stick-figure-mmo/static/physics.js

export class PhysicsSystems {
    resolveCollisions(player, platforms, groundY) {
        const totalHeight = 60;
        const halfFootW = 12;
        const EPS = 0.5;

        if (player._prevY === undefined) {
            player._prevY = player.y;
        }

        let landedOnPlatform = false;

        // one-way platforms (land only when falling and crossing from above)
        if (player.vy >= 0) {
            for (const pf of platforms) {
                const feetLeft = player.x - halfFootW;
                const feetRight = player.x + halfFootW;
                const pfLeft = pf.x, pfRight = pf.x + pf.w;

                const horizOverlap = feetRight > pfLeft && feetLeft < pfRight;
                const wasAbove = (player._prevY + totalHeight) <= pf.y + EPS;
                const nowBelowTop = (player.y + totalHeight) >= pf.y - EPS;
                const dropping = player.crouching === true; // hold 's' to drop through

                if (horizOverlap && wasAbove && nowBelowTop && !dropping) {
                    player.y = pf.y - totalHeight;
                    player.vy = 0;
                    landedOnPlatform = true;
                    break;
                }
            }
        }
        // --- ground after platforms ---
        if (!landedOnPlatform && player.y + totalHeight >= groundY) {
            player.y = groundY - totalHeight;
            player.vy = 0;
            player.onGround = true;
        } else if (landedOnPlatform) {
            player.onGround = true;
        } else {
            player.onGround = false;
        }
    }
}