import Lean.Elab.Tactic.Omega
import Mathlib.Algebra.BigOperators.Fin
import Mathlib.Algebra.BigOperators.Intervals
import Mathlib.Algebra.Order.BigOperators.Expect
import Mathlib.Basic.Real.Basic
import Mathlib.Tactic.Ring
import OAI.Computability.BinPacking.Expanders.LazySmoothing
import OAI.Computability.BinPacking.Expanders.SpectralCut

namespace OAI

noncomputable section

namespace BinPackingGames.Foundations.PCP.PoweringMoment

open scoped BigOperators

def bit (p : Prop) : ℝ := by
  classical
  exact if p then 1 else 0

def hits {I Ω : Type*} [Fintype I] (E : I → Ω → Prop) (ω : Ω) : ℝ :=
  ∑ i, bit (E i ω)

def mean {Ω : Type*} [Fintype Ω] (f : Ω → ℝ) : ℝ := Finset.univ.expect f

theorem bit_nonneg (p : Prop) : 0 ≤ bit p := by
  classical
  by_cases hp : p <;> simp [bit, hp]

theorem bit_sq (p : Prop) : bit p ^ 2 = bit p := by
  classical
  by_cases hp : p <;> simp [bit, hp]

theorem bit_mul (p q : Prop) : bit p * bit q = bit (p ∧ q) := by
  classical
  by_cases hp : p <;> by_cases hq : q <;> simp [bit, hp, hq]

theorem bit_mono {p q : Prop} (hpq : p → q) : bit p ≤ bit q := by
  classical
  by_cases hp : p
  · simp [bit, hp, hpq hp]
  · have hz : bit p = 0 := by simp [bit, hp]
    rw [hz]
    exact bit_nonneg q

theorem hits_nonneg {I Ω : Type*} [Fintype I] (E : I → Ω → Prop) (ω : Ω) :
    0 ≤ hits E ω := Finset.sum_nonneg (fun i _ => bit_nonneg (E i ω))

theorem mean_nonneg {Ω : Type*} [Fintype Ω] (f : Ω → ℝ) (hf : ∀ ω, 0 ≤ f ω) :
    0 ≤ mean f := Finset.expect_nonneg (fun ω _ => hf ω)

theorem mean_hits {I Ω : Type*} [Fintype I] [Fintype Ω] (E : I → Ω → Prop) :
    mean (hits E) = ∑ i, mean (fun ω => bit (E i ω)) := by
  exact Finset.expect_sum_comm _ _ _

theorem second_moment_eq {I Ω : Type*} [Fintype I] [Fintype Ω]
    (E : I → Ω → Prop) :
    mean (fun ω => hits E ω ^ 2) =
      ∑ i, ∑ j, mean (fun ω => bit (E i ω ∧ E j ω)) := by
  simp only [hits, pow_two, Finset.sum_mul, Finset.mul_sum, bit_mul,
    mean, Finset.expect_sum_comm]
  exact Finset.sum_comm

theorem mean_witness_sq_le {I Ω : Type*} [Fintype I] [Fintype Ω]
    (E W : I → Ω → Prop) (reject : Ω → Prop)
    (hWE : ∀ i ω, W i ω → E i ω)
    (hWR : ∀ i ω, W i ω → reject ω) :
    mean (hits W) ^ 2 ≤
      mean (fun ω => bit (reject ω)) * mean (fun ω => hits E ω ^ 2) := by
  classical
  have hNH (ω : Ω) : hits W ω ≤ hits E ω :=
    Finset.sum_le_sum (fun i _ => bit_mono (hWE i ω))
  have hzero (ω : Ω) (hr : ¬ reject ω) : hits W ω = 0 := by
    apply Finset.sum_eq_zero
    intro i _
    have hw : ¬ W i ω := fun h => hr (hWR i ω h)
    simp [bit, hw]
  have hsupport (ω : Ω) : bit (reject ω) * hits W ω = hits W ω := by
    by_cases hr : reject ω
    · simp [bit, hr]
    · rw [hzero ω hr]
      simp
  have hCS := Finset.expect_mul_sq_le_sq_mul_sq Finset.univ
    (fun ω => bit (reject ω)) (hits W)
  have hsquares : mean (fun ω => hits W ω ^ 2) ≤ mean (fun ω => hits E ω ^ 2) := by
    apply Finset.expect_le_expect
    intro ω _
    simpa only [pow_two] using
      (mul_le_mul (hNH ω) (hNH ω) (hits_nonneg W ω) (hits_nonneg E ω))
  calc
    _ ≤ mean (fun ω => bit (reject ω)) * mean (fun ω => hits W ω ^ 2) := by
      simpa only [mean, hsupport, bit_sq] using hCS
    _ ≤ _ := mul_le_mul_of_nonneg_left hsquares
      (mean_nonneg _ (fun ω => bit_nonneg (reject ω)))

