1 files changed, 79 insertions, 0 deletions
diff --git a/site/arena.mjs b/site/arena.mjs
new file mode 100644
index 0000000..c76f7dc
--- /dev/null
+++ b/site/arena.mjs
@@ -0,0 +1,79 @@
+const CANVAS_SELECTOR = 'canvas';
+const FPS_SELECTOR = '#fps';
+
+export default class extends HTMLElement {
+    static name = 'x-arena';
+    static register() {
+        console.debug('registering custom element', this.name, this);
+        self.customElements.define(this.name, this);
+    }
+    static radius = 25;
+
+    #canvas;
+    #ctx;
+    #lastTime;
+    tickMS;
+
+    connectedCallback() {
+        console.debug('connectedCallback', this);
+        this.#canvas = this.querySelector(CANVAS_SELECTOR);
+        this.#ctx = this.#canvas.getContext('2d');
+    }
+
+    invalidateFPS() {
+        document.querySelector(FPS_SELECTOR).textContent = 'n/a';
+    }
+
+    clamp(radius, x, y) {
+        const xx = Math.min(Math.max(radius, x), this.#canvas.width-radius);
+        const yy = Math.min(Math.max(radius, y), this.#canvas.height-radius);
+        return [xx, yy];
+    }
+
+    randStart(robo) {
+        const [x, y] = [this.#canvas.width, this.#canvas.height].map(len =>
+            this.constructor.radius + Math.floor(Math.random() * (len - 2*this.constructor.radius)));
+        return {...robo, x, y};
+    }
+
+    renderFPS(now) {
+        const delta = now - (this.#lastTime ?? now);
+        if (delta > 0) {
+            document.querySelector(FPS_SELECTOR).textContent = Math.floor(1_000 / delta);
+        }
+    }
+
+    render(robos, now=0) {
+        // we often get called with the same time stamp many times in
+        // a row due to fingerprint-reduction tech.
+        if (now === this.#lastTime) {
+            return;
+        }
+        this.renderFPS(now);
+        this.#lastTime = now;
+
+        this.#ctx.clearRect(0, 0, this.#canvas.width, this.#canvas.height);
+
+        robos.forEach(robo => {
+            // interpolation factor for smoother movement independent
+            // of tick rate, but never tween more than 1 tick.
+            const delta = robo.lastTick ? now - robo.lastTick : 0;
+            const timeScale = this.tickMS > 0 ? Math.min(delta / this.tickMS, 1.0) : 1;
+            let [x, y] = [robo.x, robo.y];
+            if (delta > 0) {
+                const [velx, vely] = [
+                    robo.speedx, robo.speedy
+                ].map(x => timeScale * x);
+                [x, y] = this.clamp(this.constructor.radius, robo.x + velx, robo.y + vely);
+            }
+            this.#renderRobo(x, y);
+        });
+    }
+
+    #renderRobo(x, y) {
+        this.#ctx.fillStyle = 'rgb(200 0 0)';
+        this.#ctx.beginPath();
+        this.#ctx.arc(x, y, this.constructor.radius, 0, 2 * Math.PI);
+        this.#ctx.fill();
+    }
+}