PhysLean/HepLean/Tensors/Tree/Basic.lean

/-
Copyright (c) 2024 Joseph Tooby-Smith. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Joseph Tooby-Smith
-/
import HepLean.Tensors.OverColor.Iso
import HepLean.Tensors.OverColor.Discrete
import HepLean.Tensors.OverColor.Lift
import Mathlib.CategoryTheory.Monoidal.NaturalTransformation
import LLMLean
/-!

## Tensor trees

-/

open IndexNotation
open CategoryTheory
open MonoidalCategory

/-- The sturcture of a type of tensors e.g. Lorentz tensors, Einstien tensors,
  complex Lorentz tensors.
  Note: This structure is not fully defined yet. -/
structure TensorStruct where
  /-- The colors of indices e.g. up or down. -/
  C : Type
  /-- The symmetry group acting on these tensor e.g. the Lorentz group or SL(2,ℂ). -/
  G : Type
  /-- An instance of `G` as a group. -/
  G_group : Group G
  /-- The field over which we want to consider the tensors to live in, usually `ℝ` or `ℂ`. -/
  k : Type
  /-- An instance of `k` as a commutative ring. -/
  k_commRing : CommRing k
  /-- A `MonoidalFunctor` from `OverColor C` giving the rep corresponding to a map of colors
    `X → C`. -/
  FDiscrete : Discrete C ⥤ Rep k G
  /-- A map from `C` to `C`. An involution. -/
  τ : C → C
  τ_involution : Function.Involutive τ
  /-- The natural transformation describing contraction. -/
  contr : OverColor.Discrete.pairτ FDiscrete τ ⟶ 𝟙_ (Discrete C ⥤ Rep k G)
  /-- The natural transformation describing the metric. -/
  metric : 𝟙_ (Discrete C ⥤ Rep k G) ⟶ OverColor.Discrete.pair FDiscrete
  /-- The natural transformation describing the unit. -/
  unit : 𝟙_ (Discrete C ⥤ Rep k G) ⟶ OverColor.Discrete.τPair FDiscrete τ
  /-- A specification of the dimension of each color in C. This will be used for explicit
    evaluation of tensors. -/
  evalNo : C → ℕ

noncomputable section

namespace TensorStruct

variable (S : TensorStruct)

instance : CommRing S.k := S.k_commRing

instance : Group S.G := S.G_group

/-- The lift of the functor `S.F` to a monoidal functor. -/
def F : MonoidalFunctor (OverColor S.C) (Rep S.k S.G) := (OverColor.lift).obj S.FDiscrete

/-
def metric (c : S.C) : S.F.obj (OverColor.mk ![c, c]) :=
  (OverColor.Discrete.pairIso S.FDiscrete c).hom.hom <|
  (S.metricNat.app (Discrete.mk c)).hom (1 : S.k)
-/

/-- The isomorphism of objects in `Rep S.k S.G` given an `i` in `Fin n.succ.succ` and
  a `j` in `Fin n.succ` allowing us to undertake contraction. -/
def contrIso {n : ℕ} (c : Fin n.succ.succ → S.C)
    (i : Fin n.succ.succ) (j : Fin n.succ) (h : c (i.succAbove j) = S.τ (c i)) :
    S.F.obj (OverColor.mk c) ≅ ((OverColor.Discrete.pairτ S.FDiscrete S.τ).obj
      (Discrete.mk (c i))) ⊗
      (OverColor.lift.obj S.FDiscrete).obj (OverColor.mk (c ∘ i.succAbove ∘ j.succAbove)) :=
  (S.F.mapIso (OverColor.equivToIso (OverColor.finExtractTwo i j))).trans <|
  (S.F.mapIso (OverColor.mkSum (c ∘ (OverColor.finExtractTwo i j).symm))).trans <|
  (S.F.μIso _ _).symm.trans <| by
  refine tensorIso ?_ (S.F.mapIso (OverColor.mkIso (by ext x; simp)))
  apply (S.F.mapIso (OverColor.mkSum (((c ∘ ⇑(OverColor.finExtractTwo i j).symm) ∘ Sum.inl)))).trans
  apply (S.F.μIso _ _).symm.trans
  apply tensorIso ?_ ?_
  · symm
    apply (OverColor.forgetLiftApp S.FDiscrete (c i)).symm.trans
    apply S.F.mapIso
    apply OverColor.mkIso
    funext x
    fin_cases x
    rfl
  · symm
    apply (OverColor.forgetLiftApp S.FDiscrete (S.τ (c i))).symm.trans
    apply S.F.mapIso
    apply OverColor.mkIso
    funext x
    fin_cases x
    simp [h]

