@import url(https://fonts.googleapis.com/css?family=Lato);

body {
  display: flex;
  height: 100vh;
  justify-content: center;
  align-items: center;
  background: #ECEFFC;
}

@function sample($list) {
  @return nth($list, random(length($list)));
}

@function bubbles($color, $count: 16) {
  $bubbles: ();
  // define your own bubbles here!
  $bubble-types: (
    radial-gradient(circle, $color 20%, transparent 20%),
    radial-gradient(circle, transparent 20%, $color 20%, transparent 30%)
  );
  @for $i from 1 through $count {
    $bubbles: append($bubbles, sample($bubble-types), comma);
  }
  @return $bubbles;
}

@function random_range($min, $max) {
  $rand: random();
  $random_range: $min + floor($rand * (($max - $min) + 1));
  @return $random_range;
}

@function random_sizes($count: 16) {
  $sizes: ();
  @for $i from 1 through $count {
    $sizes: append(
      $sizes,
      (random_range(10, 20) * 1%) (random_range(10, 20) * 1%),
      comma
    );
  }
  @return $sizes;
}

.btn {
  --hue: 190;
  --btn-bg-color: hsl(var(--hue), 100%, 50%);
  --btn-bg-color-darker: hsl(var(--hue), 100%, 45%);
  position: relative;
  padding: 0.75rem 1.5rem;
  margin: 1rem;
  font-size: 1rem;
  font-family: Lato, sans-serif;
  line-height: 1.5;
  color: white;
  text-decoration: none;
  background-color: var(--btn-bg-color);
  border: 1px solid var(--btn-bg-color);
  border-radius: 4px;
  box-shadow:
  0 0.1px 0.7px rgba(233, 30, 99, 0.141),
  0 0.1px 1.7px rgba(233, 30, 99, 0.202),
  0 0.3px 3.1px rgba(233, 30, 99, 0.25),
  0 0.4px 5.6px rgba(233, 30, 99, 0.298),
  0 0.8px 10.4px rgba(233, 30, 99, 0.359),
  0 2px 25px rgba(233, 30, 99, 0.5)
;
  outline: transparent;
  overflow: hidden;
  cursor: pointer;
  user-select: none;
  white-space: nowrap;
  transition: 0.25s;

  &-pink {
    --hue: 330;
  }

  &-bubbles {
    overflow: visible;
    transition: transform ease-in 0.1s, background-color ease-in 0.1s,
      box-shadow ease-in 0.25s;

    &::before {
      position: absolute;
      content: "";
      left: -2em;
      right: -2em;
      top: -2em;
      bottom: -2em;
      transition: ease-in-out 0.5s;
      background-repeat: no-repeat;
      background-image: bubbles(var(--btn-bg-color));
      background-size: random_sizes();
      background-position: 18% 40%, 20% 31%, 30% 30%, 40% 30%, 50% 30%, 57% 30%,
        65% 30%, 80% 32%, 15% 60%, 83% 60%, 18% 70%, 25% 70%, 41% 70%, 50% 70%,
        64% 70%, 80% 71%;
      animation: bubbles ease-in-out 0.75s forwards;
    }

    &:active {
      transform: scale(0.95);
      background: var(--btn-bg-color-darker);

      &::before {
        // when the clicked mouse is up, trigger the animation.
        animation: none;
        background-size: 0;
      }
    }
  }
}

@keyframes bubbles {
  0% {
    background-position: 18% 40%, 20% 31%, 30% 30%, 40% 30%, 50% 30%, 57% 30%,
      65% 30%, 80% 32%, 15% 60%, 83% 60%, 18% 70%, 25% 70%, 41% 70%, 50% 70%,
      64% 70%, 80% 71%;
  }

  50% {
    background-position: 10% 44%, 0% 20%, 15% 5%, 30% 0%, 42% 0%, 62% -2%,
      75% 0%, 95% -2%, 0% 80%, 95% 55%, 7% 100%, 24% 100%, 41% 100%, 55% 95%,
      68% 96%, 95% 100%;
  }

  100% {
    background-position: 5% 44%, -5% 20%, 7% 5%, 23% 0%, 37% 0, 58% -2%, 80% 0%,
      100% -2%, -5% 80%, 100% 55%, 2% 100%, 23% 100%, 42% 100%, 60% 95%, 70% 96%,
      100% 100%;
    background-size: 0% 0%;
  }
}