add diameter

src/IGraph/Algorithms/Structure.chs

@@ -3,6 +3,7 @@
 module IGraph.Algorithms.Structure
     ( -- * Shortest Path Related Functions
       shortestPath
+    , diameter
     , inducedSubgraph
     , isConnected
     , isStronglyConnected
@@ -64,6 +65,29 @@ shortestPath gr s t getEdgeW = unsafePerformIO $ allocaVector $ \path -> do
     , `Neimode'
     } -> `CInt' void- #}
 
+-- | Calculates the diameter of a graph (longest geodesic).
+diameter :: Graph d v e
+         -> EdgeType -- ^ whether to consider directed paths
+         -> Bool     -- ^ if unconnected,
+                     -- return largest component diameter (True)
+                     -- or number of vertices (False)
+         -> (Int,[Node])
+diameter graph directed unconn = unsafePerformIO $
+  alloca $ \pres ->
+  allocaVector $ \path -> do
+    igraphDiameter (_graph graph) pres nullPtr nullPtr path directed unconn
+    liftM2 (,) (peekIntConv pres) (toNodes path)
+{-# INLINE igraphDiameter #-}
+{#fun igraph_diameter as ^
+    { `IGraph'
+    , castPtr `Ptr CInt'
+    , castPtr `Ptr CInt'
+    , castPtr `Ptr CInt'
+    , castPtr `Ptr Vector'
+    , dirToBool `EdgeType'
+    , `Bool'
+    } -> `CInt' void- #}
+
 -- | Creates a subgraph induced by the specified vertices. This function collects
 -- the specified vertices and all edges between them to a new graph.
 inducedSubgraph :: (Ord v, Serialize v)
@@ -144,3 +168,8 @@ topSortUnsafe gr = unsafePerformIO $ allocaVectorN n $ \res -> do
     , castPtr `Ptr Vector'
     , `Neimode'
     } -> `CInt' void- #}
+
+-- Marshaller for those "treat edges as directed" booleans.
+dirToBool :: Num n => EdgeType -> n
+dirToBool D = cFromBool True
+dirToBool U = cFromBool False

src/IGraph/Internal.chs

@@ -8,6 +8,7 @@ module IGraph.Internal
     , withList
     , withListMaybe
     , toList
+    , toNodes
     , igraphVectorNull
     , igraphVectorFill
     , igraphVectorE
@@ -193,6 +194,10 @@ toList vec = do
         map realToFrac <$> peekArray n ptr
 {-# INLINE toList #-}
 
+toNodes :: Ptr Vector -> IO [Node]
+toNodes = fmap (map truncate) . toList
+{-# INLINE toNodes #-}
+
 {#fun igraph_vector_copy_to as ^ { castPtr `Ptr Vector', id `Ptr CDouble' } -> `()' #}
 
 -- Initializing elements

tests/Test/Algorithms.hs

 {-# LANGUAGE DataKinds #-}
+{-# LANGUAGE TypeApplications #-}
 module Test.Algorithms
     ( tests
     ) where
@@ -20,6 +21,7 @@ tests = testGroup "Algorithms"
     [ graphIsomorphism
     , motifTest
     , cliqueTest
+    , diameterTest
     , subGraphs
     , decomposeTest
     , pagerankTest
@@ -55,6 +57,13 @@ cliqueTest = testGroup "Clique"
         [2,3,4], [2,4,5] ]
     c4 = [[1, 2, 3, 4], [1, 2, 4, 5]]
 
+diameterTest :: TestTree
+diameterTest = testGroup "Diameters"
+    [ testCase "clique" $ fst (diameter (full @'U 10 False) U True)  @?= 1
+    , testCase "star"   $ fst (diameter (star 10)          D False) @?= 2
+    , testCase "ring"   $ fst (diameter (ring 10)          U False) @?= 5
+    ]
+
 subGraphs :: TestTree
 subGraphs = testGroup "generate induced subgraphs"
     [ testCase "" $ test case1 ]