Day 3: Squares With Three Sides



In [1]:

    
inputLines = lines <$> readFile "input/day3.txt"



In [2]:

    
-- Parse a string that contains space-separated numbers
parseNumbers :: String -> [Int]
parseNumbers = map read . words



In [3]:

    
import Data.List -- for sort



In [4]:

    
isValidTriangle :: [Int] -> Bool
isValidTriangle (a:b:c:[]) = head sortedSides < sum (tail sortedSides)
    where
        sortedSides = reverse $ sort [a, b, c]
isValidTriangle _ = False  -- too many or too few sides

Part 1: every input row contains the sides of a triangle



In [5]:

    
length . filter isValidTriangle . map parseNumbers <$> inputLines

Part 2: a column in three consecutive rows contains the sides of a triangle

Idea:

verify that the number of rows is divisible by 3
use transpose to put all numbers in a column into a list
concatenate the columms with concat
split the list into sublists of length 3 with splitEvery



In [6]:

    
-- the number of lines is divisible by 3 if the remainder is zero
(`mod` 3) . length <$> inputLines



In [7]:

    
import Data.List.Split -- for splitEvery



In [8]:

    
length . filter isValidTriangle . splitEvery 3 . concat . transpose . map parseNumbers <$> inputLines