import Mathlib
import OAI.Probability.SKGap.Model

namespace OAI

section
noncomputable section
open MeasureTheory ProbabilityTheory InformationTheory Real Set
open scoped NNReal ENNReal
open Filter
open scoped Topology
noncomputable section
open Matrix Real
open scoped BigOperators Matrix.Norms.Frobenius ENNReal NNReal
noncomputable section
open Matrix Real
open scoped BigOperators Matrix.Norms.Frobenius NNReal
noncomputable section
open MeasureTheory ProbabilityTheory Real Set Filter
open MeasureTheory.Measure
open scoped ENNReal NNReal MeasureTheory Topology
namespace SKGap

section Smoothing
open NormedSpace
open scoped Convolution ContDiff
variable {E : Type*} [NormedAddCommGroup E] [InnerProductSpace ℝ E]
  [FiniteDimensional ℝ E] [MeasurableSpace E] [BorelSpace E]

theorem lipschitz_normed_convolution {f : E → ℝ} {L : ℝ≥0}
    (hf : LipschitzWith L f) (φ : ContDiffBump (0 : E))
    (μ : Measure E) [μ.IsAddHaarMeasure] :
    LipschitzWith L (φ.normed μ ⋆[ContinuousLinearMap.lsmul ℝ ℝ, μ] f) := by
  have hcv : ConvolutionExists (φ.normed μ) f (ContinuousLinearMap.lsmul ℝ ℝ) μ :=
    (φ.hasCompactSupport_normed (μ := μ)).convolutionExists_left
      (ContinuousLinearMap.lsmul ℝ ℝ) φ.continuous_normed
      (hf.continuous.locallyIntegrable (μ := μ))
  apply LipschitzWith.of_dist_le_mul
  intro x y
  simp only [dist_eq_norm, convolution_def, ContinuousLinearMap.lsmul_apply, smul_eq_mul]
  have hx : Integrable (fun t => φ.normed μ t * f (x - t)) μ := hcv x
  have hy : Integrable (fun t => φ.normed μ t * f (y - t)) μ := hcv y
  rw [← integral_sub hx hy]
  calc
    _ ≤ ∫ t, φ.normed μ t * ((L : ℝ) * ‖x - y‖) ∂μ := by
      apply norm_integral_le_of_norm_le (φ.integrable_normed.mul_const _)
      filter_upwards [] with t
      rw [← mul_sub, norm_mul, Real.norm_eq_abs (φ.normed μ t),
        abs_of_nonneg (φ.nonneg_normed _)]
      apply mul_le_mul_of_nonneg_left _ (φ.nonneg_normed _)
      simpa only [sub_sub_sub_cancel_right] using hf.norm_sub_le (x - t) (y - t)
    _ = (L : ℝ) * ‖x - y‖ := by rw [integral_mul_const, φ.integral_normed, one_mul]

theorem exists_smooth_lipschitz_approx {f : E → ℝ} {L : ℝ≥0}
    (hf : LipschitzWith L f) {ε : ℝ} (hε : 0 < ε) :
    ∃ g : E → ℝ, ContDiff ℝ ∞ g ∧ LipschitzWith L g ∧
      ∀ x, dist (g x) (f x) ≤ (L : ℝ) * ε := by
  let μ : Measure E := addHaar
  let φ : ContDiffBump (0 : E) := ⟨ε / 2, ε, half_pos hε, half_lt_self hε⟩
  let g := φ.normed μ ⋆[ContinuousLinearMap.lsmul ℝ ℝ, μ] f
  refine ⟨g, ?_, lipschitz_normed_convolution hf φ μ, ?_⟩
  · exact φ.hasCompactSupport_normed.contDiff_convolution_left
      (ContinuousLinearMap.lsmul ℝ ℝ) φ.contDiff_normed hf.continuous.locallyIntegrable
  · intro x
    apply ContDiffBump.dist_normed_convolution_le hf.continuous.aestronglyMeasurable
    intro y hy
    apply (hf.dist_le_mul y x).trans
    exact mul_le_mul_of_nonneg_left (Metric.mem_ball.mp hy).le L.coe_nonneg

