lean/OAI/Combinatorics/IndependentSets/PCP/QueryIncidence.lean

import Mathlib
import OAI.Combinatorics.IndependentSets.PCP.ConstraintGraph

namespace OAI

namespace IndependentSetsGames.Foundations.PCP.QueryIncidence

structure Verifier (E X : Type*) (q : Nat) where
  query : E → Fin q → X
  accepts : E → (Fin q → Bool) → Bool

abbrev Label (q : Nat) := Fin q → Bool
abbrev Vertex (E X : Type*) := E ⊕ X
abbrev Dart (E : Type*) (q : Nat) := (E × Fin q) × Bool

variable {E X : Type*} {q : Nat}

def response (T : Verifier E X q) (assignment : X → Bool) (event : E) : Label q :=
  fun i => assignment (T.query event i)

def zeroSlot (positive : 0 < q) : Fin q := ⟨0, positive⟩

def decodeRight (positive : 0 < q) (label : Label q) : Bool := label (zeroSlot positive)

def encodeRight (bit : Bool) : Label q := fun _ => bit

@[simp] theorem decode_encodeRight (positive : 0 < q) (bit : Bool) :
    decodeRight positive (encodeRight bit) = bit := rfl

def canonicalSlot [DecidableEq X] (T : Verifier E X q) (event : E) (i : Fin q) : Fin q :=
  Fin.find (fun j => T.query event j = T.query event i) ⟨i, rfl⟩

theorem canonicalSlot_query [DecidableEq X] (T : Verifier E X q) (event : E) (i : Fin q) :
    T.query event (canonicalSlot T event i) = T.query event i :=
  Fin.find_spec (p := fun j => T.query event j = T.query event i) ⟨i, rfl⟩

def RepeatedConsistent (T : Verifier E X q) (event : E) (label : Label q) : Prop :=
  ∀ i j, T.query event i = T.query event j → label i = label j

instance repeatedConsistentDecidable [DecidableEq X]
    (T : Verifier E X q) (event : E) (label : Label q) :
    Decidable (RepeatedConsistent T event label) := by
  unfold RepeatedConsistent
  infer_instance

def LeftValid (T : Verifier E X q) (event : E) (label : Label q) : Prop :=
  T.accepts event label = true ∧ RepeatedConsistent T event label

instance leftValidDecidable [DecidableEq X]
    (T : Verifier E X q) (event : E) (label : Label q) :
    Decidable (LeftValid T event label) := by
  unfold LeftValid
  infer_instance

def incidenceAccept [DecidableEq X] (T : Verifier E X q) (positive : 0 < q)
    (event : E) (slot : Fin q) (left right : Label q) : Bool :=
  decide (LeftValid T event left) &&
    decide (left (canonicalSlot T event slot) = decodeRight positive right)

def reverse : Dart E q → Dart E q := fun d => (d.1, !d.2)

theorem reverse_involutive : Function.Involutive (reverse : Dart E q → Dart E q) := by
  rintro ⟨⟨event, slot⟩, orientation⟩
  cases orientation <;> rfl

def reverseEquiv : Dart E q ≃ Dart E q where
  toFun := reverse
  invFun := reverse
  left_inv := reverse_involutive
  right_inv := reverse_involutive

def tail (T : Verifier E X q) : Dart E q → Vertex E X
  | ((event, _), false) => .inl event
  | ((event, slot), true) => .inr (T.query event slot)

def predicate [DecidableEq X] (T : Verifier E X q) (positive : 0 < q) :
    Dart E q → Label q → Label q → Bool
  | ((event, slot), false), a, b => incidenceAccept T positive event slot a b
  | ((event, slot), true), a, b => incidenceAccept T positive event slot b a

def graph [DecidableEq X] (T : Verifier E X q) (positive : 0 < q) :
    ConstraintGraph (Vertex E X) (Dart E q) (Label q) where
  reverse := reverseEquiv
  reverse_involutive := reverse_involutive
  tail := tail T
  accepts := predicate T positive
  reverse_accepts := by
    rintro ⟨⟨event, slot⟩, orientation⟩ a b
    cases orientation <;> rfl

theorem edgeSatisfied_eq_incidence [DecidableEq X] (T : Verifier E X q)
    (positive : 0 < q) (labeling : Vertex E X → Label q)
    (event : E) (slot : Fin q) (orientation : Bool) :
    (graph T positive).edgeSatisfied labeling ((event, slot), orientation) =
      incidenceAccept T positive event slot (labeling (.inl event))
        (labeling (.inr (T.query event slot))) := by
  cases orientation <;> rfl

