Papure/src/lib/ink/palm.ts

// Palm rejection — ported from palm-rejection-test.html.
//
// Pens are always accepted. A touch is rejected while the pen is down, when its
// contact ellipse is palm-sized, or when it lands within 150 ms of pen activity.
// Touches that would draw are also rejected for 5 s after pen activity and while
// a pen hovers; touches that only pan/zoom are not, so navigating by hand keeps
// working between pen strokes. A touch already drawing is cut off as soon as
// pen activity appears (callers re-run `evaluate` on every move).

export interface PalmOptions {
	/** Block touches while the pen is down, and finger ink for 5 s after pen activity. */
	penSession: boolean;
	/** Reject touches with a wide contact ellipse. */
	geometry: boolean;
	/** Reject touches within 150 ms of pen activity. */
	timing: boolean;
	/** Block finger ink while 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;

/** What an accepted touch would do: ink (draw/erase) or pan/zoom. */
export type TouchIntent = 'draw' | 'navigate';

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;
	}

	/** A pen has been used since the app loaded; fingers then only navigate. */
	get penSeen() {
		return this.lastPenActivityTime > 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, intent: TouchIntent = 'draw'): 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 && this.penIsDown) {
			return { accept: false, reason: 'pen is down' };
		}
		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 (intent === 'draw') {
			if (o.penSession && since < PEN_SESSION_MS) {
				return { accept: false, reason: 'pen session active' };
			}
			if (o.hover && this.penHovering) {
				return { accept: false, reason: 'pen hovering' };
			}
		}
		return { accept: true, reason: 'touch accepted' };
	}
}