theorem rejection_lower_bound {I Ω : Type*} [Fintype I] [Fintype Ω]
    (E W : I → Ω → Prop) (reject : Ω → Prop)
    (hWE : ∀ i ω, W i ω → E i ω)
    (hWR : ∀ i ω, W i ω → reject ω)
    (a b : ℝ) (ha : 0 < a) (hb : 0 < b)
    (hfirst : a ≤ mean (hits W))
    (hsecond : mean (fun ω => hits E ω ^ 2) ≤ b) :
    a ^ 2 / b ≤ mean (fun ω => bit (reject ω)) := by
  have hN : 0 ≤ mean (hits W) := mean_nonneg _ (hits_nonneg W)
  have hρ : 0 ≤ mean (fun ω => bit (reject ω)) :=
    mean_nonneg _ (fun ω => bit_nonneg (reject ω))
  have haSq : a ^ 2 ≤ mean (hits W) ^ 2 := by
    simpa only [pow_two] using mul_le_mul hfirst hfirst ha.le hN
  have hCS := mean_witness_sq_le E W reject hWE hWR
  apply (div_le_iff₀ hb).2
  exact haSq.trans (hCS.trans (mul_le_mul_of_nonneg_left hsecond hρ))

end BinPackingGames.Foundations.PCP.PoweringMoment

end

section

namespace BinPackingGames.Foundations.PCP.PoweringWalks

variable {V D : Type*}

def lazyRotate (G : PortGraph V D) : Edge V (Bool × D) → Edge V (Bool × D)
  | (v, (false, d)) => (v, (false, d))
  | (v, (true, d)) => ((G.rot (v, d)).1, (true, (G.rot (v, d)).2))

theorem lazyRotate_involutive (G : PortGraph V D) :
    Function.Involutive (lazyRotate G) := by
  rintro ⟨v, b, d⟩
  cases b with
  | false => rfl
  | true =>
    exact congrArg (fun e : V × D => (e.1, (true, e.2))) (rot_rot G (v, d))

def lazyGraph (G : PortGraph V D) : PortGraph V (Bool × D) where
  rot := {
    toFun := lazyRotate G
    invFun := lazyRotate G
    left_inv := lazyRotate_involutive G
    right_inv := lazyRotate_involutive G }
  rot_involutive := lazyRotate_involutive G

@[simp] theorem lazyGraph_rot_false (G : PortGraph V D) (v : V) (d : D) :
    (lazyGraph G).rot (v, (false, d)) = (v, (false, d)) := rfl

@[simp] theorem lazyGraph_rot_true (G : PortGraph V D) (v : V) (d : D) :
    (lazyGraph G).rot (v, (true, d)) = ((G.rot (v, d)).1, (true, (G.rot (v, d)).2)) := rfl

end BinPackingGames.Foundations.PCP.PoweringWalks

namespace BinPackingGames.Foundations.PCP.PoweringLazy

open PoweringWalks SpectralReturn

variable {V D : Type*}

theorem operator_binomialMean [Fintype D] (G : PortGraph V D)
    (n : Nat) (g : Nat → V → ℝ) (v : V) :
    averagingOperator G (fun w => LazySmoothing.binomialMean n (fun k => g k w)) v =
      LazySmoothing.binomialMean n (fun k => averagingOperator G (g k) v) := by
  unfold averagingOperator LazySmoothing.binomialMean
  exact Finset.expect_comm Finset.univ Finset.univ
    (fun d tape => g (LazySmoothing.bitCount tape) (G.rot (v, d)).1)

theorem averagingOperator_lazy [Fintype D] [Nonempty D]
    (G : PortGraph V D) (f : V → ℝ) (v : V) :
    averagingOperator (lazyGraph G) f v = (f v + averagingOperator G f v) / 2 := by
  change mean (fun bd : Bool × D => f ((lazyGraph G).rot (v, bd)).1) = _
  rw [mean_prod]
  change Finset.univ.expect (fun b : Bool =>
    mean (fun d : D => f ((lazyGraph G).rot (v, (b, d))).1)) = _
  rw [LazySmoothing.expect_bool]
  simp only [lazyGraph_rot_false, lazyGraph_rot_true]
  rw [mean_const]
  rfl

theorem iterate_lazy_eq_binomialMean [Fintype D] [Nonempty D]
    (G : PortGraph V D) (n : Nat) (f : V → ℝ) :
    iterateOperator (lazyGraph G) n f =
      fun v => LazySmoothing.binomialMean n (fun k => iterateOperator G k f v) := by
  induction n with
  | zero =>
    funext v
    simp [iterateOperator]
  | succ n ih =>
    funext v
    change averagingOperator (lazyGraph G) (iterateOperator (lazyGraph G) n f) v = _
    rw [ih, averagingOperator_lazy, operator_binomialMean, LazySmoothing.mean_succ]
    rfl

