Add elisp/roman-numerals

elisp/roman-numerals/README.md

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+# Roman Numerals
+
+Write a function to convert from normal numbers to Roman Numerals.
+
+The Romans were a clever bunch. They conquered most of Europe and ruled
+it for hundreds of years. They invented concrete and straight roads and
+even bikinis. One thing they never discovered though was the number
+zero. This made writing and dating extensive histories of their exploits
+slightly more challenging, but the system of numbers they came up with
+is still in use today. For example the BBC uses Roman numerals to date
+their programmes.
+
+The Romans wrote numbers using letters - I, V, X, L, C, D, M. (notice
+these letters have lots of straight lines and are hence easy to hack
+into stone tablets).
+
+```
+ 1  => I
+10  => X
+ 7  => VII
+```
+
+There is no need to be able to convert numbers larger than about 3000.
+(The Romans themselves didn't tend to go any higher)
+
+Wikipedia says: Modern Roman numerals ... are written by expressing each
+digit separately starting with the left most digit and skipping any
+digit with a value of zero.
+
+To see this in practice, consider the example of 1990.
+
+In Roman numerals 1990 is MCMXC:
+
+1000=M
+900=CM
+90=XC
+
+2008 is written as MMVIII:
+
+2000=MM
+8=VIII
+
+See also: http://www.novaroma.org/via_romana/numbers.html
+## Source
+
+The Roman Numeral Kata [http://codingdojo.org/cgi-bin/index.pl?KataRomanNumerals](http://codingdojo.org/cgi-bin/index.pl?KataRomanNumerals)
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

elisp/roman-numerals/roman-numerals-test.el

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+;;; roman-numerals-test.el --- Tests for roman-numerals (exercism)
+
+;;; Commentary:
+
+;;; Code:
+
+(load-file "roman-numerals.el")
+
+(ert-deftest roman-numerals-test ()
+  (should (equal (to-roman 1) "I"))
+  (should (equal (to-roman 2) "II"))
+  (should (equal (to-roman 3) "III"))
+  (should (equal (to-roman 4) "IV"))
+  (should (equal (to-roman 5) "V"))
+  (should (equal (to-roman 6) "VI"))
+  (should (equal (to-roman 9) "IX"))
+  (should (equal (to-roman 27) "XXVII"))
+  (should (equal (to-roman 48) "XLVIII"))
+  (should (equal (to-roman 59) "LIX"))
+  (should (equal (to-roman 93) "XCIII"))
+  (should (equal (to-roman 141) "CXLI"))
+  (should (equal (to-roman 163) "CLXIII"))
+  (should (equal (to-roman 402) "CDII"))
+  (should (equal (to-roman 575) "DLXXV"))
+  (should (equal (to-roman 911) "CMXI"))
+  (should (equal (to-roman 1024) "MXXIV"))
+  (should (equal (to-roman 3000) "MMM")))
+
+(provide 'roman-numerals)
+;;; roman-numerals-test.el ends here

elisp/roman-numerals/roman-numerals.el

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+;;; roman-numerals.el --- roman-numerals Exercise (exercism)
+
+;;; Commentary:
+
+;;; Code:
+
+(provide 'roman-numerals)
+
+(defun greatest-roman-amount (n)
+  (cond ((>= n 1000) 1000)
+        ((>= n 900) 900)
+        ((>= n 500) 500)
+        ((>= n 400) 400)
+        ((>= n 100) 100)
+        ((>= n 90) 90)
+        ((>= n 50) 50)
+        ((>= n 40) 40)
+        ((>= n 10) 10)
+        ((>= n 9) 9)
+        ((>= n 5) 5)
+        ((>= n 4) 4)
+        ((>= n 1) 1)))
+
+(defun numeral-for-amount (n)
+  (cond ((= n 1000) "M")
+        ((= n 900) "CM")
+        ((= n 500) "D")
+        ((= n 400) "CD")
+        ((= n 100) "C")
+        ((= n 90) "XC")
+        ((= n 50) "L")
+        ((= n 40) "XL")
+        ((= n 10) "X")
+        ((= n 9) "IX")
+        ((= n 5) "V")
+        ((= n 4) "IV")
+        ((= n 1) "I")))
+
+(defun to-roman (n)
+"Convert whole number N to roman numeral.
+Does not handle numbers greater than 3000."
+  (cl-loop until (<= n 0)
+           concat (numeral-for-amount (greatest-roman-amount n))
+           do (setq n (- n (greatest-roman-amount n)))))
+
+;;; roman-numerals.el ends here