Add utility function to create HSL colours

lib/www/client/source/src/lib/hsl.js

@@ -0,0 +1,47 @@
+/** Return an HSL colour as a function of an input value
+  * `str`.
+  *
+  * Consider using as getHSL.bind(this) in Vue components
+  * in order to get access to the Vuetify theme configuration.
+  */
+function getHSL (str, saturation = 1, lightness = 0.25, offset = 0) {
+
+  function getHash (v) {
+    if (typeof (v??false)[Symbol.iterator] != "function") {
+      // Not an iterable, make it one
+      v = String(v);
+    }
+
+    return Math.abs([...v, ..." "].reduce( (acc, cur) => String(cur).charCodeAt(0) + ((acc << 5) - acc), 0 ));
+  }
+
+  const h = (getHash(str) + offset) % 360;
+  const s = saturation * 100;
+  const l = this?.$vuetify?.theme?.isDark
+    ? (1-lightness) * 100
+    : lightness * 100;
+
+  return {h, s, l};
+
+}
+
+/** Return a CSS hsl() or hsla() colour
+ * representation as a function of an input value.
+ *
+ * Consider using as getHSLColourFor.bind(this) – See
+ * note for getHSL() above.
+ */
+function getHSLColourFor (str, opacity = 1, saturation, lightness, offset) {
+  const _getHSL = getHSL.bind(this);
+  const {h, s, l} = _getHSL(str, saturation, lightness, offset);
+  if (opacity == 1) {
+    return `hsl(${h},${s}%,${l}%)`;
+  } else {
+    return `hsla(${h},${s}%,${l}%, ${opacity})`;
+  }
+}
+
+export {
+  getHSL,
+  getHSLColourFor
+}