Shuffle the colors on click

src/ListUtils.re

@@ -0,0 +1,34 @@
+let swap = (items, i, j) => {
+  /* find the values that need to be swapped */
+  let i_item = List.nth(items, i);
+  let j_item = List.nth(items, j);
+
+  /* partial application of mapi to create swapper function */
+  let swapper =
+    List.mapi((index, item) =>
+      switch (index) {
+      | x when x == i => j_item
+      | x when x == j => i_item
+      | _ => item
+      }
+    );
+
+  /* swap the items in the items list */
+  swapper(items);
+};
+
+/* Fisher-Yates Shuffle
+   https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle */
+let shuffle = (items: list('a)) => {
+  Random.init(int_of_float(Unix.time()));
+
+  let len = List.length(items);
+  let boxed_items = ref(items);
+
+  for (i in len - 1 downto 1) {
+    let j = Random.int(i);
+    boxed_items := swap(boxed_items^, i, j);
+  };
+
+  boxed_items^;
+};

src/index.re

@@ -22,9 +22,12 @@ let colors = [
 let width = 450;
 let height = 450;
 
-let setup = env => Env.size(~width, ~height, env);
+let setup = env => {
+  Env.size(~width, ~height, env);
+  colors;
+};
 
-let draw_circles = env => {
+let draw_circles = (colors, env) => {
   let xSpacing = width / 9;
   let ySpacing = height / 9;
 
@@ -48,9 +51,15 @@ let draw_circles = env => {
   };
 };
 
-let draw = (_state, env) => {
+let draw = (colors, env) => {
   Draw.background(Utils.color(~r=255, ~g=255, ~b=255, ~a=255), env);
-  draw_circles(env);
+  draw_circles(colors, env);
+
+  if (Env.mousePressed(env)) {
+    ListUtils.shuffle(colors);
+  } else {
+    colors;
+  };
 };
 
 run(~setup, ~draw, ());