From 643296c38200bbaf0cd9ee9a232ac0098bafb7a7 Mon Sep 17 00:00:00 2001
From: Thorn Avery <s@p7.co.nz>
Date: Wed, 9 Dec 2020 18:43:33 +1300
Subject: [PATCH] day9

---
 day9.hs  |   40 +++
 day9.txt | 1000 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 1040 insertions(+)
 create mode 100644 day9.hs
 create mode 100644 day9.txt

diff --git a/day9.hs b/day9.hs
new file mode 100644
index 0000000..347b84f
--- /dev/null
+++ b/day9.hs
@@ -0,0 +1,40 @@
+main :: IO ()
+main = do
+  raw <- readFile "day9.txt"
+  let nums = (map read $ lines raw) :: [Int]
+      ansA = solveA nums
+      ansB = solveB ansA nums
+   in do
+     putStrLn $ "day9a: " ++ (show ansA)
+     putStrLn $ "day9b: " ++ (show ansB)
+
+solveA :: [Int] -> Int
+solveA nums = runWindow' (take 25 nums) (drop 25 nums)
+
+solveB :: Int -> [Int] -> Int
+solveB t nums = (maximum ans) + (minimum ans)
+  where
+    ans = testRuns t nums
+
+runWindow' :: [Int] -> [Int] -> Int
+runWindow' _ [] = error "reached end of stream"
+runWindow' win (n:ns) =
+  if n `elem` ws
+  then runWindow' ((tail win) ++ [n]) ns
+  else n
+  where
+    ws = [ x+y | x <- ls, y <- ls, x /= y ]
+    ls = filter (<= (n - (minimum win))) win
+
+findRun :: Int -> [Int] -> Int -> [Int] -> Maybe [Int]
+findRun _ _ _ [] = Nothing
+findRun t a r (l:ls)
+  | r + l == t = Just (l:a)
+  | r + l > t = Nothing
+  | otherwise = findRun t (l:a) (r + l) ls
+
+testRuns :: Int -> [Int] -> [Int]
+testRuns _ [] = error "no answer"
+testRuns t ls = case (findRun t [] 0 ls) of
+                  Nothing -> testRuns t (tail ls)
+                  Just x -> x
diff --git a/day9.txt b/day9.txt
new file mode 100644
index 0000000..24b0baf
--- /dev/null
+++ b/day9.txt
@@ -0,0 +1,1000 @@
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