body {
  /* https://coolors.co/f06449-ede6e3-7d82b8-36382e-613f75  */
  --background-color: #ede6e3;
  --wall-color: #36382e;
  --joystick-color: #210124;
  --joystick-head-color: #f06449;
  --ball-color: #f06449;
  --end-color: #7d82b8;
  --text-color: #210124;

  font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
  background-color: var(--background-color);
}

html,
body {
  height: 100%;
  margin: 0;
}

#center {
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100%;
}

#game {
  display: grid;
  grid-template-columns: auto 150px;
  grid-template-rows: 1fr auto 1fr;
  gap: 30px;
  perspective: 600px;
}

#maze {
  position: relative;
  grid-row: 1 / -1;
  grid-column: 1;
  width: 350px;
  height: 315px;
  display: flex;
  justify-content: center;
  align-items: center;
}

#end {
  width: 65px;
  height: 65px;
  border: 5px dashed var(--end-color);
  border-radius: 50%;
}

#joystick {
  position: relative;
  background-color: var(--joystick-color);
  border-radius: 50%;
  width: 50px;
  height: 50px;
  display: flex;
  align-items: center;
  justify-content: center;
  margin: 10px 50px;
  grid-row: 2;
}

#joystick-head {
  position: relative;
  background-color: var(--joystick-head-color);
  border-radius: 50%;
  width: 20px;
  height: 20px;
  cursor: grab;

  animation-name: glow;
  animation-duration: 0.6s;
  animation-iteration-count: infinite;
  animation-direction: alternate;
  animation-timing-function: ease-in-out;
  animation-delay: 4s;
}

@keyframes glow {
  0% {
    transform: scale(1);
  }
  100% {
    transform: scale(1.2);
  }
}

.joystick-arrow:nth-of-type(1) {
  position: absolute;
  bottom: 55px;

  width: 0;
  height: 0;
  border-left: 10px solid transparent;
  border-right: 10px solid transparent;

  border-bottom: 10px solid var(--joystick-color);
}

.joystick-arrow:nth-of-type(2) {
  position: absolute;
  top: 55px;

  width: 0;
  height: 0;
  border-left: 10px solid transparent;
  border-right: 10px solid transparent;

  border-top: 10px solid var(--joystick-color);
}

.joystick-arrow:nth-of-type(3) {
  position: absolute;
  left: 55px;

  width: 0;
  height: 0;
  border-top: 10px solid transparent;
  border-bottom: 10px solid transparent;

  border-left: 10px solid var(--joystick-color);
}

.joystick-arrow:nth-of-type(4) {
  position: absolute;
  right: 55px;

  width: 0;
  height: 0;
  border-top: 10px solid transparent;
  border-bottom: 10px solid transparent;

  border-right: 10px solid var(--joystick-color);
}

#note {
  grid-row: 3;
  grid-column: 2;
  text-align: center;
  font-size: 0.8em;
  color: var(--text-color);
  transition: opacity 2s;
}

a:visited {
  color: inherit;
}

.ball {
  position: absolute;
  margin-top: -5px;
  margin-left: -5px;
  border-radius: 50%;
  background-color: var(--ball-color);
  width: 10px;
  height: 10px;
}

.wall {
  position: absolute;
  background-color: var(--wall-color);
  transform-origin: top center;
  margin-left: -5px;
}

.wall::before,
.wall::after {
  display: block;
  content: "";
  width: 10px;
  height: 10px;
  background-color: inherit;
  border-radius: 50%;
  position: absolute;
}

.wall::before {
  top: -5px;
}

.wall::after {
  bottom: -5px;
}

.black-hole {
  position: absolute;
  margin-top: -9px;
  margin-left: -9px;
  border-radius: 50%;
  background-color: black;
  width: 18px;
  height: 18px;
}

