computeBalance

formal source

  86  balance : ℤ := events.foldl (fun acc e => acc + e.flow) 0
  87
  88/-- Compute the balance of a list of events. -/
  89def computeBalance (events : List LedgerEvent) : ℤ :=
  90  events.foldl (fun acc e => acc + e.flow) 0
  91
  92/-- A **balanced ledger page** has σ = 0. -/
  93def IsBalanced (page : LedgerPage) : Prop :=
  94  computeBalance page.events = 0
  95
  96/-- The empty page is balanced. -/
  97theorem empty_balanced : IsBalanced ⟨[], 0⟩ := by
  98  simp [IsBalanced, computeBalance]
  99
 100/-- **PROVED: Adding a paired event preserves balance.** -/
 101theorem paired_preserves_balance (page : LedgerPage)
 102    (h : IsBalanced page) (e : LedgerEvent) :
 103    IsBalanced ⟨page.events ++ [e, e.conj], 0⟩ := by
 104  rcases page with ⟨events, bal⟩
 105  simp [IsBalanced, computeBalance, List.foldl_append, LedgerEvent.conj] at h ⊢
 106  omega
 107
 108/-! ## §3. The Eight-Window Constraint -/
 109
 110/-- A **window** is a contiguous block of 8 events. -/
 111structure Window8 where
 112  /-- The 8 events in this window -/
 113  events : Fin 8 → LedgerEvent
 114
 115/-- The sum over a window. -/
 116def Window8.sum (w : Window8) : ℤ :=
 117  (Finset.univ.sum fun i => (w.events i).flow)
 118
 119/-- A window is **neutral** if its sum is zero. -/