Aphorism fifteen, getting my feet wet with verlet physics

src/aphorisms/fifteen.clj

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+(ns aphorisms.fifteen
+  (:require [thi.ng.math.core :as m]
+            [thi.ng.geom.core :as g]
+            [thi.ng.geom.physics.core :as p]
+            [thi.ng.geom.vector :as v]
+            [thi.ng.geom.rect :as r]
+            [thi.ng.math.noise :as n]
+            [quil.core :as q]
+            [quil.middleware :as qm]))
+
+(defn make-world []
+  (let [[a b :as particles] [(-> (p/particle (v/vec2 250 250))
+                                 (p/lock))
+                             (p/particle (v/vec2 0 10) 10)]
+        springs [(p/spring a b 100 0.25)]
+        gravity (p/gravity (v/vec2 0 9.81))
+        world-bounds (p/shape-constraint-inside (r/rect [500 500]))]
+    (p/physics {:particles particles
+                :springs springs
+                :behaviors {:gravity gravity}
+                :constraints {:bounds world-bounds}})))
+
+(defn setup []
+  (q/frame-rate 30)
+  (q/color-mode :hsb 360 100 100)
+  (q/rect-mode :center)
+  (q/ellipse-mode :center)
+  (q/background 350)
+  {:world (make-world)})
+
+(def radius 10)
+
+(defn register-mouse [state ev]
+  (assoc state :mouse (v/vec2 ev)))
+
+(defn unregister-mouse [state _]
+  (dissoc state :mouse))
+
+(defn key-pressed [state ev]
+  (case (:key ev)
+    :r (assoc state :world (make-world))
+    state))
+
+(defn hit-particle [{:keys [mouse world]}]
+  (when mouse
+    (let [r (* radius radius)]
+      (->> (filter #(<= (g/dist-squared (p/position %) mouse) r) (:particles world))
+           (first)))))
+
+(defn move-particle-with-mouse [state]
+  (cond-> state
+    (:hit state)
+    (update-in
+      [:world :particles]
+      (fn [particles]
+        (mapv (fn [p]
+                (cond-> p
+                  (and (not (p/locked? p))
+                       (= p (:hit state)))
+                  (p/set-position (:mouse state))))
+              particles)))))
+
+(defn update-state [state]
+  #_(prn (:mouse state))
+  (->
+   (update state :world p/timestep 1)
+   (assoc :hit (hit-particle state))
+   (move-particle-with-mouse)))
+
+(defn draw-state [state]
+  (q/background 180 10 90)
+  (q/no-stroke)
+  (doseq [particle (get-in state [:world :particles])]
+    (let [[x y] (p/position particle)
+          color (if (and (not (p/locked? particle))
+                         (= particle (:hit state)))
+                  [0 80 80]
+                  [0 20 80])]
+      (apply q/fill color)
+      (q/ellipse x y radius radius))))
+
+#_:clj-kondo/ignore
+(q/defsketch fifteen
+  :title "Fifteen"
+  :size [500 500]
+  :settings #(q/pixel-density (q/display-density))
+  :setup setup
+  :key-pressed key-pressed
+  :mouse-pressed register-mouse
+  :mouse-dragged register-mouse
+  :mouse-released unregister-mouse
+  :update update-state
+  :draw draw-state
+  :renderer :p2d
+  :features [:keep-on-top :no-bind-output]
+  :middleware [qm/pause-on-error qm/fun-mode])