add density and reciprocity

src/IGraph/Algorithms/Structure.chs

@@ -18,6 +18,9 @@ module IGraph.Algorithms.Structure
     , isDag
     , topSort
     , topSortUnsafe
+      -- * Other Operations
+    , density
+    , reciprocity
       -- * Auxiliary types
     , Neimode(IgraphOut,IgraphIn,IgraphAll) -- not IgraphTotal
     ) where
@@ -248,6 +251,34 @@ topSortUnsafe gr = unsafePerformIO $ allocaVectorN n $ \res -> do
     , `Neimode'
     } -> `CInt' void- #}
 
+-- | Calculate the density of a graph.
+density :: Graph d v e
+        -> Bool -- ^ whether to include loops
+        -> Double -- ^ the ratio of edges to possible edges
+density gr loops = unsafePerformIO $ alloca $ \res -> do
+  igraphDensity (_graph gr) res loops
+  peek res
+{-# INLINE igraphDensity #-}
+{#fun igraph_density as ^
+    { `IGraph'
+    , castPtr `Ptr Double'
+    , `Bool'
+    } -> `CInt' void -#}
+
+-- | Calculates the reciprocity of a directed graph.
+reciprocity :: Graph d v e
+            -> Bool -- ^ whether to ignore loop edges
+            -> Double -- ^ the proportion of mutual connections
+reciprocity gr ignore_loops = unsafePerformIO $ alloca $ \res -> do
+  igraphReciprocity (_graph gr) res ignore_loops IgraphReciprocityDefault
+  peek res
+{#fun igraph_reciprocity as ^
+    { `IGraph'
+    , castPtr `Ptr Double'
+    , `Bool'
+    , `Reciprocity'
+    } -> `CInt' void -#}
+
 -- Marshaller for those "treat edges as directed" booleans.
 dirToBool :: Num n => EdgeType -> n
 dirToBool D = cFromBool True

src/IGraph/Internal/Constants.chs

@@ -41,3 +41,6 @@ module IGraph.Internal.Constants where
 
 {#enum igraph_degseq_t as Degseq {underscoreToCase}
     deriving (Show, Read, Eq) #}
+
+{#enum igraph_reciprocity_t as Reciprocity {underscoreToCase}
+    deriving (Show, Read, Eq) #}

tests/Test/Algorithms.hs

@@ -31,6 +31,8 @@ tests = testGroup "Algorithms"
     , bridgeTest
     , pagerankTest
     , kleinbergTest
+    , densityTest
+    , reciprocityTest
     ]
 
 graphIsomorphism :: TestTree
@@ -170,6 +172,19 @@ kleinbergTest = testGroup "Kleinberg"
         fst (authorityScore (ring 4) False) @?= replicate 4 0.5
     ]
 
+densityTest :: TestTree
+densityTest = testGroup "Density"
+    [ testCase "clique" $ density (full @'U 16 False) False @?= 1
+    , testCase "ring" $ density (ring 9) False @?= 1/4
+    ]
+
+reciprocityTest :: TestTree
+reciprocityTest = testGroup "Reciprocity"
+    [ testCase "clique" $ reciprocity (full @'D 10 False) False @?= 1
+    , testCase "ring" $ reciprocity g False @?= 0
+    ]
+  where g = fromLabeledEdges @'D [(("a","b"),()),(("b","c"),()),(("c","a"),())]
+
 -- approximate equality helper
 (@?~) :: (Ord n,Fractional n) => n -> n -> Assertion
 a @?~ b = assertBool "" $ abs (b-a) < 1/65536