/--
`contrMap` is a function that takes a natural number `n`, a function `c` from
`Fin n.succ.succ` to `S.C`, an index `i` of type `Fin n.succ.succ`, an index `j` of type
`Fin n.succ`, and a proof `h` that `c (i.succAbove j) = S.τ (c i)`. It returns a morphism
corresponding to the contraction of the `i`th index with the `i.succAbove j` index.
--/
def contrMap {n : ℕ} (c : Fin n.succ.succ → S.C)
    (i : Fin n.succ.succ) (j : Fin n.succ) (h : c (i.succAbove j) = S.τ (c i)) :
    S.F.obj (OverColor.mk c) ⟶
    (OverColor.lift.obj S.FDiscrete).obj (OverColor.mk (c ∘ i.succAbove ∘ j.succAbove)) :=
  (S.contrIso c i j h).hom ≫
  (tensorHom (S.contr.app (Discrete.mk (c i))) (𝟙 _)) ≫
  (MonoidalCategory.leftUnitor _).hom

end TensorStruct

/-- A syntax tree for tensor expressions. -/
inductive TensorTree (S : TensorStruct) : ∀ {n : ℕ}, (Fin n → S.C) → Type where
  /-- A general tensor node. -/
  | tensorNode {n : ℕ} {c : Fin n → S.C} (T : S.F.obj (OverColor.mk c)) : TensorTree S c
  /-- A node consisting of a single vector. -/
  | vecNode {c : S.C} (v : S.FDiscrete.obj (Discrete.mk c)) : TensorTree S ![c]
  /-- A node consisting of a two tensor. -/
  | twoNode {c1 c2 : S.C}
    (v : (S.FDiscrete.obj (Discrete.mk c1) ⊗ S.FDiscrete.obj (Discrete.mk c2)).V) :
    TensorTree S ![c1, c2]
  /-- A node consisting of a three tensor. -/
  | threeNode {c1 c2 c3 : S.C}
    (v : S.FDiscrete.obj (Discrete.mk c1) ⊗ S.FDiscrete.obj (Discrete.mk c2) ⊗
      S.FDiscrete.obj (Discrete.mk c3)) : TensorTree S ![c1, c2, c3]
  /-- A general constant node. -/
  | constNode {n : ℕ} {c : Fin n → S.C} (T : 𝟙_ (Rep S.k S.G) ⟶ S.F.obj (OverColor.mk c)) :
    TensorTree S c
  /-- A constant vector. -/
  | constVecNode {c : S.C} (v : 𝟙_ (Rep S.k S.G) ⟶ S.FDiscrete.obj (Discrete.mk c)) :
    TensorTree S ![c]
  /-- A constant two tensor (e.g. metric and unit). -/
  | constTwoNode {c1 c2 : S.C}
    (v : 𝟙_ (Rep S.k S.G) ⟶ S.FDiscrete.obj (Discrete.mk c1) ⊗ S.FDiscrete.obj (Discrete.mk c2)) :
    TensorTree S ![c1, c2]
  /-- A constant three tensor (e.g. Pauli-matrices). -/
  | constThreeNode {c1 c2 c3 : S.C}
    (v : 𝟙_ (Rep S.k S.G) ⟶ S.FDiscrete.obj (Discrete.mk c1) ⊗ S.FDiscrete.obj (Discrete.mk c2) ⊗
      S.FDiscrete.obj (Discrete.mk c3)) : TensorTree S ![c1, c2, c3]
  /-- A node corresponding to the addition of two tensors. -/
  | add {n : ℕ} {c : Fin n → S.C} : TensorTree S c → TensorTree S c → TensorTree S c
  /-- A node corresponding to the permutation of indices of a tensor. -/
  | perm {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
      (σ : (OverColor.mk c) ⟶ (OverColor.mk c1)) (t : TensorTree S c) : TensorTree S c1
  | prod {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    (t : TensorTree S c) (t1 : TensorTree S c1) : TensorTree S (Sum.elim c c1 ∘ finSumFinEquiv.symm)
  | smul {n : ℕ} {c : Fin n → S.C} : S.k → TensorTree S c → TensorTree S c
  /-- The negative of a node. -/
  | neg {n : ℕ} {c : Fin n → S.C} : TensorTree S c → TensorTree S c
  | contr {n : ℕ} {c : Fin n.succ.succ → S.C} : (i : Fin n.succ.succ) →
    (j : Fin n.succ) → (h : c (i.succAbove j) = S.τ (c i)) → TensorTree S c →
    TensorTree S (c ∘ Fin.succAbove i ∘ Fin.succAbove j)
  | eval {n : ℕ} {c : Fin n.succ → S.C} :
    (i : Fin n.succ) → (x : Fin (S.evalNo (c i))) → TensorTree S c →
    TensorTree S (c ∘ Fin.succAbove i)

