Advanced Nesting and Selector Techniques

// Perfect BEM with SCSS nesting
.card {
  padding: 1rem;
  background: white;
  border-radius: 8px;
  
  // BEM Element: .card__header
  &__header {
    padding: 1rem;
    border-bottom: 1px solid #ddd;
    
    // Nested element: .card__header__title
    &__title {
      font-size: 1.5rem;
      font-weight: bold;
      margin: 0;
    }
    
    &__subtitle {
      font-size: 0.875rem;
      color: #666;
    }
  }
  
  // BEM Element: .card__body
  &__body {
    padding: 1rem;
  }
  
  // BEM Element: .card__footer
  &__footer {
    padding: 1rem;
    border-top: 1px solid #ddd;
    text-align: right;
  }
  
  // BEM Modifier: .card--featured
  &--featured {
    border: 2px solid gold;
    box-shadow: 0 4px 8px rgba(0,0,0,0.2);
    
    // Modified element: .card--featured .card__header
    .card__header {
      background: gold;
    }
  }
  
  // BEM Modifier: .card--compact
  &--compact {
    padding: 0.5rem;
    
    .card__header,
    .card__body,
    .card__footer {
      padding: 0.5rem;
    }
  }
  
  // State modifier: .card.is-loading
  &.is-loading {
    opacity: 0.5;
    pointer-events: none;
  }
}

// Alternative: Element modifiers
.button {
  display: inline-block;
  padding: 10px 20px;
  
  // Element: .button__icon
  &__icon {
    margin-right: 8px;
    
    // Element modifier: .button__icon--left
    &--left {
      margin-right: 8px;
      margin-left: 0;
    }
    
    // Element modifier: .button__icon--right
    &--right {
      margin-left: 8px;
      margin-right: 0;
    }
  }
  
  // Modifier: .button--primary
  &--primary {
    background: blue;
    color: white;
  }
  
  // Modifier: .button--large
  &--large {
    padding: 15px 30px;
    font-size: 1.2rem;
  }
}

// Advanced: BEM with state classes
.nav {
  display: flex;
  
  &__item {
    padding: 10px 20px;
    
    // Combined: .nav__item--active
    &--active {
      background: blue;
      color: white;
    }
    
    // State: .nav__item.is-disabled
    &.is-disabled {
      opacity: 0.5;
      pointer-events: none;
    }
    
    // Pseudo-class: .nav__item:hover
    &:hover:not(.is-disabled) {
      background: lightblue;
    }
  }
}

// Practical: BEM mixin generator
@mixin element($name) {
  &__#{$name} {
    @content;
  }
}

@mixin modifier($name) {
  &--#{$name} {
    @content;
  }
}

.menu {
  display: flex;
  
  @include element(item) {
    padding: 10px;
    
    @include modifier(active) {
      background: blue;
      color: white;
    }
  }
}

// Output:
// .menu { display: flex; }
// .menu__item { padding: 10px; }
// .menu__item--active { background: blue; color: white; }

// Nesting depth warning (BAD - too deep)
.header {
  .container {
    .nav {
      .menu {
        .item {
          .link {
            // 6 levels - TOO DEEP!
            color: blue;
          }
        }
      }
    }
  }
}

// Better: Flatten with BEM
.header { }
.header__nav { }
.header__menu { }
.header__menu-item { }
.header__menu-link { color: blue; }

@use 'sass:selector';
@use 'sass:list';

// Dynamic BEM modifier generator
@mixin bem-modifiers($modifiers...) {
  @each $modifier in $modifiers {
    &--#{$modifier} {
      @content ($modifier);
    }
  }
}

.button {
  padding: 10px 20px;
  
  @include bem-modifiers(primary, secondary, danger) using ($mod) {
    @if $mod == primary {
      background: blue;
    } @else if $mod == secondary {
      background: gray;
    } @else if $mod == danger {
      background: red;
    }
  }
}

// Selector nesting with functions
.dynamic-nest {
  $parent: '.container';
  $child: '.item';
  
  // Programmatic nesting
  #{selector.nest($parent, $child)} {
    display: block;
  }
  // Outputs: .container .item { display: block; }
}

// Append pseudo-classes
@mixin hover-focus {
  $selector: &;
  
  #{selector.append($selector, ':hover')},
  #{selector.append($selector, ':focus')} {
    @content;
  }
}

.link {
  color: blue;
  
  @include hover-focus {
    color: darkblue;
    text-decoration: underline;
  }
}

// Generate utility classes dynamically
$sides: (top, right, bottom, left);
$sizes: (sm: 8px, md: 16px, lg: 24px, xl: 32px);

