import Mathlib.Tactic
import Mathlib.Data.Real.Basic
import Mathlib.Algebra.BigOperators.Basic

open BigOperators

namespace Chapter11

-- ===== From Section 11.1 (subset needed for Section 11.2) =====

inductive BoundedInterval where
  | Ioo (a b:ℝ) : BoundedInterval
  | Icc (a b:ℝ) : BoundedInterval
  | Ioc (a b:ℝ) : BoundedInterval
  | Ico (a b:ℝ) : BoundedInterval

open BoundedInterval

@[coe]
def BoundedInterval.toSet (I: BoundedInterval) : Set ℝ := match I with
  | Ioo a b => .Ioo a b
  | Icc a b => .Icc a b
  | Ioc a b => .Ioc a b
  | Ico a b => .Ico a b

instance BoundedInterval.inst_coeSet : Coe BoundedInterval (Set ℝ) where
  coe := toSet

instance BoundedInterval.instMembership : Membership ℝ BoundedInterval where
  mem I x := x ∈ (I:Set ℝ)

instance BoundedInterval.instSubset : HasSubset BoundedInterval where
  Subset I J := ∀ x, x ∈ I → x ∈ J

abbrev BoundedInterval.a (I: BoundedInterval) : ℝ := match I with
  | Ioo a _ => a
  | Icc a _ => a
  | Ioc a _ => a
  | Ico a _ => a

abbrev BoundedInterval.b (I: BoundedInterval) : ℝ := match I with
  | Ioo _ b => b
  | Icc _ b => b
  | Ioc _ b => b
  | Ico _ b => b

abbrev BoundedInterval.length (I: BoundedInterval) : ℝ := max (I.b - I.a) 0

/-- Using ||ₗ subscript here to not override || -/
macro:max atomic("|" noWs) a:term noWs "|ₗ" : term => `(BoundedInterval.length $a)

@[ext]
structure Partition (I: BoundedInterval) where
  intervals : Finset BoundedInterval
  exists_unique (x:ℝ) (hx : x ∈ I) : ∃! J, J ∈ intervals ∧ x ∈ J
  contains (J : BoundedInterval) (hJ : J ∈ intervals) : J ⊆ I

namespace Partition
instance instMembership (I: BoundedInterval) : Membership BoundedInterval (Partition I) where
  mem P J := J ∈ P.intervals
end Partition

-- ===== From Section 11.2 (required pieces) =====

/-- Definition 11.2.1 -/
abbrev Constant {X Y:Type} (f: X → Y) : Prop := ∃ c, ∀ x, f x = c

open Classical in
noncomputable abbrev constant_value {X Y:Type} [hY: Nonempty Y] (f:X → Y) : Y :=
  if h: Constant f then h.choose else hY.some

abbrev ConstantOn (f: ℝ → ℝ) (X: Set ℝ) : Prop := Constant (fun x : X ↦ f ↑x)

noncomputable abbrev constant_value_on (f:ℝ → ℝ) (X: Set ℝ) : ℝ := constant_value (fun x : X ↦ f ↑x)

/-- Definition 11.2.3 (Piecewise constant functions I) -/
abbrev PiecewiseConstantWith (f:ℝ → ℝ) {I: BoundedInterval} (P: Partition I) : Prop :=
  ∀ J ∈ P, ConstantOn f (J:Set ℝ)

/-- Definition 11.2.5 (Piecewise constant functions I) -/
abbrev PiecewiseConstantOn (f:ℝ → ℝ) (I: BoundedInterval) : Prop :=
  ∃ P : Partition I, PiecewiseConstantWith f P

/-- Definition 11.2.9 (Piecewise constant integral I)-/
noncomputable abbrev PiecewiseConstantWith.integ (f:ℝ → ℝ) {I: BoundedInterval} (P: Partition I)  :
  ℝ := ∑ J ∈ P.intervals, constant_value_on f (J:Set ℝ) * |J|ₗ

open Classical in
/-- Definition 11.2.14 (Piecewise constant integral II)  -/
noncomputable abbrev PiecewiseConstantOn.integ (f:ℝ → ℝ) (I: BoundedInterval) :
  ℝ := if h: PiecewiseConstantOn f I then PiecewiseConstantWith.integ f h.choose else 0

/-- Theorem 11.2.16 (a) (Laws of integration) / Exercise 11.2.4 -/
theorem PiecewiseConstantOn.integ_add {f g: ℝ → ℝ} {I: BoundedInterval}
  (hf: PiecewiseConstantOn f I) (hg: PiecewiseConstantOn g I) :
  integ (f + g) I = integ f I + integ g I := by
end Chapter11