC-style DAA epoch invariants
  //   ≥ 17560: ASERT anchor-based invariants
  // PoW: MixDigest match & XHash(h) <= targetFrom(D)