lean/OAI/Combinatorics/IndependentSets/Machines/MachineComposition.lean

import Mathlib
import OAI.Combinatorics.IndependentSets.Machines.MachineEmbedding

namespace OAI

namespace IndependentSetsGames.Foundations.Complexity.MachineComposition

def advance {σ : Type} (step : σ → Option σ) (state : Option σ) : Option σ :=
  state.bind step

@[simp] theorem advance_none {σ : Type} (step : σ → Option σ) :
    advance step none = none := rfl

@[simp] theorem advance_some {σ : Type} (step : σ → Option σ) (state : σ) :
    advance step (some state) = step state := rfl

@[simp] theorem advance_iterate_none {σ : Type} (step : σ → Option σ) (n : Nat) :
    (advance step)^[n] none = none := by
  induction n with
  | zero => rfl
  | succ n ih => rw [Function.iterate_succ_apply', ih, advance_none]

theorem liftSuccessfulTrace {σ τ : Type} (source : σ → Option σ)
    (target : τ → Option τ) (embed : σ → τ)
    (simulation : ∀ a b, source a = some b → target (embed a) = some (embed b))
    (n : Nat) (start finish : σ)
    (trace : (advance source)^[n] (some start) = some finish) :
    (advance target)^[n] (some (embed start)) = some (embed finish) := by
  induction n generalizing start with
  | zero =>
      have same := Option.some.inj trace
      cases same
      rfl
  | succ n ih =>
      rw [Function.iterate_succ_apply] at trace ⊢
      change (advance source)^[n] (source start) = some finish at trace
      change (advance target)^[n] (target (embed start)) = some (embed finish)
      cases firstStep : source start with
      | none =>
          rw [firstStep, advance_iterate_none] at trace
          contradiction
      | some intermediate =>
          rw [firstStep] at trace
          rw [simulation start intermediate firstStep]
          exact ih intermediate trace

def liftExecutionInTime {σ τ : Type} (source : σ → Option σ)
    (target : τ → Option τ) (embed : σ → τ)
    (simulation : ∀ a b, source a = some b → target (embed a) = some (embed b))
    {start finish : σ} {budget : Nat}
    (execution : StateTransition.EvalsToInTime source start (some finish) budget) :
    StateTransition.EvalsToInTime target (embed start) (some (embed finish)) budget where
  steps := execution.steps
  evals_in_steps := by
    have sourceTrace := execution.evals_in_steps
    change (advance source)^[execution.steps] (some start) = some finish at sourceTrace
    change (advance target)^[execution.steps] (some (embed start)) = some (embed finish)
    exact liftSuccessfulTrace source target embed simulation execution.steps start finish sourceTrace
  steps_le_m := execution.steps_le_m

@[simp] theorem liftExecutionInTime_steps {σ τ : Type} (source : σ → Option σ)
    (target : τ → Option τ) (embed : σ → τ)
    (simulation : ∀ a b, source a = some b → target (embed a) = some (embed b))
    {start finish : σ} {budget : Nat}
    (execution : StateTransition.EvalsToInTime source start (some finish) budget) :
    (liftExecutionInTime source target embed simulation execution).steps = execution.steps := rfl

def embeddedExecution {K E Λ Λextra σ τ : Type} {Γ : K → Type} {Δ : E → Type}
    [DecidableEq K] [DecidableEq E]
    (haltTarget : Option (Λ ⊕ Λextra)) (extraState : τ)
    (extraTapes : ∀ e, List (Δ e))
    (source : Λ → Turing.TM2.Stmt Γ Λ σ)
    (extra : Λextra → Turing.TM2.Stmt (MachineEmbedding.Alphabet Γ Δ)
      (Λ ⊕ Λextra) (σ × τ))
    {start finish : Turing.TM2.Cfg Γ Λ σ} {budget : Nat}
    (execution : StateTransition.EvalsToInTime (Turing.TM2.step source)
      start (some finish) budget) :
    StateTransition.EvalsToInTime (Turing.TM2.step (MachineEmbedding.program haltTarget source extra))
      (MachineEmbedding.configuration haltTarget extraState extraTapes start)
      (some (MachineEmbedding.configuration haltTarget extraState extraTapes finish)) budget :=
  liftExecutionInTime (Turing.TM2.step source)
    (Turing.TM2.step (MachineEmbedding.program haltTarget source extra))
    (MachineEmbedding.configuration haltTarget extraState extraTapes)
    (MachineEmbedding.step_simulation haltTarget extraState extraTapes source extra) execution

theorem natPolynomial_eval_mono (p : Polynomial Nat) {a b : Nat} (h : a ≤ b) :
    p.eval a ≤ p.eval b := by
  induction p using Polynomial.induction_on' with
  | add p q hp hq =>
      simpa only [Polynomial.eval_add] using Nat.add_le_add hp hq
  | monomial degree coefficient =>
      simp only [Polynomial.eval_monomial]
      exact Nat.mul_le_mul_left coefficient (Nat.pow_le_pow_left h degree)

noncomputable def compositionPolynomial (first second intermediate : Polynomial Nat) : Polynomial Nat :=
  first + Polynomial.C 2 * (intermediate + 1) + second.comp intermediate

theorem compositionBudget_le (first second intermediate : Polynomial Nat)
    (inputLength intermediateLength : Nat)
    (intermediateBound : intermediateLength ≤ intermediate.eval inputLength) :
    first.eval inputLength + 2 * (intermediateLength + 1) + second.eval intermediateLength ≤
      (compositionPolynomial first second intermediate).eval inputLength := by
  have secondBound := natPolynomial_eval_mono second intermediateBound
  have transferBound := Nat.mul_le_mul_left 2 (Nat.add_le_add_right intermediateBound 1)
  simp only [compositionPolynomial, Polynomial.eval_add, Polynomial.eval_mul,
    Polynomial.eval_C, Polynomial.eval_one, Polynomial.eval_comp]
  omega

def fourPhaseExecution {σ : Type} (step : σ → Option σ)
    (start afterFirst afterTransfer₁ afterTransfer₂ : σ) (finish : Option σ)
    (first second intermediate : Polynomial Nat) (inputLength intermediateLength : Nat)
    (intermediateBound : intermediateLength ≤ intermediate.eval inputLength)
    (runFirst : StateTransition.EvalsToInTime step start (some afterFirst) (first.eval inputLength))
    (transfer₁ : StateTransition.EvalsToInTime step afterFirst (some afterTransfer₁)
      (intermediateLength + 1))
    (transfer₂ : StateTransition.EvalsToInTime step afterTransfer₁ (some afterTransfer₂)
      (intermediateLength + 1))
    (runSecond : StateTransition.EvalsToInTime step afterTransfer₂ finish
      (second.eval intermediateLength)) :
    StateTransition.EvalsToInTime step start finish
      ((compositionPolynomial first second intermediate).eval inputLength) := by
  let firstTwo := StateTransition.EvalsToInTime.trans step _ _ start afterFirst
    (some afterTransfer₁) runFirst transfer₁
  let firstThree := StateTransition.EvalsToInTime.trans step _ _ start afterTransfer₁
    (some afterTransfer₂) firstTwo transfer₂
  let allFour := StateTransition.EvalsToInTime.trans step _ _ start afterTransfer₂
    finish firstThree runSecond
  refine {
    toEvalsTo := allFour.toEvalsTo
    steps_le_m := Nat.le_trans allFour.steps_le_m ?_
  }
  have bounded := compositionBudget_le first second intermediate inputLength intermediateLength
    intermediateBound
  omega

end IndependentSetsGames.Foundations.Complexity.MachineComposition

end OAI