Improve bat theme

stregsystem/static/stregsystem/themes/bat/bat.css

@@ -1,22 +1,71 @@
-.bat-container {
-    position: absolute;
-    top: 0;
-    left: 0;
-    right: 0;
-    bottom: 0;
-    pointer-events: none;
+@property --x {
+	syntax: "<number>";
+	inherits: true;
+	initial-value: 0;
 }
+@property --y {
+	syntax: "<number>";
+	inherits: true;
+	initial-value: 0;
+}
+@property --direction {
+	syntax: "<number>";
+	inherits: true;
+	initial-value: 0;
+}
+@property --delay {
+	syntax: "<number>";
+	inherits: true;
+	initial-value: 0;
+}
+
+#bat-container {
+	position: fixed;
+	inset: 1lvw 1lvh;
+	margin: 0;
+	transition: opacity 2s ease;
+	pointer-events: none;
+
+	&.stationary {
+		opacity: 0.7;
 
-.bat {
-    position: fixed;
-    left: 0;
-    top: 0;
-    opacity: 0;
-    transition-property: top, left, opacity;
-    transition-delay: 0s;
-    transition-timing-function: linear;
+		@media (prefers-contrast) or (prefers-reduced-transparency) {
+			display: none;
+		}
+	}
+
+	& .bat {
+		position: absolute;
+		width: 10%;
+		height: 5%;
+		left: 0;
+		top: 0;
+		scale: var(--direction) 1;
+		translate: calc(9% * var(--x)) calc(19% * var(--y));
+		background: url(./sprites/0.png) no-repeat center / contain;
+		animation: 600ms steps(1, end) calc(-6ms * var(--delay)) infinite bat-flap;
+		image-rendering: pixelated;
+
+		@media (prefers-reduced-motion) {
+			animation-play-state: paused !important;
+		}
+	}
 }
 
-.bat img {
-    image-rendering: pixelated;
-}
+@keyframes bat-flap {
+	0% {
+		background-image: url(sprites/0.png);
+	}
+	33.33% {
+		background-image: url(sprites/1.png);
+	}
+	50% {
+		background-image: url(sprites/2.png);
+	}
+	66.67% {
+		background-image: url(sprites/3.png);
+	}
+	83.33% {
+		background-image: url(sprites/4.png);
+	}
+}