end Smoothing

section GaussianMoments
variable {E : Type*} [NormedAddCommGroup E] [NormedSpace ℝ E]
  [MeasurableSpace E] [BorelSpace E] [SecondCountableTopology E] [CompleteSpace E]
  {μ : Measure E} [IsGaussian μ]

theorem gaussian_integrable_exp_norm (a : ℝ) :
    Integrable (fun x : E => exp (a * ‖x‖)) μ := by
  obtain ⟨C, hC, hI⟩ := IsGaussian.exists_integrable_exp_sq μ
  apply (hI.const_mul (exp (a ^ 2 / (4 * C)))).mono' (by fun_prop)
  filter_upwards [] with x
  rw [Real.norm_eq_abs, abs_of_pos (exp_pos _), ← exp_add]
  apply exp_le_exp.mpr
  have hs := sq_nonneg (‖x‖ - a / (2 * C))
  have hC0 : C ≠ 0 := ne_of_gt hC
  have heq : C * (‖x‖ - a / (2 * C)) ^ 2 =
      C * ‖x‖ ^ 2 - a * ‖x‖ + a ^ 2 / (4 * C) := by field_simp; ring
  nlinarith [mul_nonneg (le_of_lt hC) hs]

end GaussianMoments

section StandardGaussian
variable {E : Type*} [NormedAddCommGroup E] [InnerProductSpace ℝ E]
  [FiniteDimensional ℝ E] [MeasurableSpace E] [BorelSpace E]

theorem stdGaussian_map_dual (L : StrongDual ℝ E) :
    (stdGaussian E).map L = gaussianReal 0 (Real.toNNReal (‖L‖ ^ 2)) := by
  rw [IsGaussian.map_eq_gaussianReal, integral_strongDual_stdGaussian,
    variance_dual_stdGaussian]

theorem stdGaussian_integrable_exp_dual (L : StrongDual ℝ E) (r : ℝ) :
    Integrable (fun x => exp (r * L x)) (stdGaussian E) := by
  have h := integrable_exp_mul_gaussianReal (μ := 0)
    (v := Real.toNNReal (‖L‖ ^ 2)) r
  rw [← stdGaussian_map_dual L] at h
  exact h.comp_measurable L.continuous.measurable

theorem stdGaussian_integral_exp_dual (L : StrongDual ℝ E) (r : ℝ) :
    (∫ x, exp (r * L x) ∂stdGaussian E) = exp (r ^ 2 * ‖L‖ ^ 2 / 2) := by
  have hm := mgf_gaussianReal
    ⟨L.continuous.measurable.aemeasurable, stdGaussian_map_dual L⟩ r
  simpa [mgf, Real.toNNReal_of_nonneg (sq_nonneg _), mul_div_assoc, mul_comm] using hm

theorem gaussian_integrable_exp_lipschitz {μ : Measure E} [IsGaussian μ]
    {f : E → ℝ} {L : ℝ≥0} (hf : LipschitzWith L f) (r : ℝ) :
    Integrable (fun x => exp (r * f x)) μ := by
  have hfc := hf.continuous
  have hI := (gaussian_integrable_exp_norm (μ := μ) (|r| * L)).const_mul
    (exp (|r| * |f 0|))
  apply hI.mono' (by fun_prop)
  filter_upwards [] with x
  rw [Real.norm_eq_abs, abs_of_pos (exp_pos _), ← exp_add]
  apply exp_le_exp.mpr
  have hx : |f x| ≤ |f 0| + L * ‖x‖ := by
    have hh := hf.norm_sub_le x 0
    simp only [sub_zero, Real.norm_eq_abs] at hh
    have ht := abs_add_le (f x - f 0) (f 0)
    rw [sub_add_cancel] at ht
    linarith
  calc
    r * f x ≤ |r * f x| := le_abs_self _
    _ = |r| * |f x| := abs_mul _ _
    _ ≤ |r| * (|f 0| + L * ‖x‖) := mul_le_mul_of_nonneg_left hx (abs_nonneg _)
    _ = _ := by ring