def honestLabeling (T : Verifier E X q) (assignment : X → Bool) : Vertex E X → Label q
  | .inl event => response T assignment event
  | .inr address => encodeRight (assignment address)

theorem honest_leftValid (T : Verifier E X q) (assignment : X → Bool) (event : E)
    (accepted : T.accepts event (response T assignment event) = true) :
    LeftValid T event (response T assignment event) := by
  refine ⟨accepted, ?_⟩
  intro i j hij
  exact congrArg assignment hij

theorem honest_incidence [DecidableEq X] (T : Verifier E X q) (positive : 0 < q)
    (assignment : X → Bool) (event : E)
    (accepted : T.accepts event (response T assignment event) = true) (slot : Fin q) :
    incidenceAccept T positive event slot
      (honestLabeling T assignment (.inl event))
      (honestLabeling T assignment (.inr (T.query event slot))) = true := by
  simp only [incidenceAccept, Bool.and_eq_true, honestLabeling,
    decode_encodeRight]
  refine ⟨_root_.decide_eq_true (honest_leftValid T assignment event accepted), ?_⟩
  exact _root_.decide_eq_true (congrArg assignment (canonicalSlot_query T event slot))

theorem perfect_completeness [DecidableEq X] (T : Verifier E X q) (positive : 0 < q)
    (assignment : X → Bool)
    (accepted : ∀ event, T.accepts event (response T assignment event) = true) :
    ∀ dart, (graph T positive).edgeSatisfied (honestLabeling T assignment) dart = true := by
  rintro ⟨⟨event, slot⟩, orientation⟩
  rw [edgeSatisfied_eq_incidence]
  exact honest_incidence T positive assignment event (accepted event) slot

theorem satisfiable_of_verifier_satisfiable [DecidableEq X] (T : Verifier E X q)
    (positive : 0 < q)
    (sat : ∃ assignment : X → Bool,
      ∀ event, T.accepts event (response T assignment event) = true) :
    (graph T positive).Satisfiable := by
  obtain ⟨assignment, h⟩ := sat
  exact ⟨honestLabeling T assignment, perfect_completeness T positive assignment h⟩

def decodedAssignment (positive : 0 < q) (labeling : Vertex E X → Label q) : X → Bool :=
  fun address => decodeRight positive (labeling (.inr address))

theorem incidenceAccept_true [DecidableEq X] (T : Verifier E X q) (positive : 0 < q)
    (event : E) (slot : Fin q) (left right : Label q)
    (accepted : incidenceAccept T positive event slot left right = true) :
    T.accepts event left = true ∧ RepeatedConsistent T event left ∧
      left (canonicalSlot T event slot) = decodeRight positive right := by
  simp only [incidenceAccept, Bool.and_eq_true] at accepted
  have hvalid : LeftValid T event left := _root_.of_decide_eq_true accepted.1
  exact ⟨hvalid.1, hvalid.2, _root_.of_decide_eq_true accepted.2⟩

theorem all_incidences_imply_event [DecidableEq X] (T : Verifier E X q)
    (positive : 0 < q) (labeling : Vertex E X → Label q) (event : E)
    (accepted : ∀ slot, incidenceAccept T positive event slot (labeling (.inl event))
      (labeling (.inr (T.query event slot))) = true) :
    T.accepts event (response T (decodedAssignment positive labeling) event) = true := by
  have first := incidenceAccept_true T positive event (zeroSlot positive) _ _
    (accepted (zeroSlot positive))
  have labels_equal : labeling (.inl event) = response T (decodedAssignment positive labeling) event := by
    funext slot
    have info := incidenceAccept_true T positive event slot _ _ (accepted slot)
    exact (first.2.1 slot (canonicalSlot T event slot)
      (canonicalSlot_query T event slot).symm).trans info.2.2
  rw [← labels_equal]
  exact first.1