stregsystem/static/stregsystem/themes/bat/bat.js

@@ -1,47 +1,197 @@
-/*! JS Bat 2013 - v1.2 - Eric Grange - www.delphitools.info */
-
-function spawn_bat() {
-    var v = document.createElement('video');
-    var s = document.createElement('source');
-    var z = document.createElement('div');
-    var zs = z.style;
-    var a = window.innerWidth * Math.random();
-    var b = window.innerHeight * Math.random();
-    z.classList.add("bat");
-    z.appendChild(v);
-    v.appendChild(s);
-    v.width = "48";
-    v.height = "48";
-    v.autoplay = true;
-    v.loop = true;
-    v.muted = true;
-    s.src = themes_static_url + "bat/bat.webm";
-    s.type = "video/webm";
-    document.body.querySelector(".bat-container").appendChild(z);
-
-    function R(o, m) {
-        return Math.max(Math.min(o + (Math.random() - 0.5) * 400, m - 50), 50);
-    }
-
-    function A(){
-        var x = R(a, window.innerWidth);
-        var y = R(b, window.innerHeight);
-        var d = Math.round(10 * Math.sqrt((a - x) * (a - x) + (b - y) * (b - y)));
-        zs.opacity = 1;
-        zs.transitionDuration = zs.webkitTransitionDuration = d + 'ms';
-        // zs.transform = zs.webkitTransform = 'translate(' + x + 'px, ' + y + 'px)';
-        zs.left = x + 'px';
-        zs.top = y + 'px';
-        v.style.transform = v.style.webkitTransform = (a > x) ? '' : 'scaleX(-1)';
-        a = x;
-        b = y;
-        setTimeout(A,d);
-    }
-    setTimeout(A, Math.random() * 3e3);
-};
-
-const d = new Date();
-
-for(let n_bats=0; n_bats < d.getDate(); n_bats++){
-    spawn_bat();
+// Fallback for WebKit: https://caniuse.com/requestidlecallback
+// TODO: remove this crap as soon as WebKit supports the real deal
+const requestIdleCallback =
+	globalThis.requestIdleCallback ??
+	((func, { timeout }) => {
+		const maxWait = Math.min(timeout ?? Infinity, 100);
+		return setTimeout(func, maxWait);
+	});
+const cancelIdleCallback = globalThis.cancelIdleCallback ?? clearTimeout;
+
+// The minimum amount of time that CSS animations should be buffered for
+const minAnimationBuffer = 5_000;
+// The maximum amount of time that CSS animations should be buffered for
+const maxAnimationBuffer = 10_000;
+// Make this bigger to make bats go faster
+const speedMultiplier = 150;
+
+// The HTML container that our bats exist in
+const container = document.querySelector("#bat-container");
+
+// The ID of the timeout that is currently waiting to call `pointAndShoot`
+let timeoutId;
+// A queue of all the bats, ordered by when they will need new coordinates
+const batQueue = [];
+
+// Initial setup of the bats queue
+for (const element of container.querySelectorAll(".bat")) {
+	batQueue.push({
+		element,
+		nextFly: 0,
+	});
+}
+
+// Ensure that we disable this stuff if the user prefers reduced motion,
+// or if the user is not looking at the page
+const prefersReducedMotion = window.matchMedia("(prefers-reduced-motion)");
+prefersReducedMotion.addEventListener("change", handleStationaryChange);
+document.addEventListener("visibilitychange", handleStationaryChange);
+handleStationaryChange();
+function handleStationaryChange() {
+	if (prefersReducedMotion.matches || document.visibilityState === "hidden") {
+		pauseShooting();
+	} else {
+		resumeShooting();
+	}
+}
+/**
+ * Makes sure everything stops running and removes any animations.
+ */
+function pauseShooting() {
+	// Let CSS know to stop moving
+	container.classList.add("stationary");
+	// Stop shooting bats around
+	clearTimeout(timeoutId);
+	cancelIdleCallback(timeoutId);
+	timeoutId = undefined;
+}
+/**
+ * Kickstarts everything again.
+ */
+function resumeShooting() {
+	// Let CSS know to start moving again
+	container.classList.remove("stationary");
+	// Start shooting bats around again
+	prepareNextShot();
+}
+
+/**
+ * Call this function to ensure that bats are getting new coordinates.
+ * DO NOT call the `pointAndShoot` function directly, as that might
+ * cause two functions to be running simultaneously.
+ */
+function prepareNextShot() {
+	if (timeoutId !== undefined) {
+		return;
+	}
+	const bat = batQueue[0];
+	const timeToNextFly = Math.max(0, bat.nextFly - Date.now());
+	if (timeToNextFly < 10) {
+		// If the bat needs new coordinates RIGHT NOW, schedule it quick
+		// with a `setTimeout`.
+		// We do not run it directly, as running this back-to-back 30 times
+		// in a row would block user input and make the page feel janky.
+		timeoutId = setTimeout(() => {
+			timeoutId = undefined;
+			pointAndShoot();
+		}, 0);
+	} else if (timeToNextFly < maxAnimationBuffer) {
