import { clamp } from "../slider-utils/clamp";

/**
 * Sub-pixel tolerance around each edge. Scroll offsets within one pixel of
 * 0 or the maximum are treated as "at the edge" — browsers occasionally
 * report fractional offsets due to device-pixel rounding even when the
 * user perceives the content to be fully scrolled.
 */
export const SCROLL_EDGE_TOLERANCE_PX = 1;

/**
 * Maximum scroll offset available for the given scroll and client sizes.
 * Guards against negative values — when content is smaller than the
 * viewport, the max is 0, not a negative number.
 */
export function getMaxScrollOffset(
	scrollSize: number,
	clientSize: number,
): number {
	return Math.max(0, scrollSize - clientSize);
}

/**
 * Clamps `value` into `[0, max]` and snaps to whichever edge is within
 * {@link SCROLL_EDGE_TOLERANCE_PX}. When both edges are within tolerance
 * (i.e. `max <= SCROLL_EDGE_TOLERANCE_PX`) the closer edge wins, with ties
 * going to the start.
 */
export function normalizeScrollOffset(value: number, max: number): number {
	if (max <= 0) {
		return 0;
	}

	const clamped = clamp(value, 0, max);
	const startDistance = clamped;
	const endDistance = max - clamped;
	const withinStartTolerance = startDistance <= SCROLL_EDGE_TOLERANCE_PX;
	const withinEndTolerance = endDistance <= SCROLL_EDGE_TOLERANCE_PX;

	if (withinStartTolerance && withinEndTolerance) {
		return startDistance <= endDistance ? 0 : max;
	}

	if (withinStartTolerance) {
		return 0;
	}

	if (withinEndTolerance) {
		return max;
	}

	return clamped;
}