@each $side in $sides {
  @each $size-name, $size-value in $sizes {
    .m-#{$side}-#{$size-name} {
      margin-#{$side}: $size-value;
    }
    
    .p-#{$side}-#{$size-name} {
      padding-#{$side}: $size-value;
    }
  }
}

// Advanced: Context-aware selectors
@mixin theme-context($themes...) {
  @each $theme in $themes {
    .theme-#{$theme} & {
      @content ($theme);
    }
  }
}

.card {
  background: white;
  
  @include theme-context(dark, high-contrast) using ($theme) {
    @if $theme == dark {
      background: #333;
      color: white;
    } @else if $theme == high-contrast {
      background: black;
      color: yellow;
      border: 2px solid yellow;
    }
  }
}
// Outputs:
// .theme-dark .card { background: #333; color: white; }
// .theme-high-contrast .card { background: black; color: yellow; ... }

// Practical: Responsive state variants
@mixin variants($variants...) {
  @each $variant in $variants {
    &-#{$variant} {
      @content ($variant);
    }
  }
}

.text {
  @include variants(center, left, right) using ($align) {
    text-align: $align;
  }
}

// Selector replacement
$old-prefix: 'btn';
$new-prefix: 'button';

@function migrate-selector($selector) {
  @return selector.replace($selector, '.#{$old-prefix}', '.#{$new-prefix}');
}

// Practical: Attribute selector generator
@mixin data-state($states...) {
  @each $state in $states {
    &[data-state="#{$state}"] {
      @content ($state);
    }
  }
}

.component {
  @include data-state(loading, error, success) using ($state) {
    @if $state == loading {
      opacity: 0.5;
    } @else if $state == error {
      border-color: red;
    } @else if $state == success {
      border-color: green;
    }
  }
}

// Interactive states
.button {
  background: blue;
  color: white;
  transition: all 0.3s;
  
  // Hover state
  &:hover {
    background: darkblue;
    transform: translateY(-2px);
  }
  
  // Focus state (accessibility)
  &:focus {
    outline: 2px solid orange;
    outline-offset: 2px;
  }
  
  // Active (pressed) state
  &:active {
    transform: translateY(0);
    box-shadow: inset 0 2px 4px rgba(0,0,0,0.3);
  }
  
  // Disabled state
  &:disabled {
    opacity: 0.5;
    cursor: not-allowed;
    
    // No hover when disabled
    &:hover {
      background: blue;
      transform: none;
    }
  }
  
  // Combined: hover but not disabled
  &:hover:not(:disabled) {
    cursor: pointer;
  }
  
  // Focus-visible (modern accessibility)
  &:focus-visible {
    outline: 2px solid orange;
  }
  
  &:focus:not(:focus-visible) {
    outline: none;
  }
}

// Pseudo-elements
.card {
  position: relative;
  
  // Before pseudo-element
  &::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 4px;
    background: linear-gradient(90deg, blue, purple);
  }
  
  // After pseudo-element
  &::after {
    content: '→';
    margin-left: 8px;
    transition: margin-left 0.3s;
  }
  
  &:hover::after {
    margin-left: 12px;
  }
}

// Structural pseudo-classes
.list {
  &__item {
    padding: 10px;
    border-bottom: 1px solid #ddd;
    
    // First child
    &:first-child {
      border-top: 1px solid #ddd;
    }
    
    // Last child
    &:last-child {
      border-bottom: none;
    }
    
    // Nth child patterns
    &:nth-child(odd) {
      background: #f9f9f9;
    }
    
    &:nth-child(even) {
      background: white;
    }
    
    // Every 3rd item
    &:nth-child(3n) {
      font-weight: bold;
    }
    
    // First 3 items
    &:nth-child(-n + 3) {
      border-left: 3px solid blue;
    }
    
    // Last 2 items
    &:nth-last-child(-n + 2) {
      opacity: 0.7;
    }
  }
}

// Form pseudo-classes
.input {
  padding: 10px;
  border: 1px solid #ccc;
  
  &:focus {
    border-color: blue;
    box-shadow: 0 0 0 3px rgba(0,0,255,0.1);
  }
  
  &:valid {
    border-color: green;
  }
  
  &:invalid {
    border-color: red;
  }
  
  &:required {
    border-left: 3px solid orange;
  }
  
  &:disabled {
    background: #f5f5f5;
    cursor: not-allowed;
  }
  
  &:read-only {
    background: #f9f9f9;
  }
  
  &:placeholder-shown {
    border-style: dashed;
  }
  
  &::placeholder {
    color: #999;
    font-style: italic;
  }
}

// Link states (LVHA order)
.link {
  color: blue;
  text-decoration: none;
  
