import { type AABB, type Span, sweepInterval, type Vec } from "./given.ts";

/** A collision: the fraction of the frame at impact, and the surface normal. */
export type Hit = { time: number; normal: Vec };

/**
 * A point at `p` moving by `v` over this frame, against a static box.
 * Returns the entry Hit, or null if there is no collision this frame.
 *
 * Reuse sweepInterval twice (x and y), then combine:
 *   entry = max of the two entries, exit = min of the two exits.
 */
export function rayVsAABB(p: Vec, v: Vec, box: AABB): Hit | null {
	const spanX = sweepInterval(p.x, v.x, box.x, box.x + box.w);
	if (spanX === null) {
		return null;
	}
	const spanY = sweepInterval(p.y, v.y, box.y, box.y + box.h);
	if (spanY === null) {
		return null;
	}

	const span: Span = {
		entry: Math.max(spanX.entry, spanY.entry),
		exit: Math.min(spanX.exit, spanY.exit),
	};

	if (span.entry > span.exit) {
		return null;
	}

	if (span.entry >= 1 || span.exit <= 0) {
		return null;
	}

	const time = span.entry;
	const normal = {
		x: 0,
		y: 0,
	};

	if (spanX.entry > spanY.entry) {
		normal.x = v.x > 0 ? -1 : 1;
	} else {
		normal.y = v.y > 0 ? -1 : 1;
	}

	return {
		time,
		normal,
	};
}