#youtube,
#youtube-card {
  display: none;
}

@media (min-height: 425px) {
  /** Youtube logo by https://codepen.io/alvaromontoro */
  #youtube {
    z-index: 2;
    display: block;
    width: 100px;
    height: 70px;
    position: absolute;
    bottom: 20px;
    right: 20px;
    background: red;
    border-radius: 50% / 11%;
    transform: scale(0.8);
    transition: transform 0.5s;
  }

  #youtube:hover,
  #youtube:focus {
    transform: scale(0.9);
  }

  #youtube::before {
    content: "";
    display: block;
    position: absolute;
    top: 7.5%;
    left: -6%;
    width: 112%;
    height: 85%;
    background: red;
    border-radius: 9% / 50%;
  }

  #youtube::after {
    content: "";
    display: block;
    position: absolute;
    top: 20px;
    left: 40px;
    width: 45px;
    height: 30px;
    border: 15px solid transparent;
    box-sizing: border-box;
    border-left: 30px solid white;
  }

  #youtube span {
    font-size: 0;
    position: absolute;
    width: 0;
    height: 0;
    overflow: hidden;
  }

  #youtube:hover + #youtube-card {
    display: block;
    position: absolute;
    bottom: 12px;
    right: 10px;
    padding: 25px 130px 25px 25px;
    width: 300px;
    background-color: white;
  }
}

/*

If you want to know how this game works, you can find a source code walkthrough video here: https://youtu.be/bTk6dcAckuI

Follow me on twitter for more: https://twitter.com/HunorBorbely

*/

Math.minmax = (value, limit) => {
  return Math.max(Math.min(value, limit), -limit);
};

const distance2D = (p1, p2) => {
  return Math.sqrt((p2.x - p1.x) ** 2 + (p2.y - p1.y) ** 2);
};

// Angle between the two points
const getAngle = (p1, p2) => {
  let angle = Math.atan((p2.y - p1.y) / (p2.x - p1.x));
  if (p2.x - p1.x < 0) angle += Math.PI;
  return angle;
};

// The closest a ball and a wall cap can be
const closestItCanBe = (cap, ball) => {
  let angle = getAngle(cap, ball);

  const deltaX = Math.cos(angle) * (wallW / 2 + ballSize / 2);
  const deltaY = Math.sin(angle) * (wallW / 2 + ballSize / 2);

  return { x: cap.x + deltaX, y: cap.y + deltaY };
};

// Roll the ball around the wall cap
const rollAroundCap = (cap, ball) => {
  // The direction the ball can't move any further because the wall holds it back
  let impactAngle = getAngle(ball, cap);

  // The direction the ball wants to move based on it's velocity
  let heading = getAngle(
    { x: 0, y: 0 },
    { x: ball.velocityX, y: ball.velocityY }
  );

  // The angle between the impact direction and the ball's desired direction
  // The smaller this angle is, the bigger the impact
  // The closer it is to 90 degrees the smoother it gets (at 90 there would be no collision)
  let impactHeadingAngle = impactAngle - heading;

  // Velocity distance if not hit would have occurred
  const velocityMagnitude = distance2D(
    { x: 0, y: 0 },
    { x: ball.velocityX, y: ball.velocityY }
  );
  // Velocity component diagonal to the impact
  const velocityMagnitudeDiagonalToTheImpact =
    Math.sin(impactHeadingAngle) * velocityMagnitude;

  // How far should the ball be from the wall cap
  const closestDistance = wallW / 2 + ballSize / 2;

  const rotationAngle = Math.atan(
    velocityMagnitudeDiagonalToTheImpact / closestDistance
  );

  const deltaFromCap = {
    x: Math.cos(impactAngle + Math.PI - rotationAngle) * closestDistance,
    y: Math.sin(impactAngle + Math.PI - rotationAngle) * closestDistance
  };

  const x = ball.x;
  const y = ball.y;
  const velocityX = ball.x - (cap.x + deltaFromCap.x);
  const velocityY = ball.y - (cap.y + deltaFromCap.y);
  const nextX = x + velocityX;
  const nextY = y + velocityY;

  return { x, y, velocityX, velocityY, nextX, nextY };
};

