*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  background: #1a1a2e;
  padding: 16px;
  -webkit-tap-highlight-color: transparent;
}

/* ---- Calculator container ---- */
#app {
  width: clamp(260px, 90vw, 360px);
  background: #16213e;
  border-radius: 20px;
  padding: clamp(14px, 4vw, 24px);
  box-shadow:
    0 8px 32px rgba(0, 0, 0, 0.35),
    0 2px 8px rgba(0, 0, 0, 0.2);
}

/* ---- Display ---- */
#display {
  background: #0f3460;
  border-radius: 14px;
  padding: clamp(12px, 3vw, 20px);
  margin-bottom: clamp(12px, 3vw, 18px);
  min-height: 90px;
  text-align: right;
  color: #e0e0e0;
  overflow: hidden;
  position: relative;
}

#expression {
  font-size: clamp(12px, 3.5vw, 15px);
  color: #8fa8c8;
  min-height: 22px;
  word-break: break-all;
  line-height: 1.4;
  transition: color 0.2s;
}

#result {
  font-size: clamp(26px, 8vw, 36px);
  font-weight: 300;
  min-height: 44px;
  word-break: break-all;
  line-height: 1.2;
  letter-spacing: -0.02em;
  transition: color 0.15s;
  display: flex;
  align-items: flex-end;
  justify-content: flex-end;
}

/* ---- Button grid ---- */
#buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: clamp(6px, 1.8vw, 10px);
}

/* ---- Base button ---- */
button {
  font-size: clamp(17px, 5vw, 22px);
  padding: clamp(12px, 3.5vw, 18px) 0;
  border: none;
  border-radius: 12px;
  cursor: pointer;
  background: #1a1a40;
  color: #e0e0e0;
  font-weight: 400;
  transition:
    background 0.15s ease,
    transform 0.1s ease,
    box-shadow 0.15s ease;
  outline: none;
  user-select: none;
  -webkit-user-select: none;
  position: relative;
  overflow: hidden;
}

/* Ripple-like highlight on tap */
button::after {
  content: "";
  position: absolute;
  inset: 0;
  background: radial-gradient(circle at center, rgba(255,255,255,0.12) 0%, transparent 70%);
  opacity: 0;
  transition: opacity 0.2s;
  pointer-events: none;
}

button:active::after {
  opacity: 1;
}

/* Hover */
button:hover {
  background: #262655;
}

/* Active press */
button:active {
  transform: scale(0.94);
  background: #2e2e65;
}

/* Keyboard focus ring */
button:focus-visible {
  box-shadow: 0 0 0 3px rgba(83, 52, 131, 0.6);
  z-index: 1;
}

/* ---- Function keys (C, +/-, backspace) ---- */
[data-action="clear"],
[data-action="sign"],
[data-action="backspace"] {
  background: #0f3460;
  color: #8fbceb;
  font-weight: 500;
}

[data-action="clear"]:hover,
[data-action="sign"]:hover,
[data-action="backspace"]:hover {
  background: #154178;
}

[data-action="clear"]:active,
[data-action="sign"]:active,
[data-action="backspace"]:active {
  background: #1b4f8f;
  transform: scale(0.94);
}

/* ---- Operator keys ---- */
[data-action="operator"] {
  background: #533483;
  color: #d9c4f0;
  font-weight: 500;
}

[data-action="operator"]:hover {
  background: #6544a3;
}

[data-action="operator"]:active {
  background: #7654b3;
  transform: scale(0.94);
}

/* ---- Equals key ---- */
[data-action="equals"] {
  background: linear-gradient(135deg, #e94560, #d63251);
  color: #fff;
  font-weight: 600;
  box-shadow: 0 3px 12px rgba(233, 69, 96, 0.3);
}

[data-action="equals"]:hover {
  background: linear-gradient(135deg, #f05575, #e04060);
  box-shadow: 0 4px 16px rgba(233, 69, 96, 0.4);
}

[data-action="equals"]:active {
  background: linear-gradient(135deg, #ff6585, #f05070);
  transform: scale(0.94);
  box-shadow: 0 2px 8px rgba(233, 69, 96, 0.3);
}

/* ---- Zero button span ---- */
#btn-zero {
  grid-column: span 2;
}

/* ---- Responsive: narrow screens ---- */
@media (max-width: 320px) {
  #app {
    border-radius: 14px;
  }

  button {
    border-radius: 10px;
    padding: 10px 0;
  }

  #display {
    border-radius: 10px;
    min-height: 76px;
  }
}

/* ---- Responsive: wider desktop ---- */
@media (min-width: 600px) {
  #app {
    width: 360px;
  }

  button {
    padding: 20px 0;
    font-size: 22px;
  }

  #result {
    font-size: 38px;
  }

  #expression {
    font-size: 16px;
  }
}

/* ---- Reduced motion preference ---- */
@media (prefers-reduced-motion: reduce) {
  button {
    transition: none;
  }

  button::after {
    transition: none;
  }

  #expression, #result {
    transition: none;
  }
}