haskell: add [p4, p10]

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

haskell/Lib.hs

@@ -1,13 +1,19 @@
-module Lib (fib, primeFactors) where
+module Lib (fib, primes, primeFactors) where
 
 fib :: [Integer]
 fib = 1 : 2 : zipWith (+) fib (tail fib)
 
+isPrime :: Integer -> Bool
+isPrime n = n > 1 && null [[] | k <- [2 .. (floor $ sqrt $ fromIntegral n)], mod n k == 0]
+
+primes :: [Integer]
+primes = 2 : filter isPrime [3, 4 ..]
+
 primeFactors :: Integer -> [Integer]
 primeFactors n = genPrimeFactors n 2
   where
     genPrimeFactors :: Integer -> Integer -> [Integer]
     genPrimeFactors n c
-      | c == n = [n]
+      | c * c > n = [n]
       | mod n c /= 0 = genPrimeFactors n (c + 1)
       | otherwise = c : genPrimeFactors (div n c) c

haskell/p10.hs

@@ -0,0 +1,7 @@
+import Lib (primes)
+
+main :: IO ()
+main = putStr $ show solve
+
+solve :: Integer
+solve = sum $ takeWhile (< 2000000) primes

haskell/p4.hs

@@ -0,0 +1,5 @@
+main :: IO ()
+main = putStr $ show solve
+
+solve :: Integer
+solve = maximum [a * b | a <- [100 .. 999], b <- [a .. 999], let s = show $ a * b, s == reverse s]

haskell/p5.hs

@@ -0,0 +1,5 @@
+main :: IO ()
+main = putStr $ show solve
+
+solve :: Integer
+solve = foldl1 lcm [1 .. 20]

haskell/p6.hs

@@ -0,0 +1,11 @@
+main :: IO ()
+main = putStr $ show $ solve 100
+
+solve :: Integer -> Integer
+solve n = squareOfSum n - sumOfSquares n
+  where
+    squareOfSum :: Integer -> Integer
+    squareOfSum n = sum [1 .. n] ^ 2
+
+    sumOfSquares :: Integer -> Integer
+    sumOfSquares n = sum $ map (^ 2) [1 .. n]

haskell/p7.hs

@@ -0,0 +1,4 @@
+import Lib (primes)
+
+main :: IO ()
+main = putStr $ show $ primes !! 10001

haskell/p8.hs

@@ -0,0 +1,35 @@
+import Data.Char (digitToInt)
+
+main :: IO ()
+main = putStr $ show solve
+
+solve :: Int
+solve = maximum $ map (product . map digitToInt) (substrings input 13)
+  where
+    substrings :: String -> Int -> [String]
+    substrings s n
+      | length s < n = []
+      | otherwise = take n s : substrings (tail s) n
+
+input :: String
+input =
+  "73167176531330624919225119674426574742355349194934\
+  \96983520312774506326239578318016984801869478851843\
+  \85861560789112949495459501737958331952853208805511\
+  \12540698747158523863050715693290963295227443043557\
+  \66896648950445244523161731856403098711121722383113\
+  \62229893423380308135336276614282806444486645238749\
+  \30358907296290491560440772390713810515859307960866\
+  \70172427121883998797908792274921901699720888093776\
+  \65727333001053367881220235421809751254540594752243\
+  \52584907711670556013604839586446706324415722155397\
+  \53697817977846174064955149290862569321978468622482\
+  \83972241375657056057490261407972968652414535100474\
+  \82166370484403199890008895243450658541227588666881\
+  \16427171479924442928230863465674813919123162824586\
+  \17866458359124566529476545682848912883142607690042\
+  \24219022671055626321111109370544217506941658960408\
+  \07198403850962455444362981230987879927244284909188\
+  \84580156166097919133875499200524063689912560717606\
+  \05886116467109405077541002256983155200055935729725\
+  \71636269561882670428252483600823257530420752963450"

haskell/p9.hs

@@ -0,0 +1,6 @@
+main :: IO ()
+main = putStr $ show $ solve 1000
+
+-- Brute search because don't care 🧌
+solve :: Integer -> Integer
+solve p = head [a * b * c | a <- [1 .. p], b <- [a + 1 .. p], let c = p - a - b, c > 0 && c ^ 2 == a ^ 2 + b ^ 2]