/* =========================================
   1. GLOBAL RESET & VARIABLES
   ========================================= */
:root {
    --primary: #2563EB;
    --bg: #F3F4F6;
    --card: #FFFFFF;
    --text: #1F2937;
    --border: #D1D5DB;
    
    /* Calculator Theme */
    --calc-bg: #0f172a;      /* Dark Slate */
    --calc-keys-bg: #1e293b; 
    --btn-num: #334155;      
    --btn-op: #f59e0b;       
    --btn-func: #475569;     
    --btn-sci: #64748b;      
    --text-white: #F9FAFB;
}

/* Universal Box Sizing */
*, *::before, *::after {
    box-sizing: border-box;
    -webkit-tap-highlight-color: transparent;
}

html, body {
    margin: 0;
    padding: 0;
    width: 100%;
    height: 100%; /* Ensure full height for centering */
}

body {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    background-color: var(--bg);
    color: var(--text);
    /* Flex Layout for Smart Centering */
    display: flex;
    flex-direction: column;
    overflow-x: hidden;
}

/* =========================================
   2. MAIN LAYOUT CONTAINER
   ========================================= */
.main-wrapper {
    width: 100%;
    max-width: 600px;
    padding: 20px;
    /* margin: auto centers it vertically/horizontally, 
       but allows scroll if screen is too small */
    margin: auto; 
    padding-bottom: 120px; /* Space for FAB */
    display: flex;
    flex-direction: column;
    gap: 1.5rem;
}

/* =========================================
   3. CONVERTER CARD
   ========================================= */
.converter-card {
    background: var(--card);
    padding: 2.5rem;
    border-radius: 20px;
    box-shadow: 0 10px 25px -5px rgba(0, 0, 0, 0.1), 0 8px 10px -6px rgba(0, 0, 0, 0.1);
    width: 100%;
}

h2 {
    margin: 0 0 1.5rem 0;
    text-align: center;
    color: var(--primary);
    font-size: 1.75rem;
    font-weight: 800;
    letter-spacing: -0.025em;
}

.form-group {
    margin-bottom: 1.5rem;
    width: 100%;
}

label {
    display: block;
    margin-bottom: 0.5rem;
    font-size: 0.75rem;
    font-weight: 700;
    color: #6B7280;
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

/* Inputs & Selects */
select, input {
    width: 100%;
    padding: 14px;
    border: 2px solid var(--border);
    border-radius: 12px;
    font-size: 16px; /* Prevents iOS zoom */
    background-color: #fff;
    transition: all 0.2s ease;
    appearance: none;
    color: var(--text);
}

/* Custom Arrow */
select {
    background-image: url("data:image/svg+xml,%3csvg xmlns='http://www.w3.org/2000/svg' fill='none' viewBox='0 0 20 20'%3e%3cpath stroke='%236b7280' stroke-linecap='round' stroke-linejoin='round' stroke-width='1.5' d='M6 8l4 4 4-4'/%3e%3c/svg%3e");
    background-position: right 0.75rem center;
    background-repeat: no-repeat;
    background-size: 1.5em 1.5em;
    padding-right: 2.5rem;
    cursor: pointer;
}

select:focus, input:focus {
    outline: none;
    border-color: var(--primary);
    box-shadow: 0 0 0 4px rgba(37, 99, 235, 0.1);
}

.conversion-row {
    display: flex;
    gap: 1rem;
    align-items: center;
}

.input-wrapper {
    flex: 1;
    min-width: 0;
}

.input-wrapper select {
    margin-top: 10px;
    background-color: #F9FAFB;
}

.swap-icon {
    font-size: 1.5rem;
    color: var(--primary);
    cursor: pointer;
    width: 48px;
    height: 48px;
    background: #EFF6FF;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    flex-shrink: 0;
    transition: transform 0.3s ease, background-color 0.2s;
}

.swap-icon:hover {
    background-color: #DBEAFE;
    transform: rotate(180deg);
}

.swap-icon:active {
    transform: scale(0.9);
}

/* =========================================
   4. FEATURE LIST
   ========================================= */
.features-list {
    background: #fff;
    padding: 1.5rem;
    border-radius: 16px;
    border: 1px solid var(--border);
    font-size: 0.95rem;
    color: #4B5563;
}

.features-list h3 {
    margin-top: 0;
    color: var(--text);
    font-size: 1.1rem;
    margin-bottom: 0.75rem;
}

.features-list ul {
    padding-left: 1.25rem;
    margin-bottom: 0;
}

.features-list li {
    margin-bottom: 0.5rem;
    line-height: 1.5;
}

/* =========================================
   5. FLOATING ACTION BUTTON (FAB)
   ========================================= */
.calc-trigger {
    position: fixed;
    bottom: 30px;
    right: 30px;
    background: var(--calc-bg);
    color: white;
    border: none;
    padding: 16px 28px;
    border-radius: 50px;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    box-shadow: 0 10px 25px -5px rgba(0, 0, 0, 0.4);
    z-index: 2000;
    display: flex;
    align-items: center;
    gap: 10px;
    transition: transform 0.2s ease, box-shadow 0.2s ease;
}

.calc-trigger:hover {
    transform: translateY(-2px);
    box-shadow: 0 15px 30px -5px rgba(0, 0, 0, 0.5);
}

