From b835cddcf1129920608cebafbad2f9a94b8064f7 Mon Sep 17 00:00:00 2001
From: Michael Sippel <micha@fragmental.art>
Date: Sun, 4 Aug 2024 23:23:20 +0200
Subject: [PATCH] initial MorphismBase with DFS to find morphism paths

---
 src/lib.rs      |   1 +
 src/morphism.rs | 204 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 205 insertions(+)
 create mode 100644 src/morphism.rs

diff --git a/src/lib.rs b/src/lib.rs
index 970c555..00821d4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -10,6 +10,7 @@ pub mod lnf;
 pub mod pnf;
 pub mod subtype;
 pub mod unification;
+pub mod morphism;
 
 #[cfg(test)]
 mod test;
diff --git a/src/morphism.rs b/src/morphism.rs
new file mode 100644
index 0000000..31a8e31
--- /dev/null
+++ b/src/morphism.rs
@@ -0,0 +1,204 @@
+use {
+    crate::{
+        TypeTerm, TypeID,
+        unification::UnificationProblem,
+    },
+    std::collections::HashMap
+};
+
+//<<<<>>>><<>><><<>><<<*>>><<>><><<>><<<<>>>>\\
+
+pub struct MorphismType {
+    pub src_type: TypeTerm,
+    pub dst_type: TypeTerm,
+}
+
+pub struct MorphismBase<Morphism: Clone> {
+    morphisms: Vec< (MorphismType, Morphism) >,
+    list_typeid: TypeID
+}
+
+//<<<<>>>><<>><><<>><<<*>>><<>><><<>><<<<>>>>\\
+
+impl MorphismType {
+    fn normalize(self) -> Self {
+        MorphismType {
+            src_type: self.src_type.normalize(),
+            dst_type: self.dst_type.normalize()
+        }
+    }
+}
+
+//<<<<>>>><<>><><<>><<<*>>><<>><><<>><<<<>>>>\\
+
+impl<Morphism: Clone> MorphismBase<Morphism> {
+    pub fn new() -> Self {
+        MorphismBase {
+            morphisms: Vec::new(),
+
+            // FIXME: magic number
+            list_typeid: TypeID::Fun(10)
+        }
+    }
+
+    pub fn add_morphism(&mut self, morph_type: MorphismType, morphism: Morphism) {
+        self.morphisms.push( (morph_type.normalize(), morphism) );
+    }
+
+    pub fn enum_morphisms(&self, src_type: &TypeTerm)
+    -> Vec< (HashMap<TypeID, TypeTerm>, TypeTerm) >
+    {
+        let mut dst_types = Vec::new();
+
+        // first enumerate all "direct" morphisms,
+        for (ty,m) in self.morphisms.iter() {
+            if let Ok(σ) = crate::unification::unify(
+                &ty.src_type,
+                &src_type.clone().normalize()
+            ) {
+                let dst_type = 
+                    ty.dst_type.clone()
+                        .apply_substitution(
+                            &|x| σ.get(x).cloned()
+                        )
+                        .clone();
+
+                dst_types.push( (σ, dst_type) );
+            }
+        }
+
+        // ..then all "list-map" morphisms.
+        // Check if we have a List type, and if so, see what the Item type is
+
+        // TODO: function for generating fresh variables
+        let item_variable = TypeID::Var(100);
+
+        if let Ok(σ) = crate::unification::unify(
+            &TypeTerm::App(vec![
+                TypeTerm::TypeID(self.list_typeid),
+                TypeTerm::TypeID(item_variable)
+            ]),
+            &src_type.clone().param_normalize(),
+        ) {
+            let src_item_type = σ.get(&item_variable).unwrap().clone();
+
+            for (γ, dst_item_type) in self.enum_morphisms( &src_item_type ) {
+                let dst_type =
+                        TypeTerm::App(vec![
+                            TypeTerm::TypeID(self.list_typeid),
+                            dst_item_type.clone()
+                                .apply_substitution(
+                                    &|x| γ.get(x).cloned()
+                                ).clone()
+                        ]).normalize();
+
+                dst_types.push( (γ.clone(), dst_type) );
+            }
+        }
+
+        dst_types
+    }
+
+    pub fn enum_morphisms_with_subtyping(&self, src_type: &TypeTerm)
+    -> Vec< (TypeTerm, TypeTerm) >
+    {
+        let mut src_lnf = src_type.clone().get_lnf_vec();
+        let mut halo_lnf = vec![];
+        let mut dst_types = Vec::new();
+
+        while src_lnf.len() > 0 {
+            let src_type = TypeTerm::Ladder( src_lnf.clone() );
+            let halo_type = TypeTerm::Ladder( halo_lnf.clone() );
+
+            for (σ, t) in self.enum_morphisms( &src_type ) {
+                dst_types.push(
+                    (halo_type.clone()
+                        .apply_substitution(
+                            &|x| σ.get(x).cloned()
+                        ).clone(),
+                    t.clone()
+                        .apply_substitution(
+                            &|x| σ.get(x).cloned()
+                        ).clone()
+                    )
+                );
+            }
+
+            // continue with next supertype
+            halo_lnf.push(src_lnf.remove(0));
+        }
+
+        dst_types
+    }
+
+    /* performs DFS to find a morphism-path for a given type
+     * will return the first matching path, not the shortest
+     */
+    pub fn find_morphism_path(&self, ty: MorphismType)
+    -> Option< Vec<TypeTerm> >
+    {
+        let ty = ty.normalize();
+        let mut visited = Vec::new();
+        let mut queue = vec![
+            vec![ ty.src_type.clone().normalize() ]
+        ];
+
+        while let Some(current_path) = queue.pop() {       
+            let current_type = current_path.last().unwrap();
+
+            if ! visited.contains( current_type ) {
+                visited.push( current_type.clone() );
+
+                for (h, t) in self.enum_morphisms_with_subtyping(&current_type) {
+                    let tt = TypeTerm::Ladder( vec![ h, t ] ).normalize();
+
+                    if ! visited.contains( &tt ) {
+                        let unification_result = crate::unification::unify(&tt, &ty.dst_type);
+                        let mut new_path = current_path.clone();
+
+                        new_path.push( tt );
+
+                        if let Ok(σ) = unification_result {
+                            new_path = new_path.into_iter().map(
+                                |mut t: TypeTerm| t.apply_substitution(&|x| σ.get(x).cloned()).clone()
+                            ).collect::<Vec<TypeTerm>>();
+
+                            return Some(new_path);
+                        } else {
+                            queue.push( new_path );
+                        }
+                    }
+                }
+            }
+        }
+
+        None
+    }
+
+    pub fn find_morphism(&self, ty: &MorphismType)
+    -> Option< Morphism > {
+
+        // TODO
+
+        None
+    }
+
+    pub fn find_list_map_morphism(&self, item_ty: &MorphismType)
+    -> Option< Morphism > {
+
+        // TODO
+
+        None
+    }
+
+    pub fn find_morphism_with_subtyping(&self, ty: &MorphismType)
+    -> Option<( Morphism, TypeTerm, HashMap<TypeID, TypeTerm> )> {
+
+        // TODO
+
+        None
+    }
+}
+
+//<<<<>>>><<>><><<>><<<*>>><<>><><<>><<<<>>>>\\
+