From bcf20c5903b99cb8f1fdc415b20fb4792e071a71 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Na=C3=AFm=20Favier?= <n@monade.li>
Date: Tue, 19 Dec 2023 13:14:19 +0100
Subject: [PATCH] IntegerInterval: add memberCount

Returns the number of integers that lie within an integer interval.
---
 src/Data/IntegerInterval.hs | 12 ++++++++++++
 test/TestIntegerInterval.hs | 14 ++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/src/Data/IntegerInterval.hs b/src/Data/IntegerInterval.hs
index 37bbdbe..c9af8ae 100644
--- a/src/Data/IntegerInterval.hs
+++ b/src/Data/IntegerInterval.hs
@@ -50,6 +50,7 @@ module Data.IntegerInterval
   , lowerBound'
   , upperBound'
   , width
+  , memberCount
 
   -- * Universal comparison operators
   , (<!), (<=!), (==!), (>=!), (>!), (/=!)
@@ -309,6 +310,17 @@ 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 =
diff --git a/test/TestIntegerInterval.hs b/test/TestIntegerInterval.hs
index 9f00fdc..862e53a 100644
--- a/test/TestIntegerInterval.hs
+++ b/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
 --------------------------------------------------------------------}