theorem iterate_mem_Icc [Fintype D] [Nonempty D]
    (G : PortGraph V D) (f : V → ℝ) (hf : ∀ w, f w ∈ Set.Icc (0 : ℝ) 1)
    (n : Nat) (v : V) : iterateOperator G n f v ∈ Set.Icc (0 : ℝ) 1 := by
  induction n generalizing v with
  | zero => exact hf v
  | succ n ih =>
    change mean (fun d : D => iterateOperator G n f (G.rot (v, d)).1) ∈ Set.Icc (0 : ℝ) 1
    constructor
    · exact mean_nonnegative _ (fun d => (ih ((G.rot (v, d)).1)).1)
    · calc
        mean (fun d : D => iterateOperator G n f (G.rot (v, d)).1) ≤
            mean (fun _ : D => (1 : ℝ)) :=
          mean_mono (fun d => (ih ((G.rot (v, d)).1)).2)
        _ = 1 := mean_const _

theorem lazy_endpoint_modal_transfer [Fintype D] [Nonempty D]
    (G : PortGraph V D) (q M m : Nat) (hq : 1 ≤ q) (hM : 1 ≤ M)
    (hlo : (4 * q * M) ^ 2 - M ≤ m) (hhi : m ≤ (4 * q * M) ^ 2 + M)
    (f : V → ℝ) (hf : ∀ w, f w ∈ Set.Icc (0 : ℝ) 1) (v : V)
    (hmodal : 1 / (q : ℝ) ≤ iterateOperator (lazyGraph G) ((4 * q * M) ^ 2) f v) :
    1 / (2 * (q : ℝ)) ≤ iterateOperator (lazyGraph G) m f v := by
  rw [iterate_lazy_eq_binomialMean] at hmodal ⊢
  exact LazySmoothing.modal_transfer q M m hq hM hlo hhi
    (fun k => iterateOperator G k f v) (fun k => iterate_mem_Icc G f hf k v) hmodal

end BinPackingGames.Foundations.PCP.PoweringLazy

end

namespace BinPackingGames.Foundations.PCP.PoweringReturn

open PoweringWalks SpectralReturn
open PoweringMoment (bit)

variable {V D : Type*}

def portConsEquiv (D : Type*) (n : Nat) :
    (D × (Fin n → D)) ≃ (Fin (n + 1) → D) where
  toFun z := Fin.cases z.1 z.2
  invFun r := (r 0, fun j => r j.succ)
  left_inv z := by
    apply Prod.ext
    · rfl
    · funext j
      rfl
  right_inv r := by
    funext j
    exact Fin.cases rfl (fun _ => rfl) j

theorem mean_word_cons_head [Fintype D] (n : Nat)
    (F : (Fin (n + 1) → D) → ℝ) :
    mean F = mean (fun d : D => mean (fun r : Fin n → D => F (Fin.cases d r))) := by
  calc
    mean F = mean (fun z : D × (Fin n → D) => F (Fin.cases z.1 z.2)) :=
      (mean_equiv (portConsEquiv D n) F).symm
    _ = _ := mean_prod _

theorem mean_word_cons_tail [Fintype D] (n : Nat)
    (F : (Fin (n + 1) → D) → ℝ) :
    mean F = mean (fun r : Fin n → D => mean (fun d : D => F (Fin.cases d r))) := by
  calc
    mean F = mean (fun z : (Fin n → D) × D => F (Fin.cases z.2 z.1)) :=
      (mean_equiv ((Equiv.prodComm (Fin n → D) D).trans (portConsEquiv D n)) F).symm
    _ = _ := mean_prod _

theorem iterateOperator_succ_right [Fintype D] (G : PortGraph V D)
    (n : Nat) (f : V → ℝ) :
    iterateOperator G n (averagingOperator G f) = iterateOperator G (n + 1) f := by
  induction n with
  | zero => rfl
  | succ n ih => exact congrArg (averagingOperator G) ih

theorem mean_wordEnd [Fintype D] [Nonempty D] (G : PortGraph V D) :
    ∀ (n : Nat) (v : V) (f : V → ℝ),
      mean (fun r : Fin n → D => f (wordEnd G n v r)) = iterateOperator G n f v := by
  intro n
  induction n with
  | zero =>
    intro v f
    change mean (fun _ : Fin 0 → D => f v) = f v
    exact mean_const _
  | succ n ih =>
    intro v f
    rw [mean_word_cons_head]
    change mean (fun d : D => mean (fun r : Fin n → D =>
      f (wordEnd G n (next G v d) r))) = _
    calc
      _ = mean (fun d : D => iterateOperator G n f (next G v d)) := by
        congr 1
        funext d
        exact ih (next G v d) f
      _ = iterateOperator G (n + 1) f v := rfl

