IFS Common Lisp implementation (#722)

* first working version

* cleaned up the chaos-game function and added some helpfull comments

* changed the .md file and upped iterations

* Skeleton of the final solution

* Update contents/IFS/IFS.md

Co-authored-by: Eric Berquist <[email protected]>

Co-authored-by: James Schloss <[email protected]>
Co-authored-by: Eric Berquist <[email protected]>

Author: 3 people

contents/IFS/IFS.md

@@ -140,6 +140,8 @@ Here, instead of tracking children of children, we track a single individual tha
 [import:5-12, lang:"python"](code/python/IFS.py)
 {% sample lang="c" %}
 [import:18-29, lang:"c"](code/c/IFS.c)
+{% sample lang="lisp" %}
+[import:5-14, lang:"lisp"](code/clisp/ifs.lisp)
 {% sample lang="coco" %}
 [import:4-16, lang:"coconut"](code/coconut/IFS.coco)
 {% sample lang="java" %}
@@ -224,6 +226,8 @@ In addition, we have written the chaos game code to take in a set of points so t
 [import, lang:"python"](code/python/IFS.py)
 {% sample lang="c" %}
 [import, lang:"c"](code/c/IFS.c)
+{% sample lang="lisp" %}
+[import, lang:"lisp"](code/clisp/ifs.lisp)
 {%sample lang="coco" %}
 [import, lang:"coconut"](code/coconut/IFS.coco)
 {%sample lang="java" %}

contents/IFS/code/clisp/ifs.lisp

@@ -0,0 +1,28 @@
+;;;; Iterated Function System implementation
+
+(defstruct (point (:constructor make-point (x y))) x y)
+
+(defun chaos-game (iterations shape-points)
+  "Plays a chaos game with a certain shape for a determined amount of iterations"
+  (loop
+    repeat iterations
+    for rand-point = (svref shape-points (random (length shape-points)))
+    for point = (make-point (random 1.0) (random 1.0)) ; starting point
+    then (make-point
+           (* 0.5 (+ (point-x rand-point) (point-x point)))
+           (* 0.5 (+ (point-y rand-point) (point-y point)))) ; every subsequent point
+    collect point))
+
+(defparameter *shape-points*
+  (map
+    'vector
+    (lambda (e) (apply #'make-point e))
+    ;; the backquote allows us to selectively evaluate (sqrt 0.75) with the comma
+    `((0 0) (0.5 ,(sqrt 0.75)) (1 0))))
+
+;; output the data to the "out.dat" file
+(with-open-file (out "out.dat" :direction :output :if-exists :supersede)
+  (flet ((format-point (p)
+           ;; this is not very clean, but it's the simplest way to insert a tab into a string.
+           (format nil "~f~c~f" (point-x p) #\tab (point-y p))))
+    (format out "~{~a~%~}" (map 'list #'format-point (chaos-game 10000 *shape-points*)))))