.calc-trigger:active {
    transform: translateY(1px);
}

/* =========================================
   6. SCIENTIFIC CALCULATOR DRAWER
   ========================================= */
.backdrop {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.5);
    backdrop-filter: blur(4px); /* Glassmorphism effect */
    z-index: 2001;
    opacity: 0;
    pointer-events: none;
    transition: opacity 0.3s ease;
}

.backdrop.open {
    opacity: 1;
    pointer-events: auto;
}

.calc-drawer {
    position: fixed;
    bottom: 0;
    z-index: 2002;
    background: var(--calc-bg);
    box-shadow: 0 -10px 40px rgba(0,0,0,0.5);
    transition: transform 0.4s cubic-bezier(0.16, 1, 0.3, 1); /* Smooth iOS-style spring */
    
    /* Desktop Default: Centered Sheet */
    left: 50%;
    width: 100%;
    max-width: 500px; /* Prevent full width on Desktop */
    border-top-left-radius: 24px;
    border-top-right-radius: 24px;
    
    /* Hidden State (Off screen bottom + Centered X) */
    transform: translate(-50%, 110%);
    
    /* iPhone Safe Area */
    padding-bottom: max(20px, env(safe-area-inset-bottom));
}

.calc-drawer.open {
    /* Visible State (On screen bottom + Centered X) */
    transform: translate(-50%, 0);
}

.drawer-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 12px 20px;
    border-bottom: 1px solid rgba(255, 255, 255, 0.1);
    color: var(--text-white);
}

.close-btn {
    background: rgba(255,255,255,0.1);
    border: none;
    color: #94a3b8;
    width: 32px;
    height: 32px;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 1.5rem;
    line-height: 1;
    cursor: pointer;
    transition: background 0.2s;
}

.close-btn:hover {
    background: rgba(255,255,255,0.2);
    color: white;
}

.calculator {
    padding: 10px;
    width: 60%;
    margin: 0 auto;
}

#calcDisplay {
    width: 100%;
    background: transparent;
    border: none;
    color: var(--text-white);
    text-align: right;
    font-size: 2.75rem;
    padding: 0 4px 20px 4px;
    font-weight: 300;
    font-family: 'Menlo', 'Monaco', 'Courier New', monospace;
    white-space: nowrap;
    overflow-x: auto; /* Scroll long numbers */
}

/* 5-Column Grid Layout */
.calc-keys {
    display: grid;
    grid-template-columns: repeat(5, 1fr); 
    gap: 12px;
}

.calc-keys button {
    aspect-ratio: 1.15; 
    border: none;
    border-radius: 16px;
    background: var(--btn-num);
    color: white;
    font-size: 1.2rem;
    font-weight: 500;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 0;
    box-shadow: 0 2px 0 rgba(0,0,0,0.2);
    transition: all 0.1s;
}

.calc-keys button:active {
    transform: translateY(2px);
    box-shadow: none;
    filter: brightness(1.2);
}

/* Button Variants */
.key-op { background: var(--btn-op) !important; font-weight: 700; color: #fff !important; }
.key-func { background: var(--btn-func) !important; font-size: 1.1rem !important; }
.key-sci { background: var(--btn-sci) !important; font-size: 0.95rem !important; font-weight: 600; }
.key-clear { background: var(--btn-func) !important; color: #fca5a5 !important; }

/* Complex Keys */
.key-equals { 
    grid-row: span 2; 
    aspect-ratio: auto !important; 
    background: #22c55e !important; 
}

.key-zero { 
    grid-column: span 2; 
    aspect-ratio: auto !important; 
}


/* =========================================
   7. RESPONSIVE BREAKPOINTS
   ========================================= */

/* Tablet & Mobile (Below 768px) */
@media (max-width: 768px) {
    .main-wrapper {
        padding: 16px;
        padding-bottom: 100px;
    }
    
    .converter-card {
        padding: 1.5rem;
    }

    .conversion-row {
        flex-direction: column;
    }
    
    .swap-icon {
        transform: rotate(90deg);
        margin: 0;
    }
    
    .swap-icon:hover {
        transform: rotate(270deg); /* Spin on hover for mobile interaction feedback */
    }
    
    /* Center FAB on mobile */
    .calc-trigger {
        left: 50%;
        transform: translateX(-50%);
        right: auto;
        bottom: 24px;
        width: auto;
        white-space: nowrap;
        padding: 14px 24px;
    }
    .calc-trigger:hover { transform: translateX(-50%) translateY(-2px); }
    .calc-trigger:active { transform: translateX(-50%) translateY(1px); }
}

/* Mobile Drawer Behavior (Full Width) */
@media (max-width: 550px) {
    .calc-drawer {
        max-width: 100%; /* Full width on small phones */
        border-radius: 24px 24px 0 0;
    }
    
    .calculator {
        padding: 12px;
    }
    
    .calc-keys {
        gap: 6px;
    }
    
    .calc-keys button {
        border-radius: 10px;
        font-size: 1.1rem;
    }
    
    #calcDisplay {
        font-size: 2.2rem;
    }
}

/* Small Phones (iPhone SE, etc.) */
@media (max-width: 360px) {
    .calc-keys { gap: 6px; }
    .calc-keys button { font-size: 0.9rem; border-radius: 8px; }
    .key-sci { font-size: 0.8rem !important; }
    #calcDisplay { font-size: 2rem; }
}