asdf-games/lib/movement/wallslide.js

const entity = require("../utils/entity.js");

function wallslide(ent, map, x = 0, y = 0) {
	let tiles;
	let tileEdge;
	const bounds = entity.bounds(ent);

	// Final amounts of movement to allow
	let xo = x;
	let yo = y;

	// Check vertical movement
	if (y !== 0) {
		tiles = map.tilesAtCorners(bounds, 0, yo);
		const [tl, tr, bl, br] = tiles.map(t => t && t.frame.walkable);

		// Hit your head
		if (y < 0 && !(tl && tr)) {
			tileEdge = tiles[0].pos.y + tiles[0].h;
			yo = tileEdge - bounds.y;
		}
		// Hit your feet
		if (y > 0 && !(bl && br)) {
			tileEdge = tiles[2].pos.y - 1;
			yo = tileEdge - (bounds.y + bounds.h);
		}
	}

	// Check horizontal movement
	if (x !== 0) {
		tiles = map.tilesAtCorners(bounds, xo, yo);
		const [tl, tr, bl, br] = tiles.map(t => t && t.frame.walkable);

		// Hit left edge
		if (x < 0 && !(tl && bl)) {
			tileEdge = tiles[0].pos.x + tiles[0].w;
			xo = tileEdge - bounds.x;
		}
		// Hit right edge
		if (x > 0 && !(tr && br)) {
			tileEdge = tiles[1].pos.x - 1;
			xo = tileEdge - (bounds.x + bounds.w);
		}
	}
	// xo & yo contain the amount we're allowed to move by.
	return { x: xo, y: yo };
}

module.exports = {
	wallslide
};