From 0d9d0eec00cf8894ebe94ed7b969ece1dae329d6 Mon Sep 17 00:00:00 2001
From: Amneesh Singh <natto@weirdnatto.in>
Date: Sat, 7 Dec 2024 15:37:24 +0530
Subject: [PATCH] day 7

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>
---
 aoc2024.cabal | 10 +++++++---
 src/Day7.hs   | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+), 3 deletions(-)
 create mode 100644 src/Day7.hs

diff --git a/aoc2024.cabal b/aoc2024.cabal
index 70cdc63..eadbaea 100644
--- a/aoc2024.cabal
+++ b/aoc2024.cabal
@@ -64,6 +64,10 @@ executable day6
   import:         common
   hs-source-dirs: src
   main-is:        Day6.hs
-  build-depends:
-    , containers
-    , libaoc
+  build-depends:  containers
+
+executable day7
+  import:         common
+  hs-source-dirs: src
+  main-is:        Day7.hs
+  build-depends:  libaoc
diff --git a/src/Day7.hs b/src/Day7.hs
new file mode 100644
index 0000000..4e10802
--- /dev/null
+++ b/src/Day7.hs
@@ -0,0 +1,48 @@
+module Main where
+
+import qualified AoC as A (extract)
+import Text.Parsec (char, digit, many1, newline, parse, sepBy1, sepEndBy1, string)
+import Text.Parsec.String (Parser)
+
+type Equation = (Int, [Int])
+
+type Operator = Int -> Int -> Int
+
+parseEqns :: Parser [Equation]
+parseEqns = parseEqn `sepEndBy1` newline
+  where
+    parseEqn :: Parser Equation
+    parseEqn =
+      (,)
+        <$> (read <$> many1 digit <* string ": ")
+        <*> (read <$> many1 digit) `sepBy1` char ' '
+
+-- concatenate integers
+cc :: Operator
+cc a b = a * (10 ^ length b) + b
+  where
+    -- this is faster than log_10
+    length :: Int -> Int
+    length 0 = 0
+    length x = 1 + length (x `div` 10)
+
+check :: [Operator] -> Equation -> Bool
+check operators (target, operand : operands) = go operand operands
+  where
+    go :: Int -> [Int] -> Bool
+    go i [] = i == target
+    go i (x : xs)
+      | i > target = False -- prune
+      | otherwise = any (\op -> go (i `op` x) xs) operators
+
+getSum :: [Operator] -> [Equation] -> Int
+getSum ops = sum . map fst . filter (check ops)
+
+main :: IO ()
+main =
+  do
+    raw <- readFile "./inputs/day7.in"
+    let equations = A.extract $ parse parseEqns "" raw
+
+    putStr "Part 1: " >> print (getSum [(+), (*)] equations)
+    putStr "Part 2: " >> print (getSum [(+), (*), cc] equations)