/* ===== CSS RESET & BASE STYLES ===== */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

:root {
    /* Color Palette */
    --bg-primary: #1a1a2e;
    --bg-secondary: #16213e;
    --bg-calculator: #0f3460;
    --display-bg: #0a2540;
    --text-primary: #ffffff;
    --text-secondary: #e0e0e0;

    /* Button Colors */
    --btn-number: #1e3a5f;
    --btn-number-hover: #2a4a7c;
    --btn-operator: #e94560;
    --btn-operator-hover: #ff5370;
    --btn-function: #533483;
    --btn-function-hover: #6b4296;
    --btn-equals: #16a085;
    --btn-equals-hover: #1abc9c;

    /* Shadows */
    --shadow-sm: 0 2px 4px rgba(0, 0, 0, 0.2);
    --shadow-md: 0 4px 8px rgba(0, 0, 0, 0.3);
    --shadow-lg: 0 8px 16px rgba(0, 0, 0, 0.4);

    /* Transitions */
    --transition: all 0.2s ease;

    /* Border Radius */
    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 16px;
}

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
    background: linear-gradient(135deg, var(--bg-primary) 0%, var(--bg-secondary) 100%);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 20px;
    color: var(--text-primary);
}

/* ===== CALCULATOR CONTAINER ===== */
.calculator-container {
    width: 100%;
    max-width: 400px;
    display: flex;
    justify-content: center;
    align-items: center;
}

.calculator {
    width: 100%;
    background: var(--bg-calculator);
    border-radius: var(--radius-lg);
    padding: 24px;
    box-shadow: var(--shadow-lg);
    backdrop-filter: blur(10px);
}

/* ===== DISPLAY ===== */
.display {
    background: var(--display-bg);
    border-radius: var(--radius-md);
    padding: 20px;
    margin-bottom: 20px;
    min-height: 100px;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    align-items: flex-end;
    word-wrap: break-word;
    word-break: break-all;
    box-shadow: inset 0 2px 8px rgba(0, 0, 0, 0.3);
}

.display-previous {
    color: var(--text-secondary);
    font-size: 1.2rem;
    min-height: 1.5rem;
    opacity: 0.7;
    margin-bottom: 8px;
}

.display-current {
    color: var(--text-primary);
    font-size: 2.5rem;
    font-weight: 600;
    min-height: 2.5rem;
    text-align: right;
    width: 100%;
}

/* ===== BUTTONS GRID ===== */
.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 12px;
}

/* ===== BUTTON BASE STYLES ===== */
.btn {
    background: var(--btn-number);
    border: none;
    border-radius: var(--radius-sm);
    color: var(--text-primary);
    font-size: 1.5rem;
    font-weight: 600;
    padding: 24px;
    cursor: pointer;
    transition: var(--transition);
    box-shadow: var(--shadow-sm);
    user-select: none;
    -webkit-tap-highlight-color: transparent;
}

.btn:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
}

.btn:active {
    transform: translateY(0);
    box-shadow: var(--shadow-sm);
}

.btn:focus-visible {
    outline: 3px solid var(--btn-equals);
    outline-offset: 2px;
}

/* ===== BUTTON VARIANTS ===== */
.btn-number {
    background: var(--btn-number);
}

.btn-number:hover {
    background: var(--btn-number-hover);
}

.btn-operator {
    background: var(--btn-operator);
}

.btn-operator:hover {
    background: var(--btn-operator-hover);
}

.btn-function {
    background: var(--btn-function);
}

.btn-function:hover {
    background: var(--btn-function-hover);
}

.btn-equals {
    background: var(--btn-equals);
    grid-row: span 2;
    font-size: 2rem;
}

.btn-equals:hover {
    background: var(--btn-equals-hover);
}

.btn-zero {
    grid-column: span 2;
}

/* Active state for operators */
.btn-operator.active {
    background: var(--btn-operator-hover);
    box-shadow: inset 0 2px 8px rgba(0, 0, 0, 0.3);
}

/* ===== ERROR STATE ===== */
.display.error .display-current {
    color: var(--btn-operator);
    font-size: 1.8rem;
}

/* ===== RESPONSIVE DESIGN ===== */
@media (max-width: 480px) {
    .calculator {
        padding: 16px;
    }

    .display {
        padding: 16px;
        min-height: 80px;
    }

    .display-previous {
        font-size: 1rem;
    }

    .display-current {
        font-size: 2rem;
    }

    .btn {
        padding: 20px;
        font-size: 1.3rem;
    }

    .btn-equals {
        font-size: 1.8rem;
    }

    .buttons {
        gap: 8px;
    }
}

@media (max-width: 360px) {
    .calculator {
        padding: 12px;
    }

    .display-current {
        font-size: 1.8rem;
    }

    .btn {
        padding: 16px;
        font-size: 1.2rem;
    }

    .btn-equals {
        font-size: 1.6rem;
    }
}

/* ===== ACCESSIBILITY ===== */
@media (prefers-reduced-motion: reduce) {
    * {
        animation-duration: 0.01ms !important;
        animation-iteration-count: 1 !important;
        transition-duration: 0.01ms !important;
    }
}

/* High contrast mode support */
@media (prefers-contrast: high) {
    .btn {
        border: 2px solid var(--text-primary);
    }

    .display {
        border: 2px solid var(--text-primary);
    }
}

/* ===== KEYBOARD SUPPORT INDICATOR ===== */
.keyboard-user .btn:focus {
    outline: 3px solid var(--btn-equals);
    outline-offset: 2px;
}