tests

solutions/src/2023/01.hs

@@ -8,13 +8,32 @@ Maintainer  : emertens@gmail.com
 
 <https://adventofcode.com/2023/day/1>
 
+For each line in the input, create a two digit number from the first and
+last digits on the line, then sum up all of these numbers.
+
+>>> decode part1 <$> ["1abc2","pqr3stu8vwx","a1b2c3d4e5f","treb7uchet"]
+[12,38,15,77]
+
+>>> decode part2 <$> ["two1nine","eightwothree","abcone2threexyz","xtwone3four","4nineeightseven2","zoneight234","7pqrstsixteen"]
+[29,83,13,24,42,14,76]
+
 -}
 module Main where
 
 import Data.List (isPrefixOf, tails)
-import Data.Maybe (listToMaybe, mapMaybe)
 
-import Advent (format)
+import Advent (fromDigits, format)
+
+-- |
+--
+-- >>> :main
+-- 55123
+-- 55260
+main :: IO ()
+main =
+ do input <- [format|2023 1 (%s%n)*|]
+    print (sum (map (decode part1) input))
+    print (sum (map (decode part2) input))
 
 part1 :: [(String, Int)]
 part1 = [(show i, i) | i <- [0..9]]
@@ -25,15 +44,13 @@ part2 = part1 ++
     ("four" ,4),("five" ,5),("six"  ,6),
     ("seven",7),("eight",8),("nine" ,9)]
 
+earliest :: Eq k => [([k], a)] -> [k] -> a
+earliest entries haystack =
+  head [v | needle <- tails haystack, (k,v) <- entries, k `isPrefixOf` needle]
+
 decode :: [(String, Int)] -> String -> Int
-decode mapping str = 10 * head ns + last ns
+decode mapping str = fromDigits 10 [d1,d2]
   where
-    f :: String -> Maybe Int
-    f x = listToMaybe [v | (k,v) <- mapping, k `isPrefixOf` x]
-    ns = mapMaybe f (tails str)
-
-main :: IO ()
-main =
- do input <- [format|2023 1 (%s%n)*|]
-    print (sum (map (decode part1) input))
-    print (sum (map (decode part2) input))
+    d1 = earliest mapping str
+    d2 = earliest mapping' (reverse str)
+    mapping' = [(reverse k, v) | (k,v) <- mapping]