// Decreases the absolute value of a number but keeps it's sign, doesn't go below abs 0
const slow = (number, difference) => {
  if (Math.abs(number) <= difference) return 0;
  if (number > difference) return number - difference;
  return number + difference;
};

const mazeElement = document.getElementById("maze");
const joystickHeadElement = document.getElementById("joystick-head");
const noteElement = document.getElementById("note"); // Note element for instructions and game won, game failed texts

let hardMode = false;
let previousTimestamp;
let gameInProgress;
let mouseStartX;
let mouseStartY;
let accelerationX;
let accelerationY;
let frictionX;
let frictionY;

const pathW = 25; // Path width
const wallW = 10; // Wall width
const ballSize = 10; // Width and height of the ball
const holeSize = 18;

const debugMode = false;

let balls = [];
let ballElements = [];
let holeElements = [];

resetGame();

// Draw balls for the first time
balls.forEach(({ x, y }) => {
  const ball = document.createElement("div");
  ball.setAttribute("class", "ball");
  ball.style.cssText = `left: ${x}px; top: ${y}px; `;

  mazeElement.appendChild(ball);
  ballElements.push(ball);
});

// Wall metadata
const walls = [
  // Border
  { column: 0, row: 0, horizontal: true, length: 10 },
  { column: 0, row: 0, horizontal: false, length: 9 },
  { column: 0, row: 9, horizontal: true, length: 10 },
  { column: 10, row: 0, horizontal: false, length: 9 },

  // Horizontal lines starting in 1st column
  { column: 0, row: 6, horizontal: true, length: 1 },
  { column: 0, row: 8, horizontal: true, length: 1 },

  // Horizontal lines starting in 2nd column
  { column: 1, row: 1, horizontal: true, length: 2 },
  { column: 1, row: 7, horizontal: true, length: 1 },

  // Horizontal lines starting in 3rd column
  { column: 2, row: 2, horizontal: true, length: 2 },
  { column: 2, row: 4, horizontal: true, length: 1 },
  { column: 2, row: 5, horizontal: true, length: 1 },
  { column: 2, row: 6, horizontal: true, length: 1 },

  // Horizontal lines starting in 4th column
  { column: 3, row: 3, horizontal: true, length: 1 },
  { column: 3, row: 8, horizontal: true, length: 3 },

  // Horizontal lines starting in 5th column
  { column: 4, row: 6, horizontal: true, length: 1 },

  // Horizontal lines starting in 6th column
  { column: 5, row: 2, horizontal: true, length: 2 },
  { column: 5, row: 7, horizontal: true, length: 1 },

  // Horizontal lines starting in 7th column
  { column: 6, row: 1, horizontal: true, length: 1 },
  { column: 6, row: 6, horizontal: true, length: 2 },

  // Horizontal lines starting in 8th column
  { column: 7, row: 3, horizontal: true, length: 2 },
  { column: 7, row: 7, horizontal: true, length: 2 },

  // Horizontal lines starting in 9th column
  { column: 8, row: 1, horizontal: true, length: 1 },
  { column: 8, row: 2, horizontal: true, length: 1 },
  { column: 8, row: 3, horizontal: true, length: 1 },
  { column: 8, row: 4, horizontal: true, length: 2 },
  { column: 8, row: 8, horizontal: true, length: 2 },

  // Vertical lines after the 1st column
  { column: 1, row: 1, horizontal: false, length: 2 },
  { column: 1, row: 4, horizontal: false, length: 2 },

  // Vertical lines after the 2nd column
  { column: 2, row: 2, horizontal: false, length: 2 },
  { column: 2, row: 5, horizontal: false, length: 1 },
  { column: 2, row: 7, horizontal: false, length: 2 },

  // Vertical lines after the 3rd column
  { column: 3, row: 0, horizontal: false, length: 1 },
  { column: 3, row: 4, horizontal: false, length: 1 },
  { column: 3, row: 6, horizontal: false, length: 2 },

  // Vertical lines after the 4th column
  { column: 4, row: 1, horizontal: false, length: 2 },
  { column: 4, row: 6, horizontal: false, length: 1 },

