/// Convert a pixel value to an em value
///
/// For a relational value, the input is calculated based on a parent value.
/// The default parent is 16px. The parent can be changed by passing an
/// optional second value.
///
/// @param  {String} $pixel       - Pixel value to convert
/// @param  {String} $base [16px] - Base pixel value
/// @return {String}              - em value
@function em($pixel, $base: 16px) {
  @return ($pixel / $base) * 1em;
}

/// An alias for `map-get`
///
/// @param  {Map}    $map - Map name
/// @param  {String} $key - Key name
/// @return {String}      - Key value
@function get($map, $key) {
  @return map-get($map, $key);
}

/// Half gutter; requires [Susy](http://susy.oddbird.net/)
@function gutter-half() {
  @return (gutter() / 2);
}

/// Quarter gutter; requires [Susy](http://susy.oddbird.net/)
@function gutter-quarter() {
  @return (gutter() / 4);
}

/// Double gutter; requires [Susy](http://susy.oddbird.net/)
@function gutter-double() {
  @return (gutter() * 2);
}

/// Quadruple gutter; requires [Susy](http://susy.oddbird.net/)
@function gutter-quad() {
  @return (gutter() * 4);
}

/// Create a unitless line-height variable
///
/// @param  {Value}  $font-size   - Font size value in px or rem
/// @param  {Value}  $line-height - Line height value in px or rem
/// @return {String}              - Unitless value
@function create-unitless-line-height($font-size, $line-height) {
  // Check the font-size units and strip them
  @if unit($font-size) == "rem" {
    $font-size: strip-units($font-size) * 10;
  } @else {
    $font-size: strip-units($font-size);
  }

  // Check the line-height units and strip them
  @if unit($line-height) == "rem" {
    $line-height: strip-units($line-height) * 10;
  } @else {
    $line-height: strip-units($line-height);
  }

  // Calculate unitless line-height and return it
  @return ($line-height / $font-size);
}

/// Removes units from a number string
///
/// @param {string} $number The string to remove units from
@function strip-units($number) {
  @return $number / ($number * 0 + 1);
}

/// Returns a value from the $z-index map from a given key
///
/// Contains a map to hold all of the possible z-index values with a matching
/// key. Avoid declaring a z-index value in the CSS itself, but instead add
/// another key-value pair to the map.
///
/// @param  {String} $layer - Name of z layer
/// @return {String}        - z-index value
/// @example
/// .foo {
///   z-index: z("default");
/// }
@function z($layer) {
  @if not map-has-key($z-index, $layer) {
    @warn "No layer found for `#{$layer}` in $z-index map. Property omitted.";
  }

  @return map-get($z-index, $layer);
}