Fix haskell binarysearch (egonSchiele#51)

aliaksandr-s · egonSchiele · commit b515434b3f46 · 2018-02-03T11:56:41.000-08:00
diff --git a/01_introduction_to_algorithms/Haskell/01_binarysearch.hs b/01_introduction_to_algorithms/Haskell/01_binarysearch.hs
@@ -3,19 +3,20 @@ import Data.Array
 binarysearch :: Integer -> Array Integer Integer -> Integer -> Integer -> Maybe Integer
 binarysearch x arr low high
   | low > high = Nothing
-  | mid > x = binarysearch x arr low (high - 1)
-  | mid < x = binarysearch x arr (low + 1) high
+  | guess > x = binarysearch x arr low (mid - 1)
+  | guess < x = binarysearch x arr (mid + 1) high
   | otherwise = Just mid
   where
-    mid = arr ! ((low + high) `div` 2)
-
+    mid = ((low + high) `div` 2)
+    guess = arr ! mid
 
 find :: Integer -> Array Integer Integer -> Maybe Integer
 find x arr = binarysearch x arr low high
-  where p = bounds arr
-        low = fst p
-        high = snd p
+  where borders = bounds arr
+        low = fst borders
+        high = snd borders
 
 -- Usage
--- let arr = array (0, 4) [(i, i*i) | i <- [0..4]]
--- find 4 arr
+-- let arr = array (0, 4) [(i, i*i) | i <- [0..4]]  // [(0,0),(1,1),(2,4),(3,9),(4,16)]
+-- find 4 arr                                       // Just 2
+-- find 25 arr                                      // Nothing