  // Vertical lines after the 5th column
  { column: 5, row: 0, horizontal: false, length: 2 },
  { column: 5, row: 6, horizontal: false, length: 1 },
  { column: 5, row: 8, horizontal: false, length: 1 },

  // Vertical lines after the 6th column
  { column: 6, row: 4, horizontal: false, length: 1 },
  { column: 6, row: 6, horizontal: false, length: 1 },

  // Vertical lines after the 7th column
  { column: 7, row: 1, horizontal: false, length: 4 },
  { column: 7, row: 7, horizontal: false, length: 2 },

  // Vertical lines after the 8th column
  { column: 8, row: 2, horizontal: false, length: 1 },
  { column: 8, row: 4, horizontal: false, length: 2 },

  // Vertical lines after the 9th column
  { column: 9, row: 1, horizontal: false, length: 1 },
  { column: 9, row: 5, horizontal: false, length: 2 }
].map((wall) => ({
  x: wall.column * (pathW + wallW),
  y: wall.row * (pathW + wallW),
  horizontal: wall.horizontal,
  length: wall.length * (pathW + wallW)
}));

// Draw walls
walls.forEach(({ x, y, horizontal, length }) => {
  const wall = document.createElement("div");
  wall.setAttribute("class", "wall");
  wall.style.cssText = `
      left: ${x}px;
      top: ${y}px;
      width: ${wallW}px;
      height: ${length}px;
      transform: rotate(${horizontal ? -90 : 0}deg);
    `;

  mazeElement.appendChild(wall);
});

const holes = [
  { column: 0, row: 5 },
  { column: 2, row: 0 },
  { column: 2, row: 4 },
  { column: 4, row: 6 },
  { column: 6, row: 2 },
  { column: 6, row: 8 },
  { column: 8, row: 1 },
  { column: 8, row: 2 }
].map((hole) => ({
  x: hole.column * (wallW + pathW) + (wallW / 2 + pathW / 2),
  y: hole.row * (wallW + pathW) + (wallW / 2 + pathW / 2)
}));

joystickHeadElement.addEventListener("mousedown", function (event) {
  if (!gameInProgress) {
    mouseStartX = event.clientX;
    mouseStartY = event.clientY;
    gameInProgress = true;
    window.requestAnimationFrame(main);
    noteElement.style.opacity = 0;
    joystickHeadElement.style.cssText = `
        animation: none;
        cursor: grabbing;
      `;
  }
});

window.addEventListener("mousemove", function (event) {
  if (gameInProgress) {
    const mouseDeltaX = -Math.minmax(mouseStartX - event.clientX, 15);
    const mouseDeltaY = -Math.minmax(mouseStartY - event.clientY, 15);

    joystickHeadElement.style.cssText = `
        left: ${mouseDeltaX}px;
        top: ${mouseDeltaY}px;
        animation: none;
        cursor: grabbing;
      `;

    const rotationY = mouseDeltaX * 0.8; // Max rotation = 12
    const rotationX = mouseDeltaY * 0.8;

    mazeElement.style.cssText = `
        transform: rotateY(${rotationY}deg) rotateX(${-rotationX}deg)
      `;

    const gravity = 2;
    const friction = 0.01; // Coefficients of friction

    accelerationX = gravity * Math.sin((rotationY / 180) * Math.PI);
    accelerationY = gravity * Math.sin((rotationX / 180) * Math.PI);
    frictionX = gravity * Math.cos((rotationY / 180) * Math.PI) * friction;
    frictionY = gravity * Math.cos((rotationX / 180) * Math.PI) * friction;
  }
});

window.addEventListener("keydown", function (event) {
  // If not an arrow key or space or H was pressed then return
  if (![" ", "H", "h", "E", "e"].includes(event.key)) return;

  // If an arrow key was pressed then first prevent default
  event.preventDefault();

  // If space was pressed restart the game
  if (event.key == " ") {
    resetGame();
    return;
  }

