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

body {
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background: linear-gradient(135deg, #1a1a2e 0%, #16213e 50%, #0f3460 100%);
    font-family: 'Segoe UI', system-ui, -apple-system, sans-serif;
    user-select: none;
    -webkit-user-select: none;
}

/* ===== 计算器主体 ===== */
.calculator {
    width: 320px;
    background: #1c1c1e;
    border-radius: 24px;
    padding: 24px 20px 20px;
    box-shadow:
        0 20px 60px rgba(0, 0, 0, 0.5),
        0 0 0 1px rgba(255, 255, 255, 0.05) inset,
        0 1px 0 rgba(255, 255, 255, 0.1) inset;
}

/* ===== 显示区 ===== */
.display {
    background: #1c1c1e;
    border-radius: 12px;
    padding: 16px 20px;
    margin-bottom: 20px;
    min-height: 100px;
    display: flex;
    flex-direction: column;
    justify-content: flex-end;
    align-items: flex-end;
    overflow: hidden;
}

.display .expression {
    color: #8e8e93;
    font-size: 16px;
    min-height: 24px;
    word-break: break-all;
    text-align: right;
    transition: color 0.2s;
    width: 100%;
}

.display .result {
    color: #ffffff;
    font-size: 48px;
    font-weight: 300;
    letter-spacing: -1px;
    line-height: 1.1;
    word-break: break-all;
    text-align: right;
    width: 100%;
    transition: font-size 0.15s;
}

.display .result.shrink {
    font-size: 32px;
}

/* ===== 按钮网格 ===== */
.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
}

/* ===== 按钮基础样式 ===== */
button {
    height: 60px;
    border: none;
    border-radius: 50%;
    font-size: 22px;
    font-weight: 400;
    cursor: pointer;
    transition: all 0.15s ease;
    outline: none;
    position: relative;
    overflow: hidden;
    -webkit-tap-highlight-color: transparent;
}

/* 按下闪光层 */
button::after {
    content: '';
    position: absolute;
    inset: 0;
    background: radial-gradient(circle at center, rgba(255,255,255,0.3) 0%, transparent 70%);
    opacity: 0;
    transition: opacity 0.3s;
}

button:active::after {
    opacity: 1;
    transition: opacity 0s;
}

button:active {
    transform: scale(0.95);
}

/* ===== 数字按钮 ===== */
.btn-number {
    background: #333333;
    color: #ffffff;
}

.btn-number:hover {
    background: #4a4a4a;
}

/* ===== 运算符按钮 ===== */
.btn-operator {
    background: #ff9f0a;
    color: #ffffff;
    font-size: 24px;
}

.btn-operator:hover {
    background: #ffb340;
}

.btn-operator.active {
    background: #ffffff;
    color: #ff9f0a;
}

/* ===== 功能按钮 ===== */
.btn-function {
    background: #a5a5a5;
    color: #1c1c1e;
    font-size: 18px;
}

.btn-function:hover {
    background: #c7c7c7;
}

/* ===== 零按钮占两格 ===== */
.btn-zero {
    grid-column: span 2;
    border-radius: 30px;
    text-align: left;
    padding-left: 24px;
}

/* ===== 底部文字 ===== */
.footer {
    text-align: center;
    color: #8e8e93;
    margin-top: 24px;
    font-size: 12px;
    letter-spacing: 1px;
    opacity: 0.7;
}

/* ===== 波纹动画 ===== */
@keyframes ripple {
    to {
        transform: scale(4);
        opacity: 0;
    }
}

.ripple-effect {
    position: absolute;
    border-radius: 50%;
    background: rgba(255, 255, 255, 0.3);
    transform: scale(0);
    animation: ripple 0.5s ease-out;
    pointer-events: none;
}

/* ===== 键盘提示 ===== */
.keyboard-hint {
    position: fixed;
    bottom: 20px;
    right: 20px;
    background: rgba(255, 255, 255, 0.1);
    color: #8e8e93;
    padding: 6px 14px;
    border-radius: 20px;
    font-size: 11px;
    backdrop-filter: blur(10px);
    letter-spacing: 0.5px;
    pointer-events: none;
}