  // L - Link (unvisited)
  &:link {
    color: blue;
  }
  
  // V - Visited
  &:visited {
    color: purple;
  }
  
  // H - Hover
  &:hover {
    color: darkblue;
    text-decoration: underline;
  }
  
  // A - Active
  &:active {
    color: red;
  }
}

// Practical: Hover/focus mixin
@mixin interactive-state {
  &:hover,
  &:focus {
    @content;
  }
}

.card {
  @include interactive-state {
    box-shadow: 0 4px 8px rgba(0,0,0,0.2);
    transform: translateY(-2px);
  }
}

// Practical: Not disabled mixin
@mixin when-enabled {
  &:not(:disabled):not(.is-disabled) {
    @content;
  }
}

.button {
  @include when-enabled {
    cursor: pointer;
    
    &:hover {
      background: darkblue;
    }
  }
}

// CSS :has() and :is() (modern)
.card {
  // Has child selector
  &:has(> .card__image) {
    padding-top: 0;
  }
  
  // Is selector (shorthand)
  &:is(:hover, :focus) {
    outline: 2px solid blue;
  }
  
  // Where (zero specificity)
  &:where(:hover, :focus) {
    opacity: 0.9;
  }
}

@use 'sass:map';

// Breakpoint configuration
$breakpoints: (
  xs: 0,
  sm: 576px,
  md: 768px,
  lg: 992px,
  xl: 1200px,
  xxl: 1400px
);

// Mobile-first mixin (min-width)
@mixin above($breakpoint) {
  $value: map.get($breakpoints, $breakpoint);
  
  @if $value {
    @media (min-width: $value) {
      @content;
    }
  } @else {
    @media (min-width: $breakpoint) {
      @content;
    }
  }
}

// Desktop-first mixin (max-width)
@mixin below($breakpoint) {
  $value: map.get($breakpoints, $breakpoint);
  
  @if $value {
    @media (max-width: $value - 1px) {
      @content;
    }
  } @else {
    @media (max-width: $breakpoint) {
      @content;
    }
  }
}

// Between two breakpoints
@mixin between($min, $max) {
  $min-val: map.get($breakpoints, $min) or $min;
  $max-val: map.get($breakpoints, $max) or $max;
  
  @media (min-width: $min-val) and (max-width: $max-val - 1px) {
    @content;
  }
}

// Nested media queries example
.container {
  width: 100%;
  padding: 1rem;
  
  // Mobile-first responsive design
  @include above(sm) {
    max-width: 540px;
    margin: 0 auto;
  }
  
  @include above(md) {
    max-width: 720px;
    padding: 1.5rem;
  }
  
  @include above(lg) {
    max-width: 960px;
    padding: 2rem;
  }
  
  @include above(xl) {
    max-width: 1140px;
  }
  
  @include above(xxl) {
    max-width: 1320px;
  }
}

// Responsive typography
.heading {
  font-size: 1.5rem;
  line-height: 1.2;
  
  @include above(md) {
    font-size: 2rem;
  }
  
  @include above(lg) {
    font-size: 2.5rem;
  }
  
  @include above(xl) {
    font-size: 3rem;
    line-height: 1.1;
  }
}

// Responsive grid
.grid {
  display: grid;
  gap: 1rem;
  
  // Mobile: 1 column
  grid-template-columns: 1fr;
  
  // Tablet: 2 columns
  @include above(md) {
    grid-template-columns: repeat(2, 1fr);
    gap: 1.5rem;
  }
  
  // Desktop: 3 columns
  @include above(lg) {
    grid-template-columns: repeat(3, 1fr);
    gap: 2rem;
  }
  
  // Large desktop: 4 columns
  @include above(xl) {
    grid-template-columns: repeat(4, 1fr);
  }
}

// Only for specific range
.sidebar {
  display: none;
  
  // Only show between md and lg
  @include between(md, lg) {
    display: block;
    width: 200px;
  }
  
  @include above(lg) {
    display: block;
    width: 250px;
  }
}

// Media query nesting with elements
.card {
  padding: 1rem;
  
  &__image {
    width: 100%;
    height: 200px;
    
    @include above(md) {
      height: 300px;
    }
  }
  
  &__title {
    font-size: 1.25rem;
    
    @include above(md) {
      font-size: 1.5rem;
    }
  }
}

// Advanced: Orientation queries
@mixin landscape {
  @media (orientation: landscape) {
    @content;
  }
}

@mixin portrait {
  @media (orientation: portrait) {
    @content;
  }
}

.video-player {
  aspect-ratio: 16/9;
  