  // Set Hard mode
  if (event.key == "H" || event.key == "h") {
    hardMode = true;
    resetGame();
    return;
  }

  // Set Easy mode
  if (event.key == "E" || event.key == "e") {
    hardMode = false;
    resetGame();
    return;
  }
});

function resetGame() {
  previousTimestamp = undefined;
  gameInProgress = false;
  mouseStartX = undefined;
  mouseStartY = undefined;
  accelerationX = undefined;
  accelerationY = undefined;
  frictionX = undefined;
  frictionY = undefined;

  mazeElement.style.cssText = `
      transform: rotateY(0deg) rotateX(0deg)
    `;

  joystickHeadElement.style.cssText = `
      left: 0;
      top: 0;
      animation: glow;
      cursor: grab;
    `;

  if (hardMode) {
    noteElement.innerHTML = `Click the joystick to start!
        <p>Hard mode, Avoid black holes. Back to easy mode? Press E</p>`;
  } else {
    noteElement.innerHTML = `Click the joystick to start!
        <p>Move every ball to the center. Ready for hard mode? Press H</p>`;
  }
  noteElement.style.opacity = 1;

  balls = [
    { column: 0, row: 0 },
    { column: 9, row: 0 },
    { column: 0, row: 8 },
    { column: 9, row: 8 }
  ].map((ball) => ({
    x: ball.column * (wallW + pathW) + (wallW / 2 + pathW / 2),
    y: ball.row * (wallW + pathW) + (wallW / 2 + pathW / 2),
    velocityX: 0,
    velocityY: 0
  }));

  if (ballElements.length) {
    balls.forEach(({ x, y }, index) => {
      ballElements[index].style.cssText = `left: ${x}px; top: ${y}px; `;
    });
  }

  // Remove previous hole elements
  holeElements.forEach((holeElement) => {
    mazeElement.removeChild(holeElement);
  });
  holeElements = [];

  // Reset hole elements if hard mode
  if (hardMode) {
    holes.forEach(({ x, y }) => {
      const ball = document.createElement("div");
      ball.setAttribute("class", "black-hole");
      ball.style.cssText = `left: ${x}px; top: ${y}px; `;

      mazeElement.appendChild(ball);
      holeElements.push(ball);
    });
  }
}

function main(timestamp) {
  // It is possible to reset the game mid-game. This case the look should stop
  if (!gameInProgress) return;

  if (previousTimestamp === undefined) {
    previousTimestamp = timestamp;
    window.requestAnimationFrame(main);
    return;
  }

  const maxVelocity = 1.5;

  // Time passed since last cycle divided by 16
  // This function gets called every 16 ms on average so dividing by 16 will result in 1
  const timeElapsed = (timestamp - previousTimestamp) / 16;

  try {
    // If mouse didn't move yet don't do anything
    if (accelerationX != undefined && accelerationY != undefined) {
      const velocityChangeX = accelerationX * timeElapsed;
      const velocityChangeY = accelerationY * timeElapsed;
      const frictionDeltaX = frictionX * timeElapsed;
      const frictionDeltaY = frictionY * timeElapsed;

      balls.forEach((ball) => {
        if (velocityChangeX == 0) {
          // No rotation, the plane is flat
          // On flat surface friction can only slow down, but not reverse movement
          ball.velocityX = slow(ball.velocityX, frictionDeltaX);
        } else {
          ball.velocityX = ball.velocityX + velocityChangeX;
          ball.velocityX = Math.max(Math.min(ball.velocityX, 1.5), -1.5);
          ball.velocityX =
            ball.velocityX - Math.sign(velocityChangeX) * frictionDeltaX;
          ball.velocityX = Math.minmax(ball.velocityX, maxVelocity);
        }

        if (velocityChangeY == 0) {
          // No rotation, the plane is flat
          // On flat surface friction can only slow down, but not reverse movement
          ball.velocityY = slow(ball.velocityY, frictionDeltaY);
        } else {
          ball.velocityY = ball.velocityY + velocityChangeY;
          ball.velocityY =
            ball.velocityY - Math.sign(velocityChangeY) * frictionDeltaY;
          ball.velocityY = Math.minmax(ball.velocityY, maxVelocity);
        }