theorem mean_pivot_endpoints [Fintype D] [Nonempty D] (G : PortGraph V D) :
    ∀ (n : Nat) (k : Fin (n + 1)) (e : Edge V D) (φ ψ : V → ℝ),
      mean (fun r : Fin n → D =>
        φ (leftFromPivot G n k e.1 r) * ψ (rightFromPivot G n k e r)) =
        iterateOperator G k.val φ e.1 *
          iterateOperator G (n - k.val) ψ (next G e.1 e.2) := by
  intro n
  induction n with
  | zero =>
    intro k e φ ψ
    have hk : k = 0 := Fin.eq_zero k
    subst k
    change mean (fun _ : Fin 0 → D => φ e.1 * ψ (next G e.1 e.2)) =
      φ e.1 * ψ (next G e.1 e.2)
    exact mean_const _
  | succ n ih =>
    intro k e φ ψ
    refine Fin.cases ?_ (fun j => ?_) k
    · change mean (fun r : Fin (n + 1) → D =>
        φ e.1 * ψ (wordEnd G (n + 1) (next G e.1 e.2) r)) =
          φ e.1 * iterateOperator G (n + 1) ψ (next G e.1 e.2)
      rw [mean_mul_left, mean_wordEnd]
    · simp only [Fin.val_succ, Nat.add_sub_add_right]
      rw [mean_word_cons_tail]
      change mean (fun r : Fin n → D => mean (fun d : D =>
        φ (next G (leftFromPivot G n j e.1 r) d) *
          ψ (rightFromPivot G n j e r))) =
        iterateOperator G (j.val + 1) φ e.1 *
          iterateOperator G (n - j.val) ψ (next G e.1 e.2)
      calc
        _ = mean (fun r : Fin n → D =>
            averagingOperator G φ (leftFromPivot G n j e.1 r) *
              ψ (rightFromPivot G n j e r)) := by
          congr 1
          funext r
          rw [mean_mul_right]
          rfl
        _ = iterateOperator G j.val (averagingOperator G φ) e.1 *
            iterateOperator G (n - j.val) ψ (next G e.1 e.2) :=
          ih j e (averagingOperator G φ) ψ
        _ = _ := by rw [iterateOperator_succ_right]

theorem mean_edge_endpoints [Fintype V] [Fintype D] [Nonempty D]
    (G : PortGraph V D) (n : Nat) (k : Fin (n + 1)) (φ ψ : Edge V D → V → ℝ) :
    mean (fun w : Walk V D (n + 1) =>
      φ (edgeAt G n w k) w.1 * ψ (edgeAt G n w k) (endpoint G w)) =
      mean (fun e : Edge V D =>
        iterateOperator G k.val (φ e) e.1 *
          iterateOperator G (n - k.val) (ψ e) (next G e.1 e.2)) := by
  let F : Edge V D × (Fin n → D) → ℝ := fun z =>
    φ z.1 (leftFromPivot G n k z.1.1 z.2) *
      ψ z.1 (rightFromPivot G n k z.1 z.2)
  calc
    _ = mean (fun w => F (pivotEquiv G n k w)) := by
      congr 1
      funext w
      dsimp [F]
      rw [leftFromPivot_actual, rightFromPivot_actual, pivotEquiv_fst]
    _ = mean F := mean_equiv (pivotEquiv G n k) F
    _ = mean (fun e : Edge V D => mean (fun r : Fin n → D =>
        φ e (leftFromPivot G n k e.1 r) * ψ e (rightFromPivot G n k e r))) := mean_prod F
    _ = _ := by
      congr 1
      funext e
      exact mean_pivot_endpoints G n k e (φ e) (ψ e)

theorem bit_bool (b : Bool) : bit (b = true) = (if b then 1 else 0 : ℝ) := by
  cases b <;> simp [bit]

theorem mean_edge_bit [Fintype D] (bad : V × D → Bool) (v : V) :
    mean (fun d : D => bit (bad (v, d) = true)) = edgeProfile bad v := by
  simp only [bit_bool, mean, edgeProfile]

theorem mean_marked_bit [Fintype D] (G : PortGraph V D)
    (bad : V × D → Bool) (f : V → ℝ) (v : V) :
    mean (fun d : D => bit (bad (v, d) = true) * f (next G v d)) =
      markedStep G bad f v := by
  change mean (fun d : D => bit (bad (v, d) = true) * f (next G v d)) =
    mean (fun d : D => if bad (v, d) then f (next G v d) else 0)
  congr 1
  funext d
  cases bad (v, d) <;> simp [bit]

theorem edgeAt_zero (G : PortGraph V D) (n : Nat) (w : Walk V D (n + 1)) :
    edgeAt G n w 0 = (w.1, w.2 0) := by
  cases n <;> rfl

theorem word_first_hit_mean [Fintype D] [Nonempty D]
    (G : PortGraph V D) (bad : V × D → Bool) (n : Nat) (v : V) :
    mean (fun p : Fin (n + 1) → D => bit (bad (edgeAt G n (v, p) 0) = true)) =
      edgeProfile bad v := by
  rw [mean_word_cons_head]
  simp_rw [edgeAt_zero]
  change mean (fun d : D => mean (fun _ : Fin n → D => bit (bad (v, d) = true))) = _
  calc
    _ = mean (fun d : D => bit (bad (v, d) = true)) := by simp only [mean_const]
    _ = edgeProfile bad v := mean_edge_bit bad v

