// Fling detection for one-finger pans: a quick flick that ends the drag.

import type { Dir } from '$lib/model/tree';

export interface Sample {
	x: number;
	y: number;
	t: number;
}

/** Only the last stretch of the drag counts. */
const WINDOW_MS = 100;
/** px/ms; a slow drag that stops is not a fling. */
const MIN_SPEED = 0.6;
/** The main axis must dominate by this much. */
const MIN_RATIO = 2;

/**
 * Side of the canvas a fling heads for, or null. Moving the finger left pulls in
 * what's on the right, so a leftward flick returns 'right'.
 */
export function flingDir(samples: readonly Sample[]): Dir | null {
	const last = samples.at(-1);
	if (!last) return null;
	const first = samples.find((s) => last.t - s.t <= WINDOW_MS);
	if (!first || first === last) return null;
	const dt = last.t - first.t;
	if (dt <= 0) return null;
	const dx = last.x - first.x;
	const dy = last.y - first.y;
	if (Math.hypot(dx, dy) / dt < MIN_SPEED) return null;
	if (Math.abs(dx) >= Math.abs(dy) * MIN_RATIO) return dx < 0 ? 'right' : 'left';
	if (Math.abs(dy) >= Math.abs(dx) * MIN_RATIO) return dy < 0 ? 'down' : 'up';
	return null;
}

/** Keeps a short trail of pointer samples for flingDir. */
export function pushSample(samples: Sample[], s: Sample) {
	samples.push(s);
	while (samples.length > 2 && s.t - samples[0].t > WINDOW_MS * 2) samples.shift();
}