namespace TensorTree

variable {S : TensorStruct} {n : ℕ} {c : Fin n → S.C} (T : TensorTree S c)

open MonoidalCategory
open TensorProduct

/-- The node `twoNode` of a tensor tree, with all arguments explicit. -/
abbrev twoNodeE (S : TensorStruct) (c1 c2 : S.C)
    (v : (S.FDiscrete.obj (Discrete.mk c1) ⊗ S.FDiscrete.obj (Discrete.mk c2)).V) :
    TensorTree S ![c1, c2] := twoNode v

/-- The node `constTwoNodeE` of a tensor tree, with all arguments explicit. -/
abbrev constTwoNodeE (S : TensorStruct) (c1 c2 : S.C)
    (v : 𝟙_ (Rep S.k S.G) ⟶ S.FDiscrete.obj (Discrete.mk c1) ⊗ S.FDiscrete.obj (Discrete.mk c2)) :
    TensorTree S ![c1, c2] := constTwoNode v

/-- The node `constThreeNodeE` of a tensor tree, with all arguments explicit. -/
abbrev constThreeNodeE (S : TensorStruct) (c1 c2 c3 : S.C)
    (v : 𝟙_ (Rep S.k S.G) ⟶ S.FDiscrete.obj (Discrete.mk c1) ⊗ S.FDiscrete.obj (Discrete.mk c2) ⊗
      S.FDiscrete.obj (Discrete.mk c3)) : TensorTree S ![c1, c2, c3] :=
    constThreeNode v

/-- The number of nodes in a tensor tree. -/
def size : ∀ {n : ℕ} {c : Fin n → S.C}, TensorTree S c → ℕ := fun
  | tensorNode _ => 1
  | vecNode _ => 1
  | twoNode _ => 1
  | threeNode _ => 1
  | constNode _ => 1
  | constVecNode _ => 1
  | constTwoNode _ => 1
  | constThreeNode _ => 1
  | add t1 t2 => t1.size + t2.size + 1
  | perm _ t => t.size + 1
  | neg t => t.size + 1
  | smul _ t => t.size + 1
  | prod t1 t2 => t1.size + t2.size + 1
  | contr _ _ _ t => t.size + 1
  | eval _ _ t => t.size + 1

noncomputable section

/-- The underlying tensor a tensor tree corresponds to.
  Note: This function is not fully defined yet. -/
def tensor : ∀ {n : ℕ} {c : Fin n → S.C}, TensorTree S c → S.F.obj (OverColor.mk c) := fun
  | tensorNode t => t
  | constTwoNode t => (OverColor.Discrete.pairIsoSep S.FDiscrete).hom.hom (t.hom (1 : S.k))
  | add t1 t2 => t1.tensor + t2.tensor
  | perm σ t => (S.F.map σ).hom t.tensor
  | neg t => - t.tensor
  | smul a t => a • t.tensor
  | prod t1 t2 => (S.F.map (OverColor.equivToIso finSumFinEquiv).hom).hom
    ((S.F.μ _ _).hom (t1.tensor ⊗ₜ t2.tensor))
  | contr i j h t => (S.contrMap _ i j h).hom t.tensor
  | _ => 0

/-!