+		// If the bat needs new coordinates sometime before the max buffer time,
+		// schedule an idle callback, so it doesn't interfere with more important tasks.
+		timeoutId = requestIdleCallback(
+			() => {
+				timeoutId = undefined;
+				pointAndShoot();
+			},
+			{ timeout: timeToNextFly },
+		);
+	} else {
+		// If the bat needs new coordinates later than our max buffer time,
+		// take a chill pill and schedule a timeout that wakes us up when
+		// we get close to our min buffer time limit.
+		timeoutId = setTimeout(() => {
+			timeoutId = undefined;
+			prepareNextShot();
+		}, timeToNextFly - minAnimationBuffer);
+	}
+}
+/**
+ * Gives the next bat in the queue some new coordinates.
+ * DO NOT call this directly, call `prepareNextShot` instead.
+ */
+function pointAndShoot() {
+	// Get the next bat in the queue.
+	const bat = batQueue.shift();
+
+	// On first load, we need to get the bat position from the HTML
+	bat.x ??= Number(bat.element.style.getPropertyValue("--x"));
+	bat.y ??= Number(bat.element.style.getPropertyValue("--y"));
+
+	// Calculate new coordinates
+	const { coordinate: newX, direction } = newCoordinate(bat.x);
+	const { coordinate: newY } = newCoordinate(bat.y);
+	// Calculate the animation time based on how far
+	// the new coordinates are from the previous
+	const distance = Math.sqrt((bat.x - newX) ** 2 + (bat.y - newY) ** 2);
+	const flyTime = speedMultiplier * distance;
+
+	const now = Date.now();
+	// If we are late to the party, we pretend that the bat was supposed to fly right now
+	bat.nextFly = Math.max(bat.nextFly, now);
+
+	// Set the animation in the DOM
+	const batDirection = direction * -1;
+	bat.element.animate(
+		[
+			{ "--x": bat.x, "--y": bat.y, "--direction": batDirection },
+			{ "--x": newX, "--y": newY, "--direction": batDirection },
+		],
+		{
+			delay: bat.nextFly - now,
+			duration: flyTime,
+			fill: "forwards",
+		},
+	);
+
+	// Set everything in our local bat object so we know what's up next time
+	bat.nextFly += flyTime;
+	bat.x = newX;
+	bat.y = newY;
+
+	// Put it back in the bats array.
+	// Bats must be ordered such that the first element is always the next one that needs to be shot.
+	let i;
+	for (i = 0; i < batQueue.length; i++) {
+		if (batQueue[i].nextFly > bat.nextFly) {
+			break;
+		}
+	}
+	batQueue.splice(i, 0, bat);
+
+	// Move on to the next bat in need
+	prepareNextShot();
+}
+
+/**
+ * Generates a new coordinate that is at least 2% different from
+ * the previous, and at most 15% different.
+ *
+ * @param {number} previous - The previous coordinate.
+ */
+function newCoordinate(previous) {
+	// The minimum amount it can change in percentage
+	const min = 2;
+	// The amount it can change beyond the minimum, in percentage
+	const maxRange = 13;
+
+	const change = min + maxRange * Math.random();
+	let direction = Math.random() < 0.5 ? 1 : -1;
+	let coordinate = previous + change * direction;
+	// Make sure that the bat doesn't move outside the page
+	if (coordinate < 0 || 100 < coordinate) {
+		direction *= -1;
+		coordinate = previous + change * direction;
+	}
+	return { coordinate, direction };
+}
+
+/**
+ * Ensure that the value is somewhere between a min and max value.
+ *
+ * @param {number} min - The smallest permitted value.
+ * @param {number} value - The value to clamp.
+ * @param {number} max - The largest permitted value.
+ */
+function clamp(min, value, max) {
+	return Math.max(min, Math.min(max, value));
 }

stregsystem/templates/stregsystem/themes/bat/bat.html

@@ -1 +1,17 @@
-<div class="bat-container"></div>
+{% load themes %}
+{% load stregsystem_extras %}
+{% day_of_month as day %}
+
+<div id="bat-container" class="stationary">
+	{% for i in day|to_range %}
+		<div
+			class="bat"
+			style="
+				--x: {% random 0 100 %};
+				--y: {% random 0 100 %};
+				--direction: {% random_choice 1 -1 %};
+				--delay: {% random 0 100 %};
+			"
+		></div>
+	{% endfor %}
+</div>

stregsystem/templatetags/stregsystem_extras.py

@@ -1,3 +1,5 @@
+from datetime import datetime
+from random import randint, choice
 from django import template
 from django.template.loader import get_template
 from django.utils import timezone
@@ -45,3 +47,23 @@ def product_id_and_alias_string(product_id):
     else:
         #
         return str(product_id)
+
+
+@register.simple_tag
+def day_of_month():
+    return datetime.now().day
+
+
+@register.simple_tag
+def random(min, max):
+    return randint(min, max)
+
+
+@register.simple_tag
+def random_choice(str1, str2):
+    return choice([str1, str2])
+
+
+@register.filter
+def to_range(value):
+    return range(value)