notes to myself about the cloth simulation example.

info.rkt

@@ -20,6 +20,7 @@
 (define homepage "http://hashcollision.org/whalesong")
 (define scribblings '(("scribblings/manual.scrbl")))
 (define compile-omit-paths '("tests"
+                             "sandbox"
                              "examples"
                              "experiments"
                              "simulator"

make-planet-archive.sh

@@ -14,6 +14,7 @@ git archive --format=tar --prefix=$PROJNAME/ HEAD | (cd tmp && tar xf -)
 rm -rf $OLDDIR/tmp/whalesong/experiments
 rm -rf $OLDDIR/tmp/whalesong/notes
 rm -rf $OLDDIR/tmp/whalesong/simulator
+rm -rf $OLDDIR/tmp/whalesong/sandbox
 rm -rf $OLDDIR/tmp/whalesong/tests
 
 

sandbox/cloth-simulation.rkt

@@ -0,0 +1,75 @@
+#lang planet dyoo/whalesong
+
+;; A rewrite of the cloth simulation application from the Codea project
+;; into world form.
+
+;; 121 particles,
+;; 100 constraints.
+;;
+;; There are a few things about the program that aren't stated up front, but are implied
+;; by the original code:
+;;
+;; 1.  The upper edge is fixed.  The use of the orx, ory attributes is somewhat redundant.
+;; 2.  The particles
+
+(define-struct constraint (p1 ;; particle
+                           p2 ;; particle
+                           d ;; number
+                           ))
+
+(define-struct particle (x ;; number
+                         y ;; number 
+                         ox ;; number -- the very previous x position 
+                         oy ;; number -- the very previous y position
+                         fx ;; number -- force in the x direction
+                         fy ;; number -- force in the y direction
+                         orx ;; number -- the original x position
+                         ory ;; number -- the original y position 
+                         ))
+
+;; The world consists of a list of particles and constraints.
+(define-struct world (particles ;; (listof particle)
+                      constraints ;; (listof constraint)
+                      ))
+                         
+                         
+                         
+(define (new-particle x y)
+  (make-particle x y x y 0 0 x y))
+   
+
+
+;; apply-global-forces: (listof parts) number number -> (listof parts)
+;; Globally apply the forces on all the parts.
+(define (apply-global-forces parts fx fy)
+  (map (lambda (part) (apply-force part fx fy)) 
+       parts))
+
+
+;; apply-force: particle number number -> particle
+;; Update a particle with the appropriate force.
+(define (apply-force part fx fy)
+  (make-particle (particle-x part)
+                 (particle-y part)
+                 (particle-ox part)
+                 (particle-oy part)
+                 fx
+                 fy
+                 (particle-orx part)
+                 (particle-ory part)))
+
+
+;; new-constraint: point point -> constraint
+;; Construct a new initial constraint from points p1 and p2.
+(define (new-constraint p1 p2)
+  (make-constraint p1 p2 (partdist p1 p2)))
+
+
+
+;; partdist: point point -> number
+;; Computes the distance between two points.
+(define (partdist p1 p2)
+  (sqrt (sqr (- (point-x p1)
+                (point-x p2)))
+        (sqr (- (point-y p1)
+                (point-y p2)))))