  @include portrait {
    aspect-ratio: 4/3;
  }
}

// Advanced: Custom media features
@mixin prefers-dark-mode {
  @media (prefers-color-scheme: dark) {
    @content;
  }
}

@mixin prefers-reduced-motion {
  @media (prefers-reduced-motion: reduce) {
    @content;
  }
}

.theme {
  background: white;
  color: black;
  
  @include prefers-dark-mode {
    background: #1a1a1a;
    color: white;
  }
}

.animated {
  transition: all 0.3s;
  
  @include prefers-reduced-motion {
    transition: none;
  }
}

// Container queries (modern)
@mixin container($size) {
  @container (min-width: $size) {
    @content;
  }
}

.card {
  container-type: inline-size;
  
  &__title {
    font-size: 1rem;
    
    @include container(400px) {
      font-size: 1.5rem;
    }
    
    @include container(600px) {
      font-size: 2rem;
    }
  }
}

// Basic keyframe definition
@keyframes fadeIn {
  from {
    opacity: 0;
  }
  to {
    opacity: 1;
  }
}

.fade-in {
  animation: fadeIn 0.5s ease-in;
}

// Multi-step keyframes
@keyframes slideInBounce {
  0% {
    transform: translateX(-100%);
    opacity: 0;
  }
  60% {
    transform: translateX(10px);
    opacity: 1;
  }
  80% {
    transform: translateX(-5px);
  }
  100% {
    transform: translateX(0);
  }
}

// Dynamic keyframe generation
$animations: (
  fadeIn: (from: (opacity: 0), to: (opacity: 1)),
  slideDown: (from: (transform: translateY(-20px)), to: (transform: translateY(0))),
  scaleUp: (from: (transform: scale(0.8)), to: (transform: scale(1)))
);

@each $name, $steps in $animations {
  @keyframes #{$name} {
    from {
      @each $prop, $value in map.get($steps, from) {
        #{$prop}: $value;
      }
    }
    to {
      @each $prop, $value in map.get($steps, to) {
        #{$prop}: $value;
      }
    }
  }
}

// Scoped animations (nested in component)
.loader {
  // Keyframes defined within component scope
  @keyframes loader-spin {
    from { transform: rotate(0deg); }
    to { transform: rotate(360deg); }
  }
  
  width: 50px;
  height: 50px;
  border: 3px solid #f3f3f3;
  border-top: 3px solid blue;
  border-radius: 50%;
  animation: loader-spin 1s linear infinite;
}

// Complex multi-property animation
@keyframes pulse {
  0% {
    transform: scale(1);
    box-shadow: 0 0 0 0 rgba(0, 123, 255, 0.7);
  }
  50% {
    transform: scale(1.05);
    box-shadow: 0 0 0 10px rgba(0, 123, 255, 0);
  }
  100% {
    transform: scale(1);
    box-shadow: 0 0 0 0 rgba(0, 123, 255, 0);
  }
}

// Animation mixin
@mixin animate($name, $duration: 1s, $timing: ease, $iteration: 1) {
  animation-name: $name;
  animation-duration: $duration;
  animation-timing-function: $timing;
  animation-iteration-count: $iteration;
}

.button {
  @include animate(pulse, 2s, ease-out, infinite);
}

// Generate bounce animations
@for $i from 1 through 3 {
  @keyframes bounce-#{$i} {
    0%, 100% {
      transform: translateY(0);
    }
    50% {
      transform: translateY(-#{$i * 10}px);
    }
  }
  
  .bounce-#{$i} {
    animation: bounce-#{$i} 0.6s ease-in-out infinite;
  }
}

// Practical: Loading dots animation
@keyframes dot-pulse {
  0%, 80%, 100% {
    opacity: 0.3;
    transform: scale(0.8);
  }
  40% {
    opacity: 1;
    transform: scale(1);
  }
}

.loading-dots {
  display: flex;
  gap: 8px;
  
  &__dot {
    width: 10px;
    height: 10px;
    background: blue;
    border-radius: 50%;
    animation: dot-pulse 1.4s infinite ease-in-out;
    
    &:nth-child(1) {
      animation-delay: 0s;
    }
    
    &:nth-child(2) {
      animation-delay: 0.2s;
    }
    
    &:nth-child(3) {
      animation-delay: 0.4s;
    }
  }
}

// Practical: Slide animations from directions
$directions: (
  left: translateX(-100%),
  right: translateX(100%),
  top: translateY(-100%),
  bottom: translateY(100%)
);

