stick-figure-mmo/static/physics.js

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

        const feetLeft = player.x - halfFootW;
        const feetRight = player.x + halfFootW;

        const prevFeetY = (player._prevY ?? player.y) + totalHeight;
        const currFeetY = player.y + totalHeight;

        let landedOnPlatform = false;

        let landed = false;

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

                const horizOverlap = feetRight > pfLeft && feetLeft < pfRight;
                const crossedTop = (prevFeetY <= pfTop + EPS) && (currFeetY >= pfTop - EPS);
                const dropping = player.crouching === true;

                if (horizOverlap && crossedTop && !dropping) {
                    player.y = pfTop - totalHeight;
                    player.vy = 0;
                    player.onGround = true;
                    landed = true;
                    player.state = 'ground';
                    break;
                }
            }
        }
        // --- ground after platforms ---
        if (!landedOnPlatform) {
            const crossedGround = (prevFeetY <= groundY + EPS) && (currFeetY >= groundY - EPS);
            if (crossedGround) {
                player.y = groundY - totalHeight;
                player.vy = 0;
                player.onGround = true;
                player.state = 'ground';
                landed = true
            }
        }

        if (!landed) {
            player.onGround = false;
            player.state = 'air';
        }
    }
}