import OAI.Probability.GaussianRegression.Nets
import OAI.Probability.GaussianRegression.InverseMoments

namespace OAI

/-!
Affine tube covers and inverse-distance bounds.
-/

open MeasureTheory ProbabilityTheory Set Filter
open scoped ENNReal NNReal RealInnerProductSpace Topology

noncomputable section

universe u

namespace NoiselessRegression

section AffineTubes

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

open EuclideanGeometry

theorem exists_affine_tube_cover (L : AffineSubspace ℝ E) [Nonempty L]
    {r : ℝ} (hr : 0 < r) (hr1 : r ≤ 1) :
    ∃ s : Finset L.direction,
      (s.card : ℝ) ≤ (3 / r) ^ Module.finrank ℝ L.direction ∧
      ∀ t : E, ‖t‖ ≤ 1 → Metric.infDist t L ≤ r →
        ∃ y ∈ s, dist t ((y : E) + (orthogonalProjection L 0 : E)) ≤ 2 * r := by
  classical
  obtain ⟨s, _, hcard, hnet⟩ := exists_unit_ball_net (E := L.direction) hr hr1
  refine ⟨s, hcard, ?_⟩
  intro t ht hdist
  obtain ⟨y, hy, hty⟩ := hnet (L.direction.orthogonalProjectionOnto t)
    ((L.direction.norm_orthogonalProjectionOnto_apply_le t).trans ht)
  refine ⟨y, hy, ?_⟩
  have hproj : (orthogonalProjection L t : E) =
      (L.direction.orthogonalProjectionOnto t : E) + (orthogonalProjection L 0 : E) := by
    have h := (orthogonalProjection L).map_vadd (0 : E) t
    simpa only [vadd_eq_add, add_zero, orthogonalProjection_contLinear,
      AffineSubspace.coe_vadd] using congrArg (fun x : L => (x : E)) h
  have hpdist : dist (orthogonalProjection L t : E)
      ((y : E) + (orthogonalProjection L 0 : E)) ≤ r := by
    rw [hproj, dist_add_right]
    exact hty
  have hd : dist t (orthogonalProjection L t : E) ≤ r := by
    rwa [dist_orthogonalProjection_eq_infDist]
  calc
    dist t ((y : E) + (orthogonalProjection L 0 : E)) ≤
        dist t (orthogonalProjection L t : E) +
          dist (orthogonalProjection L t : E) ((y : E) + (orthogonalProjection L 0 : E)) :=
      dist_triangle _ _ _
    _ ≤ r + r := add_le_add hd hpdist
    _ = 2 * r := by ring

theorem HasLocalMassBound.affine_tube {ν : Measure E} {a : ℝ}
    (hν : HasLocalMassBound ν a) (hsupport : ν.support ⊆ Metric.closedBall 0 1)
    (L : AffineSubspace ℝ E) [Nonempty L] {r : ℝ} (hr : 0 < r) (hr1 : r ≤ 1) :
    ν {t | Metric.infDist t L ≤ r} ≤
      ENNReal.ofReal ((3 / r) ^ Module.finrank ℝ L.direction * (4 * r) ^ a) := by
  classical
  obtain ⟨s, hcard, hcover⟩ := exists_affine_tube_cover L hr hr1
  let c : L.direction → E := fun y => (y : E) + (orthogonalProjection L 0 : E)
  have hsub : {t | Metric.infDist t L ≤ r} ≤ᵐ[ν]
      ⋃ y ∈ s, Metric.closedBall (c y) (2 * r) := by
    filter_upwards [ν.support_mem_ae] with t ht
    intro hdist
    have htunit : ‖t‖ ≤ 1 := by simpa using hsupport ht
    obtain ⟨y, hy, hty⟩ := hcover t htunit hdist
    exact mem_iUnion₂.mpr ⟨y, hy, hty⟩
  calc
    ν {t | Metric.infDist t L ≤ r} ≤ ν (⋃ y ∈ s, Metric.closedBall (c y) (2 * r)) :=
      measure_mono_ae hsub
    _ ≤ ∑ y ∈ s, ν (Metric.closedBall (c y) (2 * r)) := measure_biUnion_finset_le _ _
    _ ≤ ∑ _y ∈ s, ENNReal.ofReal ((4 * r) ^ a) := by
      refine Finset.sum_le_sum fun y _ => ?_
      simpa only [show (2 : ℝ) * (2 * r) = 4 * r by ring] using
        hν.any_center (c y) (r := 2 * r) (by positivity)
    _ = (s.card : ℝ≥0∞) * ENNReal.ofReal ((4 * r) ^ a) := by simp
    _ ≤ ENNReal.ofReal ((3 / r) ^ Module.finrank ℝ L.direction) *
        ENNReal.ofReal ((4 * r) ^ a) := by
      gcongr
      exact_mod_cast ENNReal.ofReal_le_ofReal hcard
    _ = _ := (ENNReal.ofReal_mul (by positivity)).symm

