/// Function to get next map item
/// returns next map item or fallback value if map, key or next item does not exist
///
/// @author Simon Koch <agorilla@me.com>
///
/// Licensed under the MIT license.
///
/// @access public
///
/// @param {Map} $map - Sass list map
/// @param {String} $key - List map key
/// @param {Boolean} $fallback (false) - Fallback value if map, key or previous item does not exist
/// @param {String} $return (value) - Return value or key of previous list item
/// @param {Boolean} $debug (false) - Debug option
///
/// @example scss - Usage
///$map: (
///   s: 320px,
///   m: 768px,
/// );
///
/// .foo {
///   width: map-get-next($map, s);
/// }
///
/// .bar {
///    width: map-get-next($map, m, 1024px);
/// }
///
/// @example css - CSS output
/// .foo {
///   width: 768px;
/// }
///
/// .bar {
///   width: 1024px;
/// }

@function map-get-next($map, $key, $fallback: false, $return: value) {

    // Check if map is valid
    @if type-of($map) == map {

        // Check if key exists in map
        @if map-has-key($map, $key) {

            // Init index counter variable
            $i: 0;

            // Init key index
            $key-index: false;

            // Traverse map for key
            @each $map-key, $map-value in $map {
                // Update index
                $i: $i + 1;

                // If map key found, set key index
                @if $map-key == $key {
                    $key-index: $i;
                }

                // If next index return next value or key based on $return
                @if $i == $key-index + 1 {
                    @if $return == key {
                        @return $map-key;
                    } @else {
                        @return $map-value;
                    }
                }

                // If last entry return false
                @if $i == length($map) {
                    @return $fallback;
                }
            }

            @warn 'No next map item for key #{$key}';
            @return $fallback;
        }

        @warn 'No valid key #{$key} in map';
        @return $fallback;
    }

    @warn 'No valid map';
    @return $fallback;
}