From 7b22bdc170d53e83e4aa5d5f9dc24e5dba42aa77 Mon Sep 17 00:00:00 2001
From: Michael Sippel <micha@fragmental.art>
Date: Mon, 2 Oct 2023 18:48:04 +0200
Subject: [PATCH] implement ladder normalization

---
 src/lib.rs      |   1 +
 src/lnf.rs      | 112 ++++++++++++++++++++++++++++++++++++++++++++++++
 src/test/lnf.rs |  45 ++++++++++++++++++-
 3 files changed, 156 insertions(+), 2 deletions(-)
 create mode 100644 src/lnf.rs

diff --git a/src/lib.rs b/src/lib.rs
index 0319a0c..d46d35b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -5,6 +5,7 @@ pub mod term;
 pub mod lexer;
 pub mod parser;
 pub mod curry;
+pub mod lnf;
 
 #[cfg(test)]
 mod test;
diff --git a/src/lnf.rs b/src/lnf.rs
new file mode 100644
index 0000000..8acd421
--- /dev/null
+++ b/src/lnf.rs
@@ -0,0 +1,112 @@
+use crate::term::TypeTerm;
+
+//<<<<>>>><<>><><<>><<<*>>><<>><><<>><<<<>>>>\\
+
+impl TypeTerm {
+    /// does the type contain ladders (false) or is it 'flat' (true) ?
+    ///
+    /// Example:
+    /// ```<Seq <Digit 10>>``` is flat, but
+    /// ```<Digit 10>~Char``` is not
+    pub fn is_flat(&self) -> bool {
+        match self {
+            TypeTerm::TypeID(_) => true,
+            TypeTerm::Num(_) => true,
+            TypeTerm::Char(_) => true,
+            TypeTerm::App(args) => args.iter().fold(true, |s,x| s && x.is_flat()),
+            TypeTerm::Ladder(_) => false
+        }
+    }
+
+    /// transmute type into Ladder-Normal-Form (LNF)
+    ///
+    /// Example:
+    /// ```ignore
+    /// <Seq <Digit 10>~Char>
+    /// ⇒ <Seq <Digit 10>>~<Seq Char>
+    /// ```
+    pub fn normalize(self) -> Self {
+        let mut new_ladder = Vec::<TypeTerm>::new();
+
+        match self {
+            TypeTerm::Ladder(args) => {
+                for x in args.into_iter() {
+                    new_ladder.push(x.normalize());
+                }
+            }
+
+            TypeTerm::App(args) => {
+                let mut args_iter = args.into_iter();
+
+                new_ladder.push( TypeTerm::App(vec![]) );
+
+                for arg in args_iter {
+                    match arg.clone() {
+                        TypeTerm::Ladder(rungs) => {
+                            // duplicate last element for each rung
+                            let l = new_ladder.len();
+                            for _ in 1..rungs.len() {
+                                new_ladder.push( new_ladder.last().unwrap().clone() );
+                            }
+
+                            for (i,r) in new_ladder.iter_mut().enumerate() {
+                                match r {
+                                    TypeTerm::App(al) => {
+                                        if i < l {
+                                            al.push(rungs[0].clone());
+                                        } else {
+                                            al.push(rungs[i-l+1].clone());
+                                        }
+                                    }
+                                    _ => unreachable!()
+                                }
+                            }
+                        }
+                        mut other => {
+                            other = other.normalize();
+                            for rung in new_ladder.iter_mut() {
+                                match rung {
+                                    TypeTerm::App(al) => {
+                                        al.push(other.clone());
+                                    }
+                                    _ => unreachable!()
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+
+            atom => {
+                new_ladder.push(atom);
+            }
+        }
+
+        match new_ladder.len() {
+            0 => TypeTerm::unit(),
+            1 => new_ladder.into_iter().next().unwrap(),
+            _ => TypeTerm::Ladder( new_ladder )
+        }
+    }
+    
+    /// transmute type into a `Vec` containing
+    /// all rungs of the type in LNF
+    ///
+    /// Example:
+    /// ```<Seq <Digit 10>~Char>``` gives
+    /// ```ignore
+    /// vec![ <Seq <Digit 10>>,  <Seq Char> ]
+    /// ```
+    pub fn get_lnf_vec(self) -> Vec<TypeTerm> {
+        match self.normalize() {
+            TypeTerm::Ladder( v ) => {
+                v
+            },
+            _ => {
+                unreachable!();
+            }
+        }
+    }
+}
+
+//<<<<>>>><<>><><<>><<<*>>><<>><><<>><<<<>>>>\\
diff --git a/src/test/lnf.rs b/src/test/lnf.rs
index 0915387..486c0d6 100644
--- a/src/test/lnf.rs
+++ b/src/test/lnf.rs
@@ -1,6 +1,47 @@
+use crate::dict::TypeDict;
 
 #[test]
-fn test_lnf() {
-    // todo
+fn test_flat() {
+    let mut dict = TypeDict::new();
+
+    assert!( dict.parse("A").expect("parse error").is_flat() );
+    assert!( dict.parse("10").expect("parse error").is_flat() );
+    assert!( dict.parse("'x'").expect("parse error").is_flat() );
+    assert!( dict.parse("<A B 23>").expect("parse error").is_flat() );
+    assert!( dict.parse("<A <B C 'x' D>>").expect("parse error").is_flat() );
+
+    assert!( ! dict.parse("A~B").expect("parse error").is_flat() );
+    assert!( ! dict.parse("<A B~C>").expect("parse error").is_flat() );
+    assert!( ! dict.parse("<A <B C~X D>>").expect("parse error").is_flat() );
+}
+
+#[test]
+fn test_normalize() {
+    let mut dict = TypeDict::new();
+
+    assert_eq!(
+        dict.parse("A~B~C").expect("parse error").normalize(),
+        dict.parse("A~B~C").expect("parse errror"),
+    );
+
+    assert_eq!(
+        dict.parse("<A B>~C").expect("parse error").normalize(),
+        dict.parse("<A B>~C").expect("parse errror"),
+    );
+
+    assert_eq!(
+        dict.parse("<A B~C>").expect("parse error").normalize(),
+        dict.parse("<A B>~<A C>").expect("parse errror"),
+    );
+
+    assert_eq!(
+        dict.parse("<A B~C D~E>").expect("parse error").normalize(),
+        dict.parse("<A B D>~<A C D>~<A C E>").expect("parse errror"),
+    );
+
+    assert_eq!(
+        dict.parse("<Seq <Digit 10>~Char>").expect("parse error").normalize(),
+        dict.parse("<Seq <Digit 10>>~<Seq Char>").expect("parse errror"),
+    );
 }