Extract binary search into Prelude

common/src/Advent/Prelude.hs

@@ -275,3 +275,18 @@ multiline = QuasiQuoter {
 partialSums :: Num a => [a] -> [a]
 partialSums = scanl' (+) 0
 {-# Inline partialSums #-}
+
+-- | Binary search for the largest value satisfying a predicate.
+-- Finds the largest value when the predicate switches from True
+-- to False. There should only be one such point in the range.
+binSearchLargest ::
+  (Int -> Bool) {- ^ predicate    -} ->
+  Int           {- ^ small enough -} ->
+  Int           {- ^ too big      -} ->
+  Int
+binSearchLargest p lo hi
+  | lo + 1 == hi = lo
+  | p mid        = binSearchLargest p mid hi
+  | otherwise    = binSearchLargest p lo mid
+  where
+    mid = lo + (hi - lo) `div` 2

solutions/src/2019/14.hs

@@ -11,7 +11,7 @@ Maintainer  : emertens@gmail.com
 -}
 module Main (main) where
 
-import Advent.Format (format)
+import Advent.Format (format, binSearch)
 import Data.List (foldl', sortOn)
 import Data.Map (Map)
 import Data.Map qualified as Map
@@ -81,16 +81,4 @@ expSearch p lo = go (lo+1)
   where
     go hi
       | p hi      = go (2*hi)
-      | otherwise = binSearch p lo hi
-
-binSearch ::
-  (Int -> Bool) {- ^ predicate    -} ->
-  Int           {- ^ small enough -} ->
-  Int           {- ^ too big      -} ->
-  Int
-binSearch p lo hi
-  | lo + 1 == hi = lo
-  | p mid        = binSearch p mid hi
-  | otherwise    = binSearch p lo mid
-  where
-    mid = lo + (hi - lo) `div` 2
+      | otherwise = binSearchLargest p lo hi

solutions/src/2023/06.hs

@@ -25,7 +25,7 @@ Distance:  9  40  200
 -}
 module Main where
 
-import Advent (format)
+import Advent (format, binSearchLargest)
 
 -- |
 --
@@ -47,17 +47,5 @@ ways (t, d)
   where
     valid hold = (t - hold) * hold > d
     mid = t `div` 2 -- the midpoint is the best we can get
-    tooLo = binSearch (not . valid)   0 mid
-    hi    = binSearch        valid  mid   t
-
-binSearch ::
-  (Int -> Bool) {- ^ predicate    -} ->
-  Int           {- ^ small enough -} ->
-  Int           {- ^ too big      -} ->
-  Int
-binSearch p lo hi
-  | lo + 1 == hi = lo
-  | p mid        = binSearch p mid hi
-  | otherwise    = binSearch p lo mid
-  where
-    mid = lo + (hi - lo) `div` 2
+    tooLo = binSearchLargest (not . valid)   0 mid
+    hi    = binSearchLargest        valid  mid   t