        // Preliminary next ball position, only becomes true if no hit occurs
        // Used only for hit testing, does not mean that the ball will reach this position
        ball.nextX = ball.x + ball.velocityX;
        ball.nextY = ball.y + ball.velocityY;

        if (debugMode) console.log("tick", ball);

        walls.forEach((wall, wi) => {
          if (wall.horizontal) {
            // Horizontal wall

            if (
              ball.nextY + ballSize / 2 >= wall.y - wallW / 2 &&
              ball.nextY - ballSize / 2 <= wall.y + wallW / 2
            ) {
              // Ball got within the strip of the wall
              // (not necessarily hit it, could be before or after)

              const wallStart = {
                x: wall.x,
                y: wall.y
              };
              const wallEnd = {
                x: wall.x + wall.length,
                y: wall.y
              };

              if (
                ball.nextX + ballSize / 2 >= wallStart.x - wallW / 2 &&
                ball.nextX < wallStart.x
              ) {
                // Ball might hit the left cap of a horizontal wall
                const distance = distance2D(wallStart, {
                  x: ball.nextX,
                  y: ball.nextY
                });
                if (distance < ballSize / 2 + wallW / 2) {
                  if (debugMode && wi > 4)
                    console.warn("too close h head", distance, ball);

                  // Ball hits the left cap of a horizontal wall
                  const closest = closestItCanBe(wallStart, {
                    x: ball.nextX,
                    y: ball.nextY
                  });
                  const rolled = rollAroundCap(wallStart, {
                    x: closest.x,
                    y: closest.y,
                    velocityX: ball.velocityX,
                    velocityY: ball.velocityY
                  });

                  Object.assign(ball, rolled);
                }
              }

              if (
                ball.nextX - ballSize / 2 <= wallEnd.x + wallW / 2 &&
                ball.nextX > wallEnd.x
              ) {
                // Ball might hit the right cap of a horizontal wall
                const distance = distance2D(wallEnd, {
                  x: ball.nextX,
                  y: ball.nextY
                });
                if (distance < ballSize / 2 + wallW / 2) {
                  if (debugMode && wi > 4)
                    console.warn("too close h tail", distance, ball);

                  // Ball hits the right cap of a horizontal wall
                  const closest = closestItCanBe(wallEnd, {
                    x: ball.nextX,
                    y: ball.nextY
                  });
                  const rolled = rollAroundCap(wallEnd, {
                    x: closest.x,
                    y: closest.y,
                    velocityX: ball.velocityX,
                    velocityY: ball.velocityY
                  });

                  Object.assign(ball, rolled);
                }
              }

              if (ball.nextX >= wallStart.x && ball.nextX <= wallEnd.x) {
                // The ball got inside the main body of the wall
                if (ball.nextY < wall.y) {
                  // Hit horizontal wall from top
                  ball.nextY = wall.y - wallW / 2 - ballSize / 2;
                } else {
                  // Hit horizontal wall from bottom
                  ball.nextY = wall.y + wallW / 2 + ballSize / 2;
                }
                ball.y = ball.nextY;
                ball.velocityY = -ball.velocityY / 3;

                if (debugMode && wi > 4)
                  console.error("crossing h line, HIT", ball);
              }
            }
          } else {
            // Vertical wall

            if (
              ball.nextX + ballSize / 2 >= wall.x - wallW / 2 &&
              ball.nextX - ballSize / 2 <= wall.x + wallW / 2
            ) {
              // Ball got within the strip of the wall
              // (not necessarily hit it, could be before or after)

              const wallStart = {
                x: wall.x,
                y: wall.y
              };
              const wallEnd = {
                x: wall.x,
                y: wall.y + wall.length
              };

              if (
                ball.nextY + ballSize / 2 >= wallStart.y - wallW / 2 &&
                ball.nextY < wallStart.y
              ) {
                // Ball might hit the top cap of a horizontal wall
                const distance = distance2D(wallStart, {
                  x: ball.nextX,
                  y: ball.nextY
                });
                if (distance < ballSize / 2 + wallW / 2) {
                  if (debugMode && wi > 4)
                    console.warn("too close v head", distance, ball);

