From 1a19a80d8c756559e108a46129fac9489ad43d28 Mon Sep 17 00:00:00 2001
From: Determinant <ted.sybil@gmail.com>
Date: Wed, 31 May 2017 17:11:14 -0400
Subject: ...

---
 80-89/80.hs | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)
 create mode 100644 80-89/80.hs

(limited to '80-89/80.hs')

diff --git a/80-89/80.hs b/80-89/80.hs
new file mode 100644
index 0000000..4dc2956
--- /dev/null
+++ b/80-89/80.hs
@@ -0,0 +1,51 @@
+import Data.List (sort)
+
+data Graph a = Graph [a] [(a, a)]
+               deriving (Show, Eq)
+                
+data Adjacency a = Adj [(a, [a])]
+           deriving (Show, Eq)
+            
+data Friendly a = Edge [(a, a)]
+          deriving (Show, Eq)
+
+graphToAdj :: Eq a => Graph a -> Adjacency a
+
+graphToAdj (Graph [] _) = Adj []
+graphToAdj (Graph (v:vs) e) = Adj ((v, e >>= pick):l)
+    where pick (a, b)
+            | a == v = [b]
+            | b == v = [a]
+            | otherwise = []
+          Adj l = graphToAdj $ Graph vs e
+
+adjToGraph :: Eq a => Adjacency a -> Graph a
+
+adjToGraph (Adj []) = Graph [] []
+adjToGraph (Adj ((u, e):ps)) = Graph (u:us)
+                                     ((map (\v -> (u, v)) e) ++
+                                      (filter (\(a, b) -> a /= u && b /= u) es))
+    where (Graph us es) = adjToGraph $ Adj ps
+
+graphToFri :: Eq a => Graph a -> Friendly a
+
+graphToFri (Graph vs e) =
+    Edge (e ++ let g = filter (\v -> all (\(a, b) -> v /= a && v /= b) e) vs in
+                   zip g g)
+
+friToGraph :: Ord a => Friendly a -> Graph a
+
+friToGraph (Edge es) = Graph vs' (filter (\(a, b) -> a /= b) es')
+    where unique [] = []
+          unique [x] = [x]
+          unique (x:l@(y:xs))
+            | x == y = unique l
+            | otherwise = x:unique l
+          es' = unique . sort $ map (\(a, b) -> if a < b then (a, b) else (b, a)) es
+          vs' = unique . sort $ es' >>= (\(a, b) -> [a, b])
+
+adjToFri :: Eq a => Adjacency a -> Friendly a
+friToAdj :: Ord a => Friendly a -> Adjacency a
+
+adjToFri = graphToFri . adjToGraph
+friToAdj = graphToAdj . friToGraph
-- 
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