theorem gaussian_integrable_lipschitz {μ : Measure E} [IsGaussian μ]
    {f : E → ℝ} {L : ℝ≥0} (hf : LipschitzWith L f) : Integrable f μ := by
  have hI := ((IsGaussian.integrable_id (μ := μ)).norm.const_mul (L : ℝ)).add
    (integrable_const |f 0|)
  apply hI.mono' hf.continuous.aestronglyMeasurable
  filter_upwards [] with x
  change |f x| ≤ (L : ℝ) * ‖x‖ + |f 0|
  have hh := hf.norm_sub_le x 0
  simp only [sub_zero, Real.norm_eq_abs] at hh ⊢
  have ht := abs_add_le (f x - f 0) (f 0)
  rw [sub_add_cancel] at ht
  linarith

end StandardGaussian

theorem exp_interval_average_le {a : ℝ} (ha : 0 < a) {g : ℝ → ℝ}
    (hg : Continuous g) :
    exp (a⁻¹ * ∫ t in (0 : ℝ)..a, g t) ≤
      a⁻¹ * ∫ t in (0 : ℝ)..a, exp (g t) := by
  have h0 : volume (Icc (0 : ℝ) a) ≠ 0 := by
    rw [volume_Icc, sub_zero, ENNReal.ofReal_ne_zero_iff]
    exact ha
  have ht : volume (Icc (0 : ℝ) a) ≠ ∞ := by simp
  have hJ := convexOn_exp.map_set_average_le continuous_exp.continuousOn isClosed_univ
    h0 ht (by filter_upwards [] with x; exact mem_univ _)
    hg.integrableOn_Icc (continuous_exp.comp hg).integrableOn_Icc
  have hvol : volume.real (Icc (0 : ℝ) a) = a := by
    simp [measureReal_def, volume_Icc, ha.le]
  simpa only [setAverage_eq, hvol, smul_eq_mul,
    integral_Icc_eq_integral_Ioc, ← intervalIntegral.integral_of_le ha.le] using hJ

section Rotation
variable {E : Type*} [NormedAddCommGroup E] [NormedSpace ℝ E]

theorem rotation_snd_norm_le (θ : ℝ) (p : E × E) :
    ‖(ContinuousLinearMap.rotation θ p).2‖ ≤ ‖p.1‖ + ‖p.2‖ := by
  rw [ContinuousLinearMap.rotation_apply]
  calc
    ‖-sin θ • p.1 + cos θ • p.2‖ ≤ ‖-sin θ • p.1‖ + ‖cos θ • p.2‖ := norm_add_le _ _
    _ = |sin θ| * ‖p.1‖ + |cos θ| * ‖p.2‖ := by simp [norm_smul, Real.norm_eq_abs]
    _ ≤ 1 * ‖p.1‖ + 1 * ‖p.2‖ :=
      add_le_add (mul_le_mul_of_nonneg_right (abs_sin_le_one _) (norm_nonneg _))
        (mul_le_mul_of_nonneg_right (abs_cos_le_one _) (norm_nonneg _))
    _ = _ := by ring

theorem hasDerivAt_rotation_fst (θ : ℝ) (p : E × E) :
    HasDerivAt (fun t => (ContinuousLinearMap.rotation t p).1)
      (ContinuousLinearMap.rotation θ p).2 θ := by
  exact ((hasDerivAt_cos θ).smul_const p.1).add
    ((hasDerivAt_sin θ).smul_const p.2)

theorem rotation_fundamental_theorem {f : E → ℝ} {df : E → StrongDual ℝ E}
    (hf : ∀ x, HasFDerivAt f (df x) x) (hdf : Continuous df) (p : E × E) :
    (∫ θ in (0 : ℝ)..π / 2,
      df (ContinuousLinearMap.rotation θ p).1 (ContinuousLinearMap.rotation θ p).2) =
        f p.2 - f p.1 := by
  have hd : ∀ θ, HasDerivAt (fun t => f (ContinuousLinearMap.rotation t p).1)
      (df (ContinuousLinearMap.rotation θ p).1 (ContinuousLinearMap.rotation θ p).2) θ :=
    fun θ => (hf _).comp_hasDerivAt θ (hasDerivAt_rotation_fst θ p)
  have hc : Continuous (fun θ =>
      df (ContinuousLinearMap.rotation θ p).1 (ContinuousLinearMap.rotation θ p).2) := by
    simp only [ContinuousLinearMap.rotation_apply]
    fun_prop
  have hi := intervalIntegral.integral_eq_sub_of_hasDerivAt (fun θ _ => hd θ)
    (hc.intervalIntegrable 0 (π / 2))
  simpa [ContinuousLinearMap.rotation_apply] using hi