@each $direction, $transform in $directions {
  @keyframes slideIn-#{$direction} {
    from {
      transform: $transform;
      opacity: 0;
    }
    to {
      transform: translate(0, 0);
      opacity: 1;
    }
  }
  
  .slide-in-#{$direction} {
    animation: slideIn-#{$direction} 0.5s ease-out;
  }
}

// @supports nesting
.card {
  display: block;
  
  // Feature detection
  @supports (display: grid) {
    display: grid;
    grid-template-columns: 1fr 2fr;
    gap: 1rem;
  }
  
  // Fallback for older browsers
  @supports not (display: grid) {
    display: flex;
    flex-wrap: wrap;
  }
}

// @supports with multiple features
.modern-layout {
  display: block;
  
  @supports (display: grid) and (gap: 1rem) {
    display: grid;
    gap: 1rem;
  }
  
  @supports (container-type: inline-size) {
    container-type: inline-size;
  }
}

// Nested grid system
.grid-container {
  display: grid;
  grid-template-columns: repeat(12, 1fr);
  gap: 1rem;
  
  &__item {
    // Default: full width (mobile)
    grid-column: span 12;
    
    // Tablet: half width
    @media (min-width: 768px) {
      grid-column: span 6;
    }
    
    // Desktop: third width
    @media (min-width: 992px) {
      grid-column: span 4;
    }
    
    // Modifiers
    &--full {
      grid-column: 1 / -1;
    }
    
    &--half {
      @media (min-width: 768px) {
        grid-column: span 6;
      }
    }
  }
}

// Flexbox utilities with nesting
.flex {
  display: flex;
  
  // Direction modifiers
  &--row {
    flex-direction: row;
  }
  
  &--column {
    flex-direction: column;
  }
  
  // Justify content
  &--center {
    justify-content: center;
  }
  
  &--between {
    justify-content: space-between;
  }
  
  &--around {
    justify-content: space-around;
  }
  
  // Align items
  &--items-center {
    align-items: center;
  }
  
  &--items-start {
    align-items: flex-start;
  }
  
  &--items-end {
    align-items: flex-end;
  }
  
  // Responsive direction
  &--responsive {
    flex-direction: column;
    
    @media (min-width: 768px) {
      flex-direction: row;
    }
  }
}

// Container queries (modern CSS)
.card-container {
  container-type: inline-size;
  container-name: card;
  
  .card {
    padding: 1rem;
    
    // When container is at least 400px
    @container card (min-width: 400px) {
      display: grid;
      grid-template-columns: 150px 1fr;
      gap: 1rem;
      padding: 1.5rem;
    }
    
    // When container is at least 600px
    @container card (min-width: 600px) {
      grid-template-columns: 200px 1fr 200px;
      padding: 2rem;
    }
  }
}

// @layer for cascade control
@layer base, components, utilities;

@layer base {
  .button {
    padding: 10px 20px;
    border: none;
  }
}

@layer components {
  .button {
    &--primary {
      background: blue;
      color: white;
    }
  }
}

@layer utilities {
  .button {
    &.rounded {
      border-radius: 9999px;
    }
  }
}

// Practical: Responsive grid mixin
@mixin grid-columns($mobile: 1, $tablet: 2, $desktop: 3, $wide: 4) {
  display: grid;
  gap: 1rem;
  grid-template-columns: repeat($mobile, 1fr);
  
  @media (min-width: 768px) {
    grid-template-columns: repeat($tablet, 1fr);
    gap: 1.5rem;
  }
  
  @media (min-width: 992px) {
    grid-template-columns: repeat($desktop, 1fr);
    gap: 2rem;
  }
  
  @media (min-width: 1200px) {
    grid-template-columns: repeat($wide, 1fr);
  }
}

.product-grid {
  @include grid-columns(1, 2, 3, 4);
}

// Practical: Flex utilities generator
@each $justify in (start, center, end, between, around) {
  .justify-#{$justify} {
    display: flex;
    justify-content: if($justify == between, space-between,
                      if($justify == around, space-around, 
                      flex-#{$justify}));
  }
}

@each $align in (start, center, end, stretch) {
  .items-#{$align} {
    display: flex;
    align-items: if($align == start or $align == end, 
                   flex-#{$align}, 
                   $align);
  }
}

// Advanced: Subgrid support
.article-layout {
  display: grid;
  grid-template-columns: repeat(12, 1fr);
  gap: 2rem;
  
  &__sidebar {
    grid-column: 1 / 4;
  }
  
  &__content {
    grid-column: 4 / -1;
    
    // Subgrid for nested content
    @supports (grid-template-columns: subgrid) {
      display: grid;
      grid-template-columns: subgrid;
      gap: 1rem;
    }
  }
}