theorem word_hit_mean [Fintype D] [Nonempty D]
    (G : PortGraph V D) (bad : V × D → Bool) :
    ∀ (n : Nat) (v : V) (k : Fin (n + 1)),
      mean (fun p : Fin (n + 1) → D => bit (bad (edgeAt G n (v, p) k) = true)) =
        iterateOperator G k.val (edgeProfile bad) v := by
  intro n
  induction n with
  | zero =>
    intro v k
    have hk : k = 0 := Fin.eq_zero k
    subst k
    exact word_first_hit_mean G bad 0 v
  | succ n ih =>
    intro v k
    refine Fin.cases ?_ (fun j => ?_) k
    · exact word_first_hit_mean G bad (n + 1) v
    · rw [mean_word_cons_head]
      change mean (fun d : D => mean (fun p : Fin (n + 1) → D =>
        bit (bad (edgeAt G n (next G v d, p) j) = true))) =
          averagingOperator G (iterateOperator G j.val (edgeProfile bad)) v
      calc
        _ = mean (fun d : D => iterateOperator G j.val (edgeProfile bad) (next G v d)) := by
          congr 1
          funext d
          exact ih (next G v d) j
        _ = _ := rfl

theorem word_pair_hit_mean [Fintype D] [Nonempty D]
    (G : PortGraph V D) (bad : V × D → Bool) :
    ∀ (n : Nat) (v : V) (i j : Fin (n + 1)), i < j →
      mean (fun p : Fin (n + 1) → D =>
        bit (bad (edgeAt G n (v, p) i) = true) *
          bit (bad (edgeAt G n (v, p) j) = true)) =
        iterateOperator G i.val (markedStep G bad
          (iterateOperator G (j.val - i.val - 1) (edgeProfile bad))) v := by
  intro n
  induction n with
  | zero =>
    intro v i j hij
    have hi : i = 0 := Fin.eq_zero i
    have hj : j = 0 := Fin.eq_zero j
    subst i
    subst j
    exact ((lt_irrefl _) hij).elim
  | succ n ih =>
    intro v i
    refine Fin.cases ?_ (fun a => ?_) i
    · intro j
      refine Fin.cases ?_ (fun b => ?_) j
      · intro hij
        exact ((lt_irrefl _) hij).elim
      · intro _hij
        simp only [Fin.val_zero, Fin.val_succ, Nat.sub_zero, Nat.add_sub_cancel]
        rw [mean_word_cons_head]
        change mean (fun d : D => mean (fun p : Fin (n + 1) → D =>
          bit (bad (v, d) = true) * bit (bad (edgeAt G n (next G v d, p) b) = true))) =
          markedStep G bad (iterateOperator G b.val (edgeProfile bad)) v
        calc
          _ = mean (fun d : D => bit (bad (v, d) = true) *
              iterateOperator G b.val (edgeProfile bad) (next G v d)) := by
            congr 1
            funext d
            rw [mean_mul_left, word_hit_mean]
          _ = _ := mean_marked_bit G bad _ v
    · intro j
      refine Fin.cases ?_ (fun b => ?_) j
      · intro hij
        exact (Nat.not_lt_zero _ hij).elim
      · intro hij
        have hab : a < b := Nat.lt_of_succ_lt_succ hij
        simp only [Fin.val_succ, Nat.add_sub_add_right]
        rw [mean_word_cons_head]
        change mean (fun d : D => mean (fun p : Fin (n + 1) → D =>
          bit (bad (edgeAt G n (next G v d, p) a) = true) *
            bit (bad (edgeAt G n (next G v d, p) b) = true))) =
          averagingOperator G (iterateOperator G a.val (markedStep G bad
            (iterateOperator G (b.val - a.val - 1) (edgeProfile bad)))) v
        calc
          _ = mean (fun d : D => iterateOperator G a.val (markedStep G bad
              (iterateOperator G (b.val - a.val - 1) (edgeProfile bad))) (next G v d)) := by
            congr 1
            funext d
            exact ih (next G v d) a b hab
          _ = _ := rfl

theorem hit_mean [Fintype V] [Fintype D] [Nonempty V] [Nonempty D]
    (G : PortGraph V D) (bad : V × D → Bool) (n : Nat) (k : Fin (n + 1)) :
    mean (fun w : Walk V D (n + 1) => bit (bad (edgeAt G n w k) = true)) =
      edgeDensity bad := by
  calc
    _ = mean (fun v : V => mean (fun p : Fin (n + 1) → D =>
        bit (bad (edgeAt G n (v, p) k) = true))) :=
      mean_prod (A := V) (B := Fin (n + 1) → D)
        (fun w => bit (bad (edgeAt G n w k) = true))
    _ = mean (iterateOperator G k.val (edgeProfile bad)) := by
      congr 1
      funext v
      exact word_hit_mean G bad n v k
    _ = mean (edgeProfile bad) := mean_iterate G k.val _
    _ = _ := rfl