                  // Ball hits the left cap of a horizontal wall
                  const closest = closestItCanBe(wallStart, {
                    x: ball.nextX,
                    y: ball.nextY
                  });
                  const rolled = rollAroundCap(wallStart, {
                    x: closest.x,
                    y: closest.y,
                    velocityX: ball.velocityX,
                    velocityY: ball.velocityY
                  });

                  Object.assign(ball, rolled);
                }
              }

              if (
                ball.nextY - ballSize / 2 <= wallEnd.y + wallW / 2 &&
                ball.nextY > wallEnd.y
              ) {
                // Ball might hit the bottom cap of a horizontal wall
                const distance = distance2D(wallEnd, {
                  x: ball.nextX,
                  y: ball.nextY
                });
                if (distance < ballSize / 2 + wallW / 2) {
                  if (debugMode && wi > 4)
                    console.warn("too close v tail", distance, ball);

                  // Ball hits the right cap of a horizontal wall
                  const closest = closestItCanBe(wallEnd, {
                    x: ball.nextX,
                    y: ball.nextY
                  });
                  const rolled = rollAroundCap(wallEnd, {
                    x: closest.x,
                    y: closest.y,
                    velocityX: ball.velocityX,
                    velocityY: ball.velocityY
                  });

                  Object.assign(ball, rolled);
                }
              }

              if (ball.nextY >= wallStart.y && ball.nextY <= wallEnd.y) {
                // The ball got inside the main body of the wall
                if (ball.nextX < wall.x) {
                  // Hit vertical wall from left
                  ball.nextX = wall.x - wallW / 2 - ballSize / 2;
                } else {
                  // Hit vertical wall from right
                  ball.nextX = wall.x + wallW / 2 + ballSize / 2;
                }
                ball.x = ball.nextX;
                ball.velocityX = -ball.velocityX / 3;

                if (debugMode && wi > 4)
                  console.error("crossing v line, HIT", ball);
              }
            }
          }
        });

        // Detect is a ball fell into a hole
        if (hardMode) {
          holes.forEach((hole, hi) => {
            const distance = distance2D(hole, {
              x: ball.nextX,
              y: ball.nextY
            });

            if (distance <= holeSize / 2) {
              // The ball fell into a hole
              holeElements[hi].style.backgroundColor = "red";
              throw Error("The ball fell into a hole");
            }
          });
        }

        // Adjust ball metadata
        ball.x = ball.x + ball.velocityX;
        ball.y = ball.y + ball.velocityY;
      });

      // Move balls to their new position on the UI
      balls.forEach(({ x, y }, index) => {
        ballElements[index].style.cssText = `left: ${x}px; top: ${y}px; `;
      });
    }

    // Win detection
    if (
      balls.every(
        (ball) => distance2D(ball, { x: 350 / 2, y: 315 / 2 }) < 65 / 2
      )
    ) {
      noteElement.innerHTML = `Congrats, you did it!
        ${!hardMode ? "<p>Press H for hard mode</p>" : ""}
        <p>
          Follow me
          <a href="https://twitter.com/HunorBorbely" , target="_top"
            >@HunorBorbely</a
          >
        </p>`;
      noteElement.style.opacity = 1;
      gameInProgress = false;
    } else {
      previousTimestamp = timestamp;
      window.requestAnimationFrame(main);
    }
  } catch (error) {
    if (error.message == "The ball fell into a hole") {
      noteElement.innerHTML = `A ball fell into a black hole! Press space to reset the game.
        <p>
          Back to easy? Press E
        </p>`;
      noteElement.style.opacity = 1;
      gameInProgress = false;
    } else throw error;
  }
}