## Tensor on different nodes.

-/

@[simp]
lemma tensoreNode_tensor {c : Fin n → S.C} (T : S.F.obj (OverColor.mk c)) :
    (tensorNode T).tensor = T := rfl

@[simp]
lemma constTwoNode_tensor {c1 c2 : S.C}
    (v : 𝟙_ (Rep S.k S.G) ⟶ S.FDiscrete.obj (Discrete.mk c1) ⊗ S.FDiscrete.obj (Discrete.mk c2)) :
    (constTwoNode v).tensor = (OverColor.Discrete.pairIsoSep S.FDiscrete).hom.hom (v.hom (1 : S.k)) :=
  rfl

lemma prod_tensor {c1 : Fin n → S.C} {c2 : Fin m → S.C} (t1 : TensorTree S c1) (t2 : TensorTree S c2) :
    (prod t1 t2).tensor = (S.F.map (OverColor.equivToIso finSumFinEquiv).hom).hom
    ((S.F.μ _ _).hom (t1.tensor ⊗ₜ t2.tensor))  := rfl

lemma add_tensor (t1 t2 : TensorTree S c) : (add t1 t2).tensor = t1.tensor + t2.tensor := rfl

lemma perm_tensor (σ : (OverColor.mk c) ⟶ (OverColor.mk c1)) (t : TensorTree S c) :
    (perm σ t).tensor = (S.F.map σ).hom t.tensor := rfl

lemma contr_tensor {n : ℕ} {c : Fin n.succ.succ → S.C}  {i : Fin n.succ.succ} {j : Fin n.succ} {h : c (i.succAbove j) = S.τ (c i)}
    (t : TensorTree S c) : (contr i j h t).tensor = (S.contrMap c i j h).hom t.tensor := rfl

lemma neg_tensor (t : TensorTree S c) : (neg t).tensor = - t.tensor := rfl

/-!

## Equality of tensors and rewrites.

-/
lemma contr_tensor_eq {n : ℕ} {c : Fin n.succ.succ → S.C} {T1 T2 : TensorTree S c}
    (h : T1.tensor = T2.tensor) {i : Fin n.succ.succ} {j : Fin n.succ}
    {h' : c (i.succAbove j) = S.τ (c i)} :
    (contr i j h' T1).tensor = (contr i j h' T2).tensor := by
  simp only [Nat.succ_eq_add_one, contr_tensor]
  rw [h]

lemma prod_tensor_eq_fst {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    {T1  T1' : TensorTree S c} { T2 : TensorTree S c1}
    (h : T1.tensor = T1'.tensor) :
    (prod T1 T2).tensor = (prod T1' T2).tensor := by
  simp [prod_tensor]
  rw [h]

lemma prod_tensor_eq_snd {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    {T1 : TensorTree S c} {T2 T2' : TensorTree S c1}
    (h : T2.tensor = T2'.tensor) :
    (prod T1 T2).tensor = (prod T1 T2').tensor := by
  simp [prod_tensor]
  rw [h]

/-!

## Negation lemmas

We define the simp lemmas here so that negation is always moved to the top of the tree.
-/

@[simp]
lemma neg_neg (t : TensorTree S c) : (neg (neg t)).tensor = t.tensor := by
  simp only [neg_tensor, _root_.neg_neg]

@[simp]
lemma neg_fst_prod  {c1 : Fin n → S.C} {c2 : Fin m → S.C} (T1 : TensorTree S c1)
    (T2 : TensorTree S c2) :
    (prod (neg T1) T2).tensor = (neg (prod T1 T2)).tensor := by
  simp only [prod_tensor, Functor.id_obj, Action.instMonoidalCategory_tensorObj_V,
    Equivalence.symm_inverse, Action.functorCategoryEquivalence_functor,
    Action.FunctorCategoryEquivalence.functor_obj_obj, neg_tensor, neg_tmul, map_neg]

@[simp]
lemma neg_snd_prod  {c1 : Fin n → S.C} {c2 : Fin m → S.C} (T1 : TensorTree S c1)
    (T2 : TensorTree S c2) :
    (prod T1 (neg T2)).tensor = (neg (prod T1 T2)).tensor := by
  simp only [prod_tensor, Functor.id_obj, Action.instMonoidalCategory_tensorObj_V,
    Equivalence.symm_inverse, Action.functorCategoryEquivalence_functor,
    Action.FunctorCategoryEquivalence.functor_obj_obj, neg_tensor, tmul_neg, map_neg]

@[simp]
lemma neg_contr {n : ℕ} {c : Fin n.succ.succ → S.C} {i : Fin n.succ.succ} {j : Fin n.succ} {h : c (i.succAbove j) = S.τ (c i)}
    (t : TensorTree S c) : (contr i j h (neg t)).tensor = (neg (contr i j h t)).tensor := by
  simp only [Nat.succ_eq_add_one, contr_tensor, neg_tensor, map_neg]

end

end TensorTree

end