theorem pair_hit_mean [Fintype V] [Fintype D] [Nonempty V] [Nonempty D]
    (G : PortGraph V D) (bad : V × D → Bool) (n : Nat)
    (i j : Fin (n + 1)) (hij : i < j) :
    mean (fun w : Walk V D (n + 1) => bit (bad (edgeAt G n w i) = true) *
      bit (bad (edgeAt G n w j) = true)) = returnMass G bad (j.val - i.val - 1) := by
  calc
    _ = mean (fun v : V => mean (fun p : Fin (n + 1) → D =>
        bit (bad (edgeAt G n (v, p) i) = true) *
          bit (bad (edgeAt G n (v, p) j) = true))) :=
      mean_prod (A := V) (B := Fin (n + 1) → D)
        (fun w => bit (bad (edgeAt G n w i) = true) *
          bit (bad (edgeAt G n w j) = true))
    _ = mean (iterateOperator G i.val (markedStep G bad
        (iterateOperator G (j.val - i.val - 1) (edgeProfile bad)))) := by
      congr 1
      funext v
      exact word_pair_hit_mean G bad n v i j hij
    _ = mean (markedStep G bad
        (iterateOperator G (j.val - i.val - 1) (edgeProfile bad))) := mean_iterate G i.val _
    _ = _ := rfl

theorem pair_event_mean [Fintype V] [Fintype D] [Nonempty V] [Nonempty D]
    (G : PortGraph V D) (bad : V × D → Bool) (n : Nat)
    (i j : Fin (n + 1)) (hij : i < j) :
    mean (fun w : Walk V D (n + 1) =>
      bit (bad (edgeAt G n w i) = true ∧ bad (edgeAt G n w j) = true)) =
        returnMass G bad (j.val - i.val - 1) := by
  simpa only [PoweringMoment.bit_mul] using pair_hit_mean G bad n i j hij

end BinPackingGames.Foundations.PCP.PoweringReturn

namespace BinPackingGames.Foundations.PCP.PoweringWitness

open PoweringWalks SpectralReturn PoweringReturn PoweringLazy
open PoweringMoment (bit)

variable {V D : Type*}

theorem iterate_mul_const [Fintype D] (G : PortGraph V D)
    (n : Nat) (c : ℝ) (h : V → ℝ) :
    iterateOperator G n (fun x => c * h x) =
      fun x => c * iterateOperator G n h x := by
  induction n with
  | zero => rfl
  | succ n ih =>
    change averagingOperator G (iterateOperator G n (fun x => c * h x)) =
      fun x => c * averagingOperator G (iterateOperator G n h) x
    rw [ih]
    funext v
    exact mean_mul_left c (fun d => iterateOperator G n h (G.rot (v, d)).1)

noncomputable def endpointWitness (G : PortGraph V D) (bad : Edge V D → Bool)
    (φ ψ : Edge V D → V → ℝ) (n : Nat) (k : Fin (n + 1))
    (w : Walk V D (n + 1)) : ℝ :=
  bit (bad (edgeAt G n w k) = true) *
    φ (edgeAt G n w k) w.1 * ψ (edgeAt G n w k) (endpoint G w)

noncomputable def vertexWitness (G : PortGraph V D) (bad : Edge V D → Bool)
    (f : V → V → ℝ) (n : Nat) (k : Fin (n + 1)) :
    Walk V D (n + 1) → ℝ :=
  endpointWitness G bad (fun e => f e.1) (fun e => f (next G e.1 e.2)) n k

theorem endpoint_lower_bound [Fintype V] [Fintype D] [Nonempty D]
    (G : PortGraph V D) (n : Nat) (k : Fin (n + 1)) (bad : Edge V D → Bool)
    (φ ψ : Edge V D → V → ℝ) (a : ℝ) (ha : 0 ≤ a)
    (hL : ∀ e, bad e = true → a ≤ iterateOperator G k.val (φ e) e.1)
    (hR : ∀ e, bad e = true →
      a ≤ iterateOperator G (n - k.val) (ψ e) (next G e.1 e.2)) :
    edgeDensity bad * a ^ 2 ≤ mean (endpointWitness G bad φ ψ n k) := by
  have hfactor : mean (endpointWitness G bad φ ψ n k) =
      mean (fun e : Edge V D =>
        (bit (bad e = true) * iterateOperator G k.val (φ e) e.1) *
          iterateOperator G (n - k.val) (ψ e) (next G e.1 e.2)) := by
    unfold endpointWitness
    rw [mean_edge_endpoints G n k (fun e x => bit (bad e = true) * φ e x) ψ]
    simp_rw [iterate_mul_const]
  have hbitMean : mean (fun e : Edge V D => bit (bad e = true)) = edgeDensity bad := by
    calc
      _ = mean (fun v : V => mean (fun d : D => bit (bad (v, d) = true))) :=
        mean_prod (A := V) (B := D) (fun e => bit (bad e = true))
      _ = edgeDensity bad := by simp only [mean_edge_bit, edgeDensity]
  calc
    edgeDensity bad * a ^ 2 =
        mean (fun e : Edge V D => bit (bad e = true) * a ^ 2) := by
      rw [mean_mul_right, hbitMean]
    _ ≤ mean (fun e : Edge V D =>
        (bit (bad e = true) * iterateOperator G k.val (φ e) e.1) *
          iterateOperator G (n - k.val) (ψ e) (next G e.1 e.2)) := by
      apply mean_mono
      intro e
      cases hb : bad e with
      | false => simp [bit]
      | true =>
        have hprod := mul_le_mul (hL e hb) (hR e hb) ha (ha.trans (hL e hb))
        simpa [bit, hb, pow_two] using hprod
    _ = mean (endpointWitness G bad φ ψ n k) := hfactor.symm