theorem rotation_exp_difference_le {f : E → ℝ} {df : E → StrongDual ℝ E}
    (hf : ∀ x, HasFDerivAt f (df x) x) (hdf : Continuous df) (r : ℝ) (p : E × E) :
    exp (r * (f p.2 - f p.1)) ≤ (π / 2)⁻¹ *
      ∫ θ in (0 : ℝ)..π / 2, exp (r * (π / 2) *
        df (ContinuousLinearMap.rotation θ p).1 (ContinuousLinearMap.rotation θ p).2) := by
  have hc : Continuous (fun θ => r * (π / 2) *
      df (ContinuousLinearMap.rotation θ p).1 (ContinuousLinearMap.rotation θ p).2) := by
    simp only [ContinuousLinearMap.rotation_apply]
    fun_prop
  have hJ := exp_interval_average_le (half_pos pi_pos) hc
  rw [intervalIntegral.integral_const_mul, rotation_fundamental_theorem hf hdf] at hJ
  convert hJ using 1
  congr 1
  field_simp

theorem exp_dual_le {L : ℝ≥0} (D : StrongDual ℝ E) (hD : ‖D‖ ≤ L) (r : ℝ) (y : E) :
    exp (r * D y) ≤ exp (|r| * L * ‖y‖) := by
  apply exp_le_exp.mpr
  calc
    r * D y ≤ |r * D y| := le_abs_self _
    _ = |r| * ‖D y‖ := by rw [abs_mul, Real.norm_eq_abs]
    _ ≤ |r| * (‖D‖ * ‖y‖) := mul_le_mul_of_nonneg_left (D.le_opNorm y) (abs_nonneg _)
    _ ≤ |r| * ((L : ℝ) * ‖y‖) := by gcongr
    _ = _ := by ring

theorem rotation_exp_dual_le {L : ℝ≥0} {df : E → StrongDual ℝ E}
    (hdf : ∀ x, ‖df x‖ ≤ L) (r θ : ℝ) (p : E × E) :
    exp (r * df (ContinuousLinearMap.rotation θ p).1 (ContinuousLinearMap.rotation θ p).2) ≤
      exp (|r| * L * ‖p.1‖) * exp (|r| * L * ‖p.2‖) := by
  calc
    _ ≤ exp (|r| * L * ‖(ContinuousLinearMap.rotation θ p).2‖) := exp_dual_le _ (hdf _) _ _
    _ ≤ exp (|r| * L * (‖p.1‖ + ‖p.2‖)) := by
      gcongr
      exact rotation_snd_norm_le θ p
    _ = _ := by rw [mul_add, exp_add]

end Rotation

section StandardGaussianRotation
variable {E : Type*} [NormedAddCommGroup E] [InnerProductSpace ℝ E]
  [FiniteDimensional ℝ E] [MeasurableSpace E] [BorelSpace E]

theorem stdGaussian_integrable_variable_dual {df : E → StrongDual ℝ E} (hc : Continuous df)
    {L : ℝ≥0} (hdf : ∀ x, ‖df x‖ ≤ L) (r : ℝ) :
    Integrable (fun p : E × E => exp (r * df p.1 p.2))
      ((stdGaussian E).prod (stdGaussian E)) := by
  apply ((gaussian_integrable_exp_norm (μ := stdGaussian E) (|r| * L)).comp_snd
    (stdGaussian E)).mono' (by fun_prop)
  filter_upwards [] with p
  rw [Real.norm_eq_abs, abs_of_pos (exp_pos _)]
  exact exp_dual_le _ (hdf _) _ _

