Kata: Happy Numbers in Clojure

I've just dome the Happy Numbers kata in Clojure.

It's a simple kata that can be a lot of fun.

I used a mix of TDD and REPL-driven development to code it.

These are the resulting tests in Midje:

	(ns happy-numbers.core-test
	(:use midje.sweet)
	(:use [happy-numbers.core]))
	(facts
	(facts
	"about detecting happy numbers"
	(happy? 1) => true
	(happy? 4) => false
	(happy? 7) => true
	(happy? 10) => true
	(happy? 31) => true)
	(facts
	"about happy numbers under a given number
	(see http://mathworld.wolfram.com/HappyNumber.html)"
	(happy-numbers-under 100) =>
	'(1 7 10 13 19 23 28 31 32 44 49 68 70 79 82 86 91 94 97)))

view raw happy-numbers-tests.clj hosted with ❤ by GitHub

and this is the code:

	(ns happy-numbers.core)
	(defn- parse-int [ch]
	(Integer/parseInt (str ch)))
	(def ^:private square
	(partial map #(* % %)))
	(defn- digits-in [n]
	(map parse-int (str n)))
	(defn- sum-squares-of [digits]
	(reduce + (square digits)))
	(defn- one-digit? [n]
	(= 1 (count (digits-in n))))
	(defn- happy-num? [n]
	(let [sum-squared-digits (sum-squares-of (digits-in n))]
	(if (one-digit? sum-squared-digits)
	(= sum-squared-digits 1)
	(recur sum-squared-digits))))
	(def happy? (memoize happy-num?))
	(defn happy-numbers-under [n]
	(filter happy? (range 1 n)))

view raw happy-numbers.clj hosted with ❤ by GitHub

To document the process I commited the code after every passing test and every refactoring. I also commited the REPL history.

You can find the commits step by step here and the code in this repository in GitHub.

I used memoize to avoid repeated computations.

In the next days I will try other suggested optimizations based on the properties of happy numbers.