1 files changed, 38 insertions, 0 deletions
diff --git a/2020/07b.hs b/2020/07b.hs
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--- /dev/null
+++ b/2020/07b.hs
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+module Day7A where
+
+import System.Environment (getArgs)
+import Text.Regex.PCRE
+
+type Rule = (String, [(Int, String)])
+
+parseRule :: String -> Rule
+parseRule xs = (parent, children)
+                where parentMatch = xs =~ "^([a-z ]*?) bag" :: [[String]]
+                      parent = head parentMatch!!1
+                      childrenMatch = xs =~ "([0-9]+) ([a-z ]*) bag" :: [[String]]
+                      childrenNs = map (read . (!! 1)) childrenMatch
+                      childrenNames = map (!! 2) childrenMatch 
+                      children = zip childrenNs childrenNames
+
+-- Keep applying f to a until the output stops changing
+untilUnchanged :: Eq a => (a -> a) -> a -> a
+untilUnchanged f a | n /= a    = untilUnchanged f n
+                   | otherwise = n
+                     where n = f a
+
+ruleFor :: String -> [Rule] -> Rule
+-- ruleFor x rs | null r = (x, [])
+--              | otherwise = head r
+--                  where r = filter ((== x) . fst) rs
+ruleFor x = head . filter ((== x) . fst)
+
+nBags :: [Rule] -> String -> Int
+nBags rs x = 1 + sum [n * nBags rs c | (n, c) <- snd $ ruleFor x rs]
+
+-- Usage: runghc --ghc-arg="-package regex-pcre-builtin" 7a.hs inputs/day7
+main :: IO ()
+main = do 
+        args <- getArgs;
+        content <- readFile $ head args;
+        let rules = map parseRule $ lines content;
+        print $ nBags rules "shiny gold" - 1;