theorem stdGaussian_integral_variable_dual_le {df : E → StrongDual ℝ E} (hc : Continuous df)
    {L : ℝ≥0} (hdf : ∀ x, ‖df x‖ ≤ L) (r : ℝ) :
    (∫ p : E × E, exp (r * df p.1 p.2) ∂(stdGaussian E).prod (stdGaussian E)) ≤
      exp (r ^ 2 * (L : ℝ) ^ 2 / 2) := by
  have hi := stdGaussian_integrable_variable_dual hc hdf r
  have hio := hi.integral_prod_left
  simp only [stdGaussian_integral_exp_dual] at hio
  rw [integral_prod _ hi]
  simp only [stdGaussian_integral_exp_dual]
  calc
    _ ≤ ∫ _x : E, exp (r ^ 2 * (L : ℝ) ^ 2 / 2) ∂stdGaussian E := by
      apply integral_mono hio (integrable_const _)
      intro x
      apply exp_le_exp.mpr
      gcongr
      exact hdf x
    _ = _ := by simp

theorem stdGaussian_integral_rotation_variable_dual_le {df : E → StrongDual ℝ E}
    (hc : Continuous df) {L : ℝ≥0} (hdf : ∀ x, ‖df x‖ ≤ L) (r θ : ℝ) :
    (∫ p : E × E, exp (r * df (ContinuousLinearMap.rotation θ p).1
        (ContinuousLinearMap.rotation θ p).2) ∂(stdGaussian E).prod (stdGaussian E)) ≤
      exp (r ^ 2 * (L : ℝ) ^ 2 / 2) := by
  have hm := IsGaussian.map_rotation_eq_self (μ := stdGaussian E) (by simp) θ
  rw [← integral_map (ContinuousLinearMap.rotation θ).continuous.aemeasurable
    (f := fun p : E × E => exp (r * df p.1 p.2)) (by fun_prop), hm]
  exact stdGaussian_integral_variable_dual_le hc hdf r

theorem stdGaussian_integrable_rotation_variable_dual {df : E → StrongDual ℝ E}
    (hc : Continuous df) {L : ℝ≥0} (hdf : ∀ x, ‖df x‖ ≤ L) (r : ℝ)
    (ν : Measure ℝ) [IsFiniteMeasure ν] :
    Integrable (fun z : ℝ × (E × E) =>
      exp (r * df (ContinuousLinearMap.rotation z.1 z.2).1
        (ContinuousLinearMap.rotation z.1 z.2).2))
      (ν.prod ((stdGaussian E).prod (stdGaussian E))) := by
  have hI := gaussian_integrable_exp_norm (μ := stdGaussian E) (|r| * L)
  apply ((hI.mul_prod hI).comp_snd ν).mono'
  · simp only [ContinuousLinearMap.rotation_apply]
    fun_prop
  · filter_upwards [] with z
    rw [Real.norm_eq_abs, abs_of_pos (exp_pos _)]
    exact rotation_exp_dual_le hdf _ _ _

omit [FiniteDimensional ℝ E] [MeasurableSpace E] [BorelSpace E] in
theorem lipschitz_of_bounded_derivative {f : E → ℝ} {df : E → StrongDual ℝ E}
    (hf : ∀ x, HasFDerivAt f (df x) x) {L : ℝ≥0} (hdf : ∀ x, ‖df x‖ ≤ L) :
    LipschitzWith L f := by
  apply lipschitzWith_of_nnnorm_fderiv_le (fun x => (hf x).differentiableAt)
  intro x
  rw [(hf x).fderiv]
  exact hdf x

lemma exp_mul_sub (r a b : ℝ) : exp (r * (a - b)) = exp (-r * b) * exp (r * a) := by
  rw [← exp_add]
  congr 1
  ring

theorem stdGaussian_integrable_exp_difference {f : E → ℝ} {L : ℝ≥0}
    (hf : LipschitzWith L f) (r : ℝ) :
    Integrable (fun p : E × E => exp (r * (f p.2 - f p.1)))
      ((stdGaussian E).prod (stdGaussian E)) := by
  simp_rw [exp_mul_sub]
  exact (gaussian_integrable_exp_lipschitz hf (-r)).mul_prod
    (gaussian_integrable_exp_lipschitz hf r)