lemma tube_radius_factors (ℓ : ℕ) (a : ℝ) {r : ℝ} (hr : 0 < r) :
    (3 / r) ^ ℓ * (4 * r) ^ a =
      (3 : ℝ) ^ ℓ * (4 : ℝ) ^ a * r ^ (a - ℓ) := by
  rw [div_pow, Real.mul_rpow (by norm_num) hr.le,
    Real.rpow_sub_natCast hr.ne']
  ring

theorem HasLocalMassBound.affine_tube_pow {ν : Measure E} {a : ℝ}
    (hν : HasLocalMassBound ν a) (hsupport : ν.support ⊆ Metric.closedBall 0 1)
    (L : AffineSubspace ℝ E) [Nonempty L] (k : ℕ)
    (hmargin : (Module.finrank ℝ L.direction : ℝ) + k ≤ a)
    {r : ℝ} (hr : 0 < r) (hr1 : r ≤ 1) :
    ν {t | Metric.infDist t L ≤ r} ≤
      ENNReal.ofReal ((3 : ℝ) ^ Module.finrank ℝ L.direction * (4 : ℝ) ^ a) *
        ENNReal.ofReal (r ^ k) := by
  refine (hν.affine_tube hsupport L hr hr1).trans ?_
  rw [tube_radius_factors _ _ hr, ← ENNReal.ofReal_mul (by positivity)]
  apply ENNReal.ofReal_le_ofReal
  apply mul_le_mul_of_nonneg_left _ (by positivity)
  rw [← Real.rpow_natCast]
  exact Real.rpow_le_rpow_of_exponent_ge hr hr1 (by linarith)

theorem HasLocalMassBound.affine_inverse_moment {ν : Measure E} {a : ℝ}
    (hν : HasLocalMassBound ν a) (hsupport : ν.support ⊆ Metric.closedBall 0 1)
    (L : AffineSubspace ℝ E) [Nonempty L] (m : ℕ)
    (hmargin : (Module.finrank ℝ L.direction : ℝ) + (m + 1 : ℕ) ≤ a) :
    ∫⁻ t, ENNReal.ofReal ((Metric.infDist t L)⁻¹ ^ m) ∂ν ≤
      ν Set.univ +
        ENNReal.ofReal ((3 : ℝ) ^ Module.finrank ℝ L.direction * (4 : ℝ) ^ a) *
          (2 : ℝ≥0∞) ^ (m + 1) := by
  apply inverse_moment_of_tube_bound ν _ m (Metric.continuous_infDist_pt _).measurable
    (fun _ => Metric.infDist_nonneg)
  intro r hr hr1
  exact hν.affine_tube_pow hsupport L (m + 1) hmargin hr hr1

theorem HasLocalMassBound.affine_null {ν : Measure E} {a : ℝ}
    (hν : HasLocalMassBound ν a) (hsupport : ν.support ⊆ Metric.closedBall 0 1)
    (L : AffineSubspace ℝ E) [Nonempty L]
    (hmargin : (Module.finrank ℝ L.direction : ℝ) + 1 ≤ a) : ν L = 0 := by
  let C : ℝ := (3 : ℝ) ^ Module.finrank ℝ L.direction * (4 : ℝ) ^ a
  have hC : 0 ≤ C := by dsimp [C]; positivity
  have hb (n : ℕ) : ν L ≤ ENNReal.ofReal (C * (1 / 2 : ℝ) ^ n) := by
    have hn : 0 < (1 / 2 : ℝ) ^ n := by positivity
    have hn1 : (1 / 2 : ℝ) ^ n ≤ 1 := pow_le_one₀ (by norm_num) (by norm_num)
    calc
      ν L ≤ ν {t | Metric.infDist t L ≤ (1 / 2 : ℝ) ^ n} := by
        apply measure_mono
        intro t ht
        simp only [mem_ofPred_eq, Metric.infDist_zero_of_mem ht]
        exact hn.le
      _ ≤ ENNReal.ofReal C * ENNReal.ofReal (((1 / 2 : ℝ) ^ n) ^ 1) :=
        hν.affine_tube_pow hsupport L 1 (by simpa using hmargin) hn hn1
      _ = _ := by rw [pow_one, ENNReal.ofReal_mul hC]
  have hlim : Tendsto (fun n : ℕ => ENNReal.ofReal (C * (1 / 2 : ℝ) ^ n))
      atTop (𝓝 0) := by
    convert ENNReal.tendsto_ofReal ((tendsto_const_nhds (x := C)).mul
      (tendsto_pow_atTop_nhds_zero_of_lt_one (by norm_num : (0 : ℝ) ≤ 1 / 2)
        (by norm_num : (1 / 2 : ℝ) < 1))) using 1
    simp
  exact le_antisymm (ge_of_tendsto' hlim hb) (by positivity)

end AffineTubes

end NoiselessRegression

end

end OAI