theorem lazy_middle_witness_mean [Fintype V] [Fintype D] [Nonempty D]
    (G : PortGraph V D) (q M : Nat) (hq : 1 ≤ q) (hM : 1 ≤ M)
    (k : Fin (2 * (4 * q * M) ^ 2 + 1))
    (hlo : (4 * q * M) ^ 2 - M ≤ k.val)
    (hhi : k.val ≤ (4 * q * M) ^ 2 + M)
    (bad : Edge V (Bool × D) → Bool) (f : V → V → ℝ)
    (hf : ∀ u w, f u w ∈ Set.Icc (0 : ℝ) 1)
    (hmodal : ∀ u, 1 / (q : ℝ) ≤
      iterateOperator (lazyGraph G) ((4 * q * M) ^ 2) (f u) u) :
    edgeDensity bad / (4 * (q : ℝ) ^ 2) ≤
      mean (vertexWitness (lazyGraph G) bad f (2 * (4 * q * M) ^ 2) k) := by
  have hk : k.val ≤ 2 * (4 * q * M) ^ 2 := Nat.le_of_lt_succ k.isLt
  have hslo : (4 * q * M) ^ 2 - M ≤ 2 * (4 * q * M) ^ 2 - k.val := by omega
  have hshi : 2 * (4 * q * M) ^ 2 - k.val ≤ (4 * q * M) ^ 2 + M := by omega
  have ha : (0 : ℝ) ≤ 1 / (2 * (q : ℝ)) := by positivity
  have hbound := endpoint_lower_bound (lazyGraph G) (2 * (4 * q * M) ^ 2) k bad
    (fun e => f e.1) (fun e => f (next (lazyGraph G) e.1 e.2))
    (1 / (2 * (q : ℝ))) ha
    (by
      intro e _
      exact lazy_endpoint_modal_transfer G q M k.val hq hM hlo hhi
        (f e.1) (hf e.1) e.1 (hmodal e.1))
    (by
      intro e _
      exact lazy_endpoint_modal_transfer G q M (2 * (4 * q * M) ^ 2 - k.val)
        hq hM hslo hshi (f (next (lazyGraph G) e.1 e.2))
        (hf (next (lazyGraph G) e.1 e.2)) (next (lazyGraph G) e.1 e.2)
        (hmodal (next (lazyGraph G) e.1 e.2)))
  have hscale : edgeDensity bad * (1 / (2 * (q : ℝ))) ^ 2 =
      edgeDensity bad / (4 * (q : ℝ) ^ 2) := by ring
  simpa only [vertexWitness, hscale] using hbound

end BinPackingGames.Foundations.PCP.PoweringWitness

namespace BinPackingGames.Foundations.PCP.PoweringMomentBound

open scoped BigOperators
open PoweringWalks SpectralReturn PoweringReturn
open PoweringMoment (bit hits)

