From 92db31e76aa74655e936fa254012723dc6fba5c7 Mon Sep 17 00:00:00 2001
From: Trashtalk <trashtalk217@gmail.com>
Date: Fri, 10 Aug 2018 19:31:54 +0200
Subject: [PATCH] Made euclid-sub and -mod work with negatives and refactored.

---
 .../code/lisp/euclidean_algorithm.lisp        | 33 +++++++++++++++++++
 .../euclidean_algorithm.md                    |  6 ++++
 2 files changed, 39 insertions(+)
 create mode 100644 contents/euclidean_algorithm/code/lisp/euclidean_algorithm.lisp

diff --git a/contents/euclidean_algorithm/code/lisp/euclidean_algorithm.lisp b/contents/euclidean_algorithm/code/lisp/euclidean_algorithm.lisp
new file mode 100644
index 000000000..cff5ab29f
--- /dev/null
+++ b/contents/euclidean_algorithm/code/lisp/euclidean_algorithm.lisp
@@ -0,0 +1,33 @@
+;;;; Euclid algorithm implementation
+
+(defun euclid-sub (a b)
+  (euclid-sub* (abs a) (abs b)))
+
+(defun euclid-sub* (a b)
+  (if (eq a b)
+    a
+    (if (> a b)
+      (euclid-sub* (- a b) b)
+      (euclid-sub* a (- b a)))))
+
+(defun euclid-mod (a b)
+  (if (zerop b)
+    (abs a)
+    (euclid-mod b (mod a b))))
+
+(print 
+  (euclid-sub (* 64 67)
+              (* 64 81)))
+(print 
+  (euclid-mod (* 128 12)
+              (* 128 77)))
+
+;; built-in function: (gcd 80 40)
+;; quick test
+(assert 
+  (=  (euclid-sub (* 64 67) (* 64 81))
+      (gcd (* 64 67) (* 64 81))))
+
+(assert 
+  (=  (euclid-mod (* 64 67) (* 64 81))
+      (gcd (* 64 67) (* 64 81))))
diff --git a/contents/euclidean_algorithm/euclidean_algorithm.md b/contents/euclidean_algorithm/euclidean_algorithm.md
index 7cfe043a5..712f90295 100644
--- a/contents/euclidean_algorithm/euclidean_algorithm.md
+++ b/contents/euclidean_algorithm/euclidean_algorithm.md
@@ -17,6 +17,8 @@ The algorithm is a simple way to find the *greatest common divisor* (GCD) of two
 [import:3-16, lang="java"](code/java/EuclideanAlgo.java)
 {% sample lang="js" %}
 [import:15-29, lang="javascript"](code/javascript/euclidean_example.js)
+{% sample lang="lisp" %}
+[import:3-12, lang="lisp"](code/lisp/euclidean_algorithm.lisp)
 {% sample lang="py" %}
 [import:11-22, lang="python"](code/python/euclidean_example.py)
 {% sample lang="haskell" %}
@@ -58,6 +60,8 @@ Modern implementations, though, often use the modulus operator (%) like so
 [import:18-26, lang="java"](code/java/EuclideanAlgo.java)
 {% sample lang="js" %}
 [import:1-13, lang="javascript"](code/javascript/euclidean_example.js)
+{% sample lang="lisp" %}
+[import:13-17, lang="lisp"](code/lisp/euclidean_algorithm.lisp)
 {% sample lang="py" %}
 [import:1-9, lang="python"](code/python/euclidean_example.py)
 {% sample lang="haskell" %}
@@ -104,6 +108,8 @@ Program.cs
 [import, lang="java"](code/java/EuclideanAlgo.java)
 {% sample lang="js" %}
 [import, lang="javascript"](code/javascript/euclidean_example.js)
+{% sample lang="lisp" %}
+[import, lang="lisp"](code/lisp/euclidean_algorithm.lisp)
 {% sample lang="py" %}
 [import, lang="python"](code/python/euclidean_example.py)
 {% sample lang="haskell" %}