src/task0/task0.hs

   1 import qualified Data.HashMap.Strict as M
   2 import Data.List (foldl')        -- import the strict fold
   3 import Data.List.Split (splitOn)
   4
   5 -- a key-value store for recording how often each name is seen
   6 type NameCount = M.HashMap String Int
   7
   8 main :: IO ()
   9 main = do
  10         -- text <- readFile "data/small_invites.txt"
  11         text <- readFile "data/00-invites.txt"
  12         print $ part1 text
  13         print $ part2 text
  14
  15 -- use `words` here as I don't need to worry about the comma delimiters
  16 part1 :: String -> Int
  17 part1  = maximum . map (length . words) . lines
  18
  19 part2 :: String -> Int
  20 part2 inviteText = M.size popular
  21     where -- split names on the delimieter, use `concatMap` to give just one long list
  22           invites = concatMap (splitOn ", ") $ lines inviteText
  23           -- count the names
  24           invited = nameCounts invites
  25           -- popular are those mentioned more than once
  26           popular = M.filter (> 1) invited
  27
  28
  29 -- count how many times each name appears.
  30 -- It's a fold over the list of names, creating the name count HashMap.
  31 -- As each name is taken from the list, we find how many times we've seen it
  32 -- before, add one to it, then update the HashMap with the new count.
  33 nameCounts :: [String] -> NameCount
  34 nameCounts names = foldl' includeName M.empty names
  35     where includeName nc name = M.insert name (1 + count name nc) nc
  36           -- a default count of 0 for each name
  37           count name = M.lookupDefault 0 name