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src/lib/ink/palm.ts

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+// Palm rejection — ported from palm-rejection-test.html.
+//
+// Pens are always accepted. A touch is rejected when a pen session is active
+// (stroke in progress or pen activity within the last 5 s), when its contact
+// ellipse is palm-sized, when it lands within 150 ms of pen activity, or while
+// a pen hovers over the screen. A touch already drawing is cut off as soon as
+// pen activity appears (callers re-run `evaluate` on every move).
+
+export interface PalmOptions {
+	/** Block all touches while a pen session is active (5 s window). */
+	penSession: boolean;
+	/** Reject touches with a wide contact ellipse. */
+	geometry: boolean;
+	/** Reject touches within 150 ms of pen activity. */
+	timing: boolean;
+	/** Arm the lockout as soon as the pen hovers. */
+	hover: boolean;
+}
+
+export const DEFAULT_PALM: PalmOptions = { penSession: true, geometry: true, timing: true, hover: true };
+
+const PEN_SESSION_MS = 5000;
+const PEN_TIMING_MS = 150;
+const PALM_CONTACT_PX = 35;
+
+export interface Decision {
+	accept: boolean;
+	reason: string;
+}
+
+export class PalmRejector {
+	options: PalmOptions;
+	lastPenActivityTime = 0;
+	private activePens = new Set<number>();
+	private penHovering = false;
+
+	constructor(options: PalmOptions = DEFAULT_PALM) {
+		this.options = options;
+	}
+
+	get penIsDown() {
+		return this.activePens.size > 0;
+	}
+
+	/** Feed every pointerdown/move/up/leave so pen state stays current. */
+	track(e: PointerEvent) {
+		if (e.pointerType !== 'pen') return;
+		const now = Date.now();
+		switch (e.type) {
+			case 'pointerdown':
+				this.activePens.add(e.pointerId);
+				this.lastPenActivityTime = now;
+				break;
+			case 'pointermove':
+			case 'pointerover':
+			case 'pointerenter':
+				// Hover (buttons == 0) counts as "pen nearby" too.
+				this.lastPenActivityTime = now;
+				this.penHovering = !this.activePens.has(e.pointerId);
+				break;
+			case 'pointerup':
+			case 'pointercancel':
+				this.activePens.delete(e.pointerId);
+				this.lastPenActivityTime = now;
+				break;
+			case 'pointerleave':
+			case 'pointerout':
+				this.penHovering = false;
+				this.lastPenActivityTime = now;
+				break;
+		}
+	}
+
+	evaluate(e: PointerEvent): Decision {
+		if (e.pointerType === 'pen') return { accept: true, reason: 'pen input' };
+		if (e.pointerType !== 'touch') return { accept: true, reason: 'non-touch pointer' };
+
+		const o = this.options;
+		const since = Date.now() - this.lastPenActivityTime;
+
+		if (o.penSession && (since < PEN_SESSION_MS || this.penIsDown)) {
+			return { accept: false, reason: 'pen session active' };
+		}
+		if (o.geometry) {
+			const contact = Math.max(e.width || 0, e.height || 0);
+			if (contact > PALM_CONTACT_PX) {
+				return { accept: false, reason: `wide contact (${contact.toFixed(0)}px)` };
+			}
+		}
+		if (o.timing && since < PEN_TIMING_MS) {
+			return { accept: false, reason: 'within 150ms of pen activity' };
+		}
+		if (o.hover && this.penHovering) {
+			return { accept: false, reason: 'pen hovering' };
+		}
+		return { accept: true, reason: 'touch accepted' };
+	}
+}