IFS Common Lisp implementation

contents/IFS/IFS.md

@@ -134,6 +134,8 @@ 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:5-13, lang:"lisp"](code/clisp/ifs.lisp)
 {% endmethod %}
 
 If we set the initial points to the on the equilateral triangle we saw before, we can see the Sierpinski triangle again after a few thousand iterations, as shown below:
@@ -203,6 +205,8 @@ 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)
 {% endmethod %}
 
 ### Bibliography

contents/IFS/code/clisp/ifs.lisp

@@ -0,0 +1,26 @@
+;;;; Iterated Function System implementation
+
+(defstruct (point (:constructor make-point (x y))) x y)
+
+(defun chaos-game (iterations shape-points)
+ (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)
+ (format nil "~f~c~f" (point-x p) #\tab (point-y p)))) ; this is a bit ugly, but it works
+ (format out "~{~a~%~}" (map 'list #'format-point (chaos-game 10000 *shape-points*)))))