Solve 'How Many Numbers?' kata

Author: borisskert

src/Codewars/G964/Howmany.hs

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+module Codewars.G964.Howmany (selNumber) where
+
+-- https://www.codewars.com/kata/55d8aa568dec9fb9e200004a/train/haskell
+
+selNumber :: Int -> Int -> Int
+selNumber n d = length $ makeNumbers n d
+
+makeNumbers :: Int -> Int -> [Int]
+makeNumbers n d = filter valid numbers
+  where
+    numbers
+      | n < d = [n + 1 .. d]
+      | otherwise = [d .. n - 1]
+    valid = isValid d
+
+isValid :: Int -> Int -> Bool
+isValid d n
+  | n < 10 = False
+  | otherwise = isAscending d n
+
+isAscending :: Int -> Int -> Bool
+isAscending d n = all (\x -> x <= d && x > 0) . zipWith (flip (-)) digits . tail $ digits
+  where
+    digits = toDigits n
+
+toDigits :: (Integral a) => a -> [a]
+toDigits 0 = []
+toDigits n = toDigits (n `div` 10) ++ [n `mod` 10]

test/Codewars/G964/HowmanySpec.hs

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+module Codewars.G964.HowmanySpec where
+
+import Codewars.G964.Howmany
+import Data.Char
+import Data.List
+import Test.Hspec
+import Test.QuickCheck
+import Text.Printf (printf)
+
+testNb :: Int -> Int -> Int -> Spec
+testNb n d s =
+  it (printf "should return selNumber for n d result : %d %d --> %d \n" n d s) $
+    selNumber n d `shouldBe` s
+
+spec :: Spec
+spec = do
+  describe "selNumber small values" $ do
+    testNb 0 1 0
+    testNb 3 1 0
+    testNb 13 1 1
+    testNb 15 1 1
+    testNb 20 2 2
+    testNb 30 2 4
+    testNb 44 2 6
+    testNb 50 3 12
+    testNb 100 3 21