theorem rejected_event_forces_slot [DecidableEq X] (T : Verifier E X q)
    (positive : 0 < q) (labeling : Vertex E X → Label q) (event : E)
    (rejected : T.accepts event (response T (decodedAssignment positive labeling) event) = false) :
    ∃ slot, incidenceAccept T positive event slot (labeling (.inl event))
      (labeling (.inr (T.query event slot))) = false := by
  classical
  by_contra none
  have all : ∀ slot, incidenceAccept T positive event slot (labeling (.inl event))
      (labeling (.inr (T.query event slot))) = true := by
    intro slot
    cases h : incidenceAccept T positive event slot (labeling (.inl event))
        (labeling (.inr (T.query event slot))) with
    | false => exact False.elim (none ⟨slot, h⟩)
    | true => rfl
  have h := all_incidences_imply_event T positive labeling event all
  rw [rejected] at h
  cases h

def rejectedEvents [Fintype E] (T : Verifier E X q) (assignment : X → Bool) : Finset E :=
  Finset.univ.filter (fun event => T.accepts event (response T assignment event) = false)

def verifierRejectionCount [Fintype E] (T : Verifier E X q) (assignment : X → Bool) : Nat :=
  (rejectedEvents T assignment).card

theorem rejection_count_bound [Fintype E] [DecidableEq X]
    (T : Verifier E X q) (positive : 0 < q) (labeling : Vertex E X → Label q) :
    2 * verifierRejectionCount T (decodedAssignment positive labeling) ≤
      (graph T positive).rejectionCount labeling := by
  classical
  let failed := rejectedEvents T (decodedAssignment positive labeling)
  have witnesses : ∀ event, event ∈ failed →
      ∃ slot, incidenceAccept T positive event slot (labeling (.inl event))
        (labeling (.inr (T.query event slot))) = false := by
    intro event he
    apply rejected_event_forces_slot T positive labeling event
    simpa [failed, rejectedEvents] using he
  let chosen := fun event he => Classical.choose (witnesses event he)
  let inject : (failed.product (Finset.univ : Finset Bool)) →
      (graph T positive).rejectedDarts labeling := fun p =>
    ⟨((p.val.1, chosen p.val.1 (Finset.mem_product.mp p.property).1), p.val.2), by
      apply (ConstraintGraph.mem_rejectedDarts _ _ _).mpr
      rw [edgeSatisfied_eq_incidence]
      exact Classical.choose_spec (witnesses p.val.1 (Finset.mem_product.mp p.property).1)⟩
  have injective : Function.Injective inject := by
    intro a b h
    apply Subtype.ext
    apply Prod.ext
    · exact congrArg (fun d => d.val.1.1) h
    · exact congrArg (fun d => d.val.2) h
  have bound := Finset.card_le_card_of_injective injective
  simpa [failed, verifierRejectionCount, ConstraintGraph.rejectionCount,
    Finset.card_product, Nat.mul_comm] using bound

@[simp] theorem card_label (q : Nat) : Fintype.card (Label q) = 2 ^ q := by
  simp [Label]

@[simp] theorem card_vertex [Fintype E] [Fintype X] :
    Fintype.card (Vertex E X) = Fintype.card E + Fintype.card X := by
  simp [Vertex, Fintype.card_sum]

@[simp] theorem card_dart [Fintype E] :
    Fintype.card (Dart E q) = 2 * q * Fintype.card E := by
  simp [Dart, Fintype.card_prod, Nat.mul_comm, Nat.mul_left_comm, Nat.mul_assoc]

theorem gap_transfer [Fintype E] [DecidableEq X] (T : Verifier E X q)
    (positive : 0 < q) (a b : Nat)
    (sourceGap : ∀ assignment : X → Bool,
      a * Fintype.card E ≤ b * verifierRejectionCount T assignment)
    (labeling : Vertex E X → Label q) :
    a * Fintype.card (Dart E q) ≤ (b * q) * (graph T positive).rejectionCount labeling := by
  have source := Nat.mul_le_mul_left (2 * q) (sourceGap (decodedAssignment positive labeling))
  have incidences := Nat.mul_le_mul_left (b * q) (rejection_count_bound T positive labeling)
  rw [card_dart]
  calc
    a * (2 * q * Fintype.card E) = (2 * q) * (a * Fintype.card E) := by ac_rfl
    _ ≤ (2 * q) * (b * verifierRejectionCount T (decodedAssignment positive labeling)) := source
    _ = (b * q) * (2 * verifierRejectionCount T (decodedAssignment positive labeling)) := by ac_rfl
    _ ≤ (b * q) * (graph T positive).rejectionCount labeling := incidences

theorem six_query_alphabet : Fintype.card (Label 6) = 64 := by
  simp

end IndependentSetsGames.Foundations.PCP.QueryIncidence

end OAI