Added a tail recursive compound interest function

lisp/compound-interest-test.lisp

@@ -0,0 +1,49 @@
+(load "unit-test.lisp")
+(load "compound-interest.lisp")
+
+(unit
+  (list
+
+    (defun many-years-with-no-interest-rate ()
+      (eq (compound-interest 100000 0 10) 100000)
+    )
+
+    (defun no-years-with-positive-interest-rate ()
+      (eq (compound-interest 100000 10 0) 100000)
+    )
+
+    (defun one-year-with-positive-interest-rate ()
+      (eq (compound-interest 100000 5 1) 105000)
+    )
+
+    (defun two-years-with-positive-interest-rate ()
+      (eq (compound-interest 100000 5 2) 110250)
+    )
+
+    (defun three-years-with-positive-interest-rate ()
+      (eq (compound-interest 100000 5 3) 115763)
+    )
+
+    (defun four-years-with-positive-interest-rate ()
+      (eq (compound-interest 100000 5 4) 121551)
+    )
+
+    (defun one-year-with-negative-interest-rate ()
+      (eq (compound-interest 100000 (- 5) 1) 95000)
+    )
+
+    (defun two-years-with-negative-interest-rate ()
+      (eq (compound-interest 100000 (- 5) 2) 90250)
+    )
+
+    (defun three-years-with-negative-interest-rate ()
+      (eq (compound-interest 100000 (- 5) 3) 85737)
+    )
+
+    (defun four-years-with-negative-interest-rate ()
+      (eq (compound-interest 100000 (- 5) 4) 81450)
+    )
+
+  )
+)
+

lisp/compound-interest.lisp

@@ -0,0 +1,19 @@
+(defun decrement (n) (- n 1))
+
+(defun percent (n percent)
+  (cond
+    ((> percent 0) (/ (+ (* n percent) 50) 100))
+    (t (/ (- (* n percent) 50) 100))
+  )
+)
+
+(defun compound-interest (principal rate years)
+  (cond
+    ((= years 0) principal)
+    (t (compound-interest (+ principal (percent principal rate))
+                          rate
+                          (decrement years)
+       )
+    )
+  )
+)