theorem symmetric_gap_matrix_sum (ε : ℝ) (r : Nat → ℝ) :
    ∀ (m : Nat) (f : Fin m → Fin m → ℝ),
      (∀ i j, f i j = f j i) → (∀ i, f i i = ε) →
      (∀ i j, i.val < j.val → f i j = r (j.val - i.val - 1)) →
      (∑ i, ∑ j, f i j) = (m : ℝ) * ε +
        2 * ∑ j ∈ Finset.range m, ∑ gap ∈ Finset.range j, r gap := by
  intro m
  induction m with
  | zero => intro f _hSym _hDiag _hUpper; simp
  | succ m ih =>
    intro f hSym hDiag hUpper
    have hOld : (∑ i : Fin m, ∑ j : Fin m, f i.castSucc j.castSucc) =
        (m : ℝ) * ε + 2 * ∑ j ∈ Finset.range m, ∑ gap ∈ Finset.range j, r gap := by
      apply ih (fun i j => f i.castSucc j.castSucc)
      · intro i j; exact hSym i.castSucc j.castSucc
      · intro i; exact hDiag i.castSucc
      · intro i j hij; exact hUpper i.castSucc j.castSucc hij
    have hLast : (∑ i : Fin m, f i.castSucc (Fin.last m)) =
        ∑ gap ∈ Finset.range m, r gap := by
      calc
        _ = ∑ i : Fin m, r (m - 1 - i.val) := by
          apply Finset.sum_congr rfl
          intro i _
          rw [hUpper i.castSucc (Fin.last m) i.isLt]
          simp only [Fin.val_last, Fin.val_castSucc]
          congr 1
          omega
        _ = ∑ i ∈ Finset.range m, r (m - 1 - i) :=
          Fin.sum_univ_eq_sum_range (fun i => r (m - 1 - i)) m
        _ = _ := Finset.sum_range_reflect r m
    have hLastRow : (∑ j : Fin m, f (Fin.last m) j.castSucc) =
        ∑ gap ∈ Finset.range m, r gap := by
      calc
        _ = ∑ j : Fin m, f j.castSucc (Fin.last m) := by
          apply Finset.sum_congr rfl
          intro j _
          exact hSym (Fin.last m) j.castSucc
        _ = _ := hLast
    calc
      (∑ i, ∑ j, f i j) =
          (∑ i : Fin m, ∑ j : Fin m, f i.castSucc j.castSucc) +
            (∑ i : Fin m, f i.castSucc (Fin.last m)) +
            (∑ j : Fin m, f (Fin.last m) j.castSucc) + f (Fin.last m) (Fin.last m) := by
        simp only [Fin.sum_univ_castSucc, Finset.sum_add_distrib]
        ring
      _ = (m : ℝ) * ε +
          2 * (∑ j ∈ Finset.range m, ∑ gap ∈ Finset.range j, r gap) +
          (∑ gap ∈ Finset.range m, r gap) + (∑ gap ∈ Finset.range m, r gap) + ε := by
        rw [hOld, hLast, hLastRow, hDiag]
      _ = ((m + 1 : Nat) : ℝ) * ε +
          2 * ∑ j ∈ Finset.range (m + 1), ∑ gap ∈ Finset.range j, r gap := by
        rw [Finset.sum_range_succ]
        simp only [Nat.cast_add, Nat.cast_one]
        ring

def windowIndex (n start m : Nat) (h : start + m ≤ n + 1) (i : Fin m) : Fin (n + 1) :=
  ⟨start + i.val, (Nat.add_lt_add_left i.isLt start).trans_le h⟩

variable {V D : Type*} [Fintype V] [Fintype D] [Nonempty V] [Nonempty D]

def windowEvent (G : PortGraph V D) (bad : V × D → Bool)
    (n start m : Nat) (h : start + m ≤ n + 1) (i : Fin m) (w : Walk V D (n + 1)) : Prop :=
  bad (edgeAt G n w (windowIndex n start m h i)) = true

theorem window_second_moment_eq (G : PortGraph V D) (bad : V × D → Bool)
    (n start m : Nat) (h : start + m ≤ n + 1) :
    mean (fun w => hits (windowEvent G bad n start m h) w ^ 2) =
      (m : ℝ) * edgeDensity bad +
        2 * ∑ j ∈ Finset.range m, ∑ gap ∈ Finset.range j, returnMass G bad gap := by
  rw [show mean (fun w => hits (windowEvent G bad n start m h) w ^ 2) =
      ∑ i, ∑ j, mean (fun w => bit (windowEvent G bad n start m h i w ∧
        windowEvent G bad n start m h j w)) from
    PoweringMoment.second_moment_eq (windowEvent G bad n start m h)]
  apply symmetric_gap_matrix_sum
  · intro i j
    congr 1
    funext w
    simp only [and_comm]
  · intro i
    simp only [and_self]
    exact hit_mean G bad n (windowIndex n start m h i)
  · intro i j hij
    have hwin : windowIndex n start m h i < windowIndex n start m h j :=
      Nat.add_lt_add_left hij start
    have hp := pair_event_mean G bad n (windowIndex n start m h i)
      (windowIndex n start m h j) hwin
    simpa only [windowEvent, windowIndex, Nat.add_sub_add_left] using hp

theorem window_second_moment_le (G : PortGraph V D) (lambda : ℝ)
    (certificate : SpectralCertificate G lambda) (bad : V × D → Bool)
    (reversal : ∀ e, bad (G.rot e) = bad e)
    (n start m : Nat) (h : start + m ≤ n + 1) :
    mean (fun w => hits (windowEvent G bad n start m h) w ^ 2) ≤
      (m : ℝ) * edgeDensity bad *
        (1 + 2 / (1 - lambda) + ((m : ℝ) - 1) * edgeDensity bad) := by
  rw [window_second_moment_eq]
  exact secondMoment_return_envelope G lambda certificate bad reversal m

end BinPackingGames.Foundations.PCP.PoweringMomentBound

end OAI