From b042d24ceeb355a106d2c01ef18024f53988eb2f Mon Sep 17 00:00:00 2001
From: Richard Hull <rm_hull@yahoo.co.uk>
Date: Sun, 20 Mar 2016 15:13:21 +0000
Subject: [PATCH] Rarest letter

---
 src/ars_magna/letter_frequencies.clj       | 72 +++++++++++++---------
 test/ars_magna/letter_frequencies_test.clj |  9 +++
 2 files changed, 52 insertions(+), 29 deletions(-)
 create mode 100644 test/ars_magna/letter_frequencies_test.clj

diff --git a/src/ars_magna/letter_frequencies.clj b/src/ars_magna/letter_frequencies.clj
index b3cea55..22d87e0 100644
--- a/src/ars_magna/letter_frequencies.clj
+++ b/src/ars_magna/letter_frequencies.clj
@@ -2,36 +2,50 @@
 
 ; From: https://en.wikipedia.org/wiki/Letter_frequency#Relative_frequencies_of_letters_in_the_English_language
 
-(def en-GB {
-  \a  0.08167
-  \b  0.01492
-  \c  0.02782
-  \d  0.04253
-  \e  0.12702
-  \f  0.02228
-  \g  0.02015
-  \h  0.06094
-  \i  0.06966
-  \j  0.00153
-  \k  0.00772
-  \l  0.04025
-  \m  0.02406
-  \n  0.06749
-  \o  0.07507
-  \p  0.01929
-  \q  0.00095
-  \r  0.05987
-  \s  0.06327
-  \t  0.09056
-  \u  0.02758
-  \v  0.00978
-  \w  0.02361
-  \x  0.00150
-  \y  0.01974
-  \z  0.00074 })
+(def letter-frequencies
+  {
+    :en-GB {
+      \a  0.08167
+      \b  0.01492
+      \c  0.02782
+      \d  0.04253
+      \e  0.12702
+      \f  0.02228
+      \g  0.02015
+      \h  0.06094
+      \i  0.06966
+      \j  0.00153
+      \k  0.00772
+      \l  0.04025
+      \m  0.02406
+      \n  0.06749
+      \o  0.07507
+      \p  0.01929
+      \q  0.00095
+      \r  0.05987
+      \s  0.06327
+      \t  0.09056
+      \u  0.02758
+      \v  0.00978
+      \w  0.02361
+      \x  0.00150
+      \y  0.01974
+      \z  0.00074 }})
 
-(defn by-ascending-frequency [letter-table]
+(defn by-ascending-frequency [letter-freq]
   (->>
-    letter-table
+    letter-freq
     (sort-by second)
     (map first)))
+
+(defn rarest-letter
+  ([word] (rarest-letter word :en-GB))
+  ([word lang]
+    (let [letters (set word)]
+      (->>
+        (get letter-frequencies lang)
+        (filter #(letters (first %)))
+        by-ascending-frequency
+        first))))
+
+(rarest-letter "compute")
diff --git a/test/ars_magna/letter_frequencies_test.clj b/test/ars_magna/letter_frequencies_test.clj
new file mode 100644
index 0000000..3af5231
--- /dev/null
+++ b/test/ars_magna/letter_frequencies_test.clj
@@ -0,0 +1,9 @@
+(ns ars-magna.letter-frequencies-test
+  (:require
+    [clojure.test :refer :all]
+    [ars-magna.letter-frequencies :refer :all]))
+
+(deftest check-rarest-letter
+  (is (= \p (rarest-letter "compute")))
+  (is (nil? (rarest-letter "compute" :ru-RU)))
+  (is (nil? (rarest-letter nil))))