theorem stdGaussian_mgf_difference_le {f : E → ℝ} {df : E → StrongDual ℝ E}
    (hf : ∀ x, HasFDerivAt f (df x) x) (hc : Continuous df)
    {L : ℝ≥0} (hdf : ∀ x, ‖df x‖ ≤ L) (r : ℝ) :
    (∫ p : E × E, exp (r * (f p.2 - f p.1)) ∂(stdGaussian E).prod (stdGaussian E)) ≤
      exp (π ^ 2 * r ^ 2 * (L : ℝ) ^ 2 / 8) := by
  let ν : Measure ℝ := volume.restrict (Ioc (0 : ℝ) (π / 2))
  have hν : IsFiniteMeasure ν := by dsimp [ν]; infer_instance
  let R := r * (π / 2)
  let G : ℝ → E × E → ℝ := fun θ p => exp (R *
    df (ContinuousLinearMap.rotation θ p).1 (ContinuousLinearMap.rotation θ p).2)
  have hI : Integrable (Function.uncurry G) (ν.prod ((stdGaussian E).prod (stdGaussian E))) :=
    stdGaussian_integrable_rotation_variable_dual hc hdf R ν
  have hD := stdGaussian_integrable_exp_difference (lipschitz_of_bounded_derivative hf hdf) r
  have hpos : 0 < π / 2 := half_pos pi_pos
  have hvol : ν.real univ = π / 2 := by
    simp [ν, measureReal_def, Real.volume_Ioc, hpos.le]
  have hpoint (p : E × E) : exp (r * (f p.2 - f p.1)) ≤
      (π / 2)⁻¹ * ∫ θ, G θ p ∂ν := by
    simpa only [intervalIntegral.integral_of_le hpos.le] using rotation_exp_difference_le hf hc r p
  calc
    _ ≤ ∫ p : E × E, (π / 2)⁻¹ * (∫ θ, G θ p ∂ν)
        ∂(stdGaussian E).prod (stdGaussian E) :=
      integral_mono hD (hI.integral_prod_right.const_mul _) hpoint
    _ = (π / 2)⁻¹ * ∫ θ, (∫ p : E × E, G θ p
        ∂(stdGaussian E).prod (stdGaussian E)) ∂ν := by
      rw [integral_const_mul, ← integral_integral_swap hI]
    _ ≤ (π / 2)⁻¹ * ∫ _θ, exp (R ^ 2 * (L : ℝ) ^ 2 / 2) ∂ν := by
      apply mul_le_mul_of_nonneg_left _ (inv_nonneg.mpr hpos.le)
      apply integral_mono hI.integral_prod_left (integrable_const _)
      intro θ
      exact stdGaussian_integral_rotation_variable_dual_le hc hdf R θ
    _ = exp (π ^ 2 * r ^ 2 * (L : ℝ) ^ 2 / 8) := by
      rw [integral_const, smul_eq_mul, hvol, ← mul_assoc, inv_mul_cancel₀ hpos.ne', one_mul]
      congr 1
      dsimp [R]
      ring

theorem stdGaussian_mgf_centered_le {f : E → ℝ} {df : E → StrongDual ℝ E}
    (hf : ∀ x, HasFDerivAt f (df x) x) (hc : Continuous df)
    {L : ℝ≥0} (hdf : ∀ x, ‖df x‖ ≤ L) (r : ℝ) :
    (∫ x, exp (r * (f x - ∫ y, f y ∂stdGaussian E)) ∂stdGaussian E) ≤
      exp (π ^ 2 * r ^ 2 * (L : ℝ) ^ 2 / 8) := by
  have hLip := lipschitz_of_bounded_derivative hf hdf
  have hF := gaussian_integrable_lipschitz (μ := stdGaussian E) hLip
  have hG := gaussian_integrable_exp_lipschitz (μ := stdGaussian E) hLip (-r)
  have hpair := stdGaussian_integrable_exp_difference hLip (-r)
  have hcenter : Integrable (fun x => exp (r * (f x - ∫ y, f y ∂stdGaussian E)))
      (stdGaussian E) := by
    simp_rw [exp_mul_sub]
    exact (gaussian_integrable_exp_lipschitz hLip r).const_mul _
  have hj (x : E) : exp (r * (f x - ∫ y, f y ∂stdGaussian E)) ≤
      ∫ y, exp (r * (f x - f y)) ∂stdGaussian E := by
    have hsub := ((integrable_const (f x)).sub hF).const_mul r
    have he : Integrable (fun y => exp (r * (f x - f y))) (stdGaussian E) := by
      simp_rw [exp_mul_sub]
      exact hG.mul_const _
    have h := convexOn_exp.map_integral_le continuous_exp.continuousOn isClosed_univ
      (by filter_upwards [] with y; exact mem_univ _) hsub he
    simp only [Pi.sub_apply] at h
    simpa only [integral_const_mul, integral_sub (integrable_const _) hF,
      integral_const, probReal_univ, one_smul] using h
  have heq (x y : E) : r * (f x - f y) = -r * (f y - f x) := by ring
  simp_rw [heq] at hj
  calc
    _ ≤ ∫ x, (∫ y, exp (-r * (f y - f x)) ∂stdGaussian E) ∂stdGaussian E :=
      integral_mono hcenter hpair.integral_prod_left hj
    _ = ∫ p : E × E, exp (-r * (f p.2 - f p.1))
        ∂(stdGaussian E).prod (stdGaussian E) := (integral_prod _ hpair).symm
    _ ≤ exp (π ^ 2 * r ^ 2 * (L : ℝ) ^ 2 / 8) := by
      simpa using stdGaussian_mgf_difference_le hf hc hdf (-r)

