IntegerInterval: add memberCount

Returns the number of integers that lie within an integer interval.

src/Data/IntegerInterval.hs

@@ -309,6 +309,17 @@ width x
         (Finite lb, Finite ub) -> ub - lb
         _ -> error "Data.IntegerInterval.width: unbounded interval"
 
+-- | How many integers lie within the interval.
+-- Equal to @width + 1@ for non-empty intervals.
+-- @memberCount@ of an unbounded interval is @undefined@.
+memberCount :: IntegerInterval -> Integer
+memberCount x
+  | null x = 0
+  | otherwise =
+      case (lowerBound x, upperBound x) of
+        (Finite lb, Finite ub) -> ub - lb + 1
+        _ -> error "Data.IntegerInterval.memberCount: unbounded interval"
+
 -- | pick up an element from the interval if the interval is not empty.
 pickup :: IntegerInterval -> Maybe Integer
 pickup x =

test/TestIntegerInterval.hs

@@ -275,6 +275,20 @@ prop_width_singleton =
   forAll arbitrary $ \x ->
     IntegerInterval.width (IntegerInterval.singleton x) == 0
 
+{--------------------------------------------------------------------
+  memberCount
+--------------------------------------------------------------------}
+
+case_memberCount_null =
+  IntegerInterval.memberCount IntegerInterval.empty @?= 0
+
+case_memberCount_positive =
+  IntegerInterval.memberCount (0 <=..< 10) @?= 10
+
+prop_memberCount_singleton =
+  forAll arbitrary $ \x ->
+    IntegerInterval.memberCount (IntegerInterval.singleton x) == 1
+
 {--------------------------------------------------------------------
   map
 --------------------------------------------------------------------}