From b2ea7ebefce50179f51913aae736670658190db6 Mon Sep 17 00:00:00 2001
From: Kjetil Orbekk <kj@orbekk.com>
Date: Sun, 21 Nov 2021 14:03:26 -0500
Subject: implement roman numerals

---
 .../emacs-lisp/roman-numerals/roman-numerals.el    | 47 ++++++++++++++++++++++
 exercism/emacs-lisp/run-exercises.el               |  4 ++
 2 files changed, 51 insertions(+)

diff --git a/exercism/emacs-lisp/roman-numerals/roman-numerals.el b/exercism/emacs-lisp/roman-numerals/roman-numerals.el
index 7ee7f15..9b556fd 100644
--- a/exercism/emacs-lisp/roman-numerals/roman-numerals.el
+++ b/exercism/emacs-lisp/roman-numerals/roman-numerals.el
@@ -4,7 +4,54 @@
 
 ;;; Code:
 
+(require 'cl-macs)
 
+(defun rn/unfold (fn x)
+  (let ((res (funcall fn x)))
+    (when (car res)
+      (cons (car res) (rn/unfold fn (cdr res))))))
+
+(defun rn/decimal-digits (number)
+  "Return the decimal digits of NUMBER as a little-endian list."
+  (rn/unfold (lambda (x)
+            (when (> x 0)
+              (cons (% x 10) (/ x 10)))) number))
+
+(defun rn/zip (l1 l2)
+  (when (and l1 l2)
+    (cons (cons (car l1) (car l2))
+          (rn/zip (cdr l1) (cdr l2)))))
+
+(defun rn/roman-digit (number symbols)
+  (cl-destructuring-bind (one five ten) symbols
+    (cond
+     ((eql  0 number) '())
+     ((eql  1 number) (list one))
+     ((eql  2 number) (list one one))
+     ((eql  3 number) (list one one one))
+     ((eql  4 number) (list one five))
+     ((eql  5 number) (list five))
+     ((eql  6 number) (list five one))
+     ((eql  7 number) (list five one one))
+     ((eql  8 number) (list five one one one))
+     ((eql  9 number) (list one ten))
+     ((eql 10 number) (list ten)))))
+
+(defvar rn/roman-symbols
+  '(("I" "V" "X")
+    ("X" "L" "C")
+    ("C" "D" "M")
+    ("M" nil nil)))
+
+(defun to-roman (number)
+  (when (> number 3000)
+    (error 'number-too-big))
+  (let* ((decimal-digits (rn/decimal-digits number))
+         (roman-digits
+          (mapcar (lambda (digit_syms)
+                    (rn/roman-digit (car digit_syms) (cdr digit_syms)))
+                  (rn/zip decimal-digits rn/roman-symbols))))
+    (apply 'concat (mapcan 'identity (reverse roman-digits)))))
 
 (provide 'roman-numerals)
 ;;; roman-numerals.el ends here
diff --git a/exercism/emacs-lisp/run-exercises.el b/exercism/emacs-lisp/run-exercises.el
index 983c8f8..44831a6 100644
--- a/exercism/emacs-lisp/run-exercises.el
+++ b/exercism/emacs-lisp/run-exercises.el
@@ -47,4 +47,8 @@
              "allergy"
              '("gallery" "ballerina" "regally" "clergy" "largely" "leading")))
 
+(load-file "roman-numerals/roman-numerals.el")
+(kj-display "Roman numerals"
+            (to-roman 1990))
+
 (provide 'hello)
-- 
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