theorem centered_mgf_le_of_uniform_dist {f g : E → ℝ} {L M : ℝ≥0}
    (hf : LipschitzWith L f) (hg : LipschitzWith M g) {δ : ℝ}
    (hfg : ∀ x, dist (f x) (g x) ≤ δ) (r : ℝ) :
    (∫ x, exp (r * (f x - ∫ y, f y ∂stdGaussian E)) ∂stdGaussian E) ≤
      exp (2 * |r| * δ) *
        (∫ x, exp (r * (g x - ∫ y, g y ∂stdGaussian E)) ∂stdGaussian E) := by
  let μ := stdGaussian E
  have hfi : Integrable f μ := gaussian_integrable_lipschitz hf
  have hgi : Integrable g μ := gaussian_integrable_lipschitz hg
  have hmean : |(∫ y, f y ∂μ) - ∫ y, g y ∂μ| ≤ δ := by
    have hi : ‖∫ x, f x - g x ∂μ‖ ≤ δ * μ.real univ :=
      norm_integral_le_of_norm_le_const (μ := μ)
        (Eventually.of_forall (fun x => by simpa only [dist_eq_norm] using hfg x))
    rw [integral_sub hfi hgi] at hi
    simpa only [Real.norm_eq_abs, probReal_univ, mul_one] using hi
  have he (h : E → ℝ) {K : ℝ≥0} (hh : LipschitzWith K h) :
      Integrable (fun x => exp (r * (h x - ∫ y, h y ∂μ))) μ := by
    simp_rw [exp_mul_sub]
    exact (gaussian_integrable_exp_lipschitz hh r).const_mul _
  rw [← integral_const_mul]
  apply integral_mono (he f hf) ((he g hg).const_mul _)
  intro x
  dsimp only
  rw [← exp_add]
  apply exp_le_exp.mpr
  have hdiff : |(f x - ∫ y, f y ∂μ) - (g x - ∫ y, g y ∂μ)| ≤ 2 * δ := by
    have hx : |f x - g x| ≤ δ := by simpa only [Real.dist_eq] using hfg x
    calc
      _ = |(f x - g x) - ((∫ y, f y ∂μ) - ∫ y, g y ∂μ)| := by congr 1; ring
      _ ≤ |f x - g x| + |(∫ y, f y ∂μ) - ∫ y, g y ∂μ| := by
        simpa only [Real.norm_eq_abs] using norm_sub_le
          (f x - g x) ((∫ y, f y ∂μ) - ∫ y, g y ∂μ)
      _ ≤ 2 * δ := by linarith
  have hh := le_abs_self (r * ((f x - ∫ y, f y ∂μ) - (g x - ∫ y, g y ∂μ)))
  rw [abs_mul] at hh
  have hb := mul_le_mul_of_nonneg_left hdiff (abs_nonneg r)
  dsimp [μ] at *
  nlinarith

end StandardGaussianRotation
end SKGap
end
end
end
end
end

end OAI