/* ============================================
   Design System — Minimal Dark Calculator
   ============================================ */

:root {
    /* --- palette --- */
    --color-bg:        #0a0a0a;
    --color-surface:   #141414;
    --color-display:   #0d0d0d;
    --color-key-num:   #2a2a2a;
    --color-key-num-h: #3d3d3d;
    --color-key-op:    #f59e0b;
    --color-key-op-h:  #fbbf24;
    --color-key-func:  #1f1f1f;
    --color-key-func-h:#333333;
    --color-key-eq:    #f59e0b;
    --color-key-eq-h:  #fbbf24;
    --color-text:      #fafafa;
    --color-text-dim:  #6b6b6b;
    --color-text-info: #404040;

    /* --- spacing --- */
    --space-xs: 4px;
    --space-sm: 8px;
    --space-md: 12px;
    --space-lg: 20px;
    --space-xl: 24px;

    /* --- radius --- */
    --radius-sm: 12px;
    --radius-md: 18px;
    --radius-full: 999px;

    /* --- typography --- */
    --font-mono: 'Roboto Mono', 'SF Mono', 'Consolas', monospace;
    --font-xs:  clamp(10px, 2.6vw, 11px);
    --font-sm:  clamp(12px, 3.2vw, 14px);
    --font-md:  clamp(18px, 4.8vw, 22px);
    --font-lg:  clamp(22px, 5.6vw, 26px);
    --font-xl:  clamp(28px, 7.5vw, 44px);

    /* --- motion --- */
    --ease-out: cubic-bezier(0.16, 1, 0.3, 1);
    --duration-fast: 120ms;
    --duration-normal: 200ms;
}

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

body {
    font-family: var(--font-mono);
    background: var(--color-bg);
    min-height: 100vh;
    min-height: 100dvh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: var(--space-md);
    -webkit-tap-highlight-color: transparent;
    -webkit-user-select: none;
    user-select: none;
}

/* --- layout --- */
.calculator {
    width: 100%;
    max-width: 360px;
    display: flex;
    flex-direction: column;
    gap: var(--space-md);
}

/* --- display --- */
.display {
    background: var(--color-display);
    border: 1px solid rgba(255, 255, 255, 0.04);
    border-radius: var(--radius-md);
    padding: var(--space-lg) var(--space-xl);
    min-height: 110px;
    display: flex;
    flex-direction: column;
    justify-content: flex-end;
    align-items: flex-end;
    overflow: hidden;
}

.expression {
    color: var(--color-text-dim);
    font-family: var(--font-mono);
    font-size: var(--font-sm);
    font-weight: 300;
    word-break: break-all;
    text-align: right;
    width: 100%;
    line-height: 1.5;
    min-height: 1.5em;
    margin-bottom: var(--space-xs);
    letter-spacing: -0.01em;
}

.result {
    color: var(--color-text);
    font-family: var(--font-mono);
    font-size: var(--font-xl);
    font-weight: 200;
    word-break: break-all;
    text-align: right;
    width: 100%;
    line-height: 1.15;
    transition: font-size var(--duration-normal) var(--ease-out);
    letter-spacing: -0.02em;
}

.result.small {
    font-size: var(--font-lg);
}

/* --- keypad grid --- */
.keypad {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: var(--space-sm);
}

/* --- button base --- */
.btn {
    aspect-ratio: 1;
    border: none;
    border-radius: var(--radius-full);
    font-family: var(--font-mono);
    font-size: var(--font-md);
    font-weight: 350;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: background var(--duration-fast) var(--ease-out),
                transform var(--duration-fast) var(--ease-out);
    -webkit-tap-highlight-color: transparent;
    outline: none;
    line-height: 1;
    letter-spacing: -0.01em;
}

.btn:active {
    transform: scale(0.94);
}

/* --- numeric keys --- */
.btn.num {
    background: var(--color-key-num);
    color: var(--color-text);
}
.btn.num:hover  { background: var(--color-key-num-h); }
.btn.num:active { background: #555; }

.btn.num.zero {
    grid-column: span 2;
    aspect-ratio: auto;
    border-radius: var(--radius-full);
    padding-left: 26px;
    justify-content: flex-start;
}

/* --- operator keys --- */
.btn.op {
    background: var(--color-key-op);
    color: #fff;
    font-size: var(--font-lg);
    font-weight: 450;
}
.btn.op:hover  { background: var(--color-key-op-h); }
.btn.op.active { background: #fff; color: var(--color-key-op); }

/* --- function keys --- */
.btn.func {
    background: var(--color-key-func);
    color: var(--color-text);
    font-size: var(--font-sm);
}
.btn.func:hover  { background: var(--color-key-func-h); }
.btn.func:active { background: #444; }

/* --- equals key --- */
.btn.eq {
    background: var(--color-key-eq);
    color: #fff;
    font-size: var(--font-lg);
    font-weight: 450;
}
.btn.eq:hover  { background: var(--color-key-eq-h); }
.btn.eq:active { background: #fcd34d; }

/* --- footer --- */
.footer {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: var(--space-sm);
}

.info {
    color: var(--color-text-info);
    font-family: var(--font-mono);
    font-size: var(--font-xs);
    font-weight: 300;
    letter-spacing: 0.04em;
    text-transform: uppercase;
}

.mute-toggle {
    background: none;
    border: none;
    font-size: 14px;
    cursor: pointer;
    opacity: 0.3;
    transition: opacity var(--duration-fast) var(--ease-out);
    padding: 2px;
    line-height: 1;
}
.mute-toggle:hover { opacity: 0.7; }
.mute-toggle.muted  { opacity: 0.15; }

/* --- responsive --- */
@media (max-width: 380px) {
    .keypad { gap: 6px; }
    .display {
        padding: 16px 18px;
        min-height: 96px;
        border-radius: var(--radius-sm);
    }
    .btn.num.zero { padding-left: 22px; }
}

@media (min-width: 400px) {
    .calculator { max-width: 380px; }
}