/// Function to get previous map item
/// returns previous map item or fallback value if map, key or previous 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
/// $map: (
///   s: 320px,
///   m: 768px,
/// );
///
/// .foo {
///   width: map-get-prev($map, m);
/// }
///
/// .bar {
///    width: map-get-prev($map, s, 240px);
/// }
///
/// @example css - CSS output
/// .foo {
///   width: 320px;
/// }
///
/// .bar {
///   width: 240px;
/// }

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

    // Check if map is valid
    @if type-of($map) == map {
        @if $debug {
            @debug 'Map exists #{$map}';
        }

        // Check if key exists in map
        @if map-has-key($map, $key) {
            @if $debug {
                @debug 'Map has key #{$key}';
            }

            // Init index counter variable
            $i: 1;

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

            // Traverse map for key
            @each $map-key, $map-value in $map {
                @if $debug {
                    @debug 'map-key: #{$map-key}, map-value: #{$map-value}, i: #{$i}';
                }

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

                // Update index
                $i: $i + 1;
            }

            // If the key-index exists, iterate through the map again
            @if $key-index != false {
                $previous-index: $key-index - 1;
                $i: 1;

                // If the previous key is less than one, use the fallback
                @if $previous-index < 1 {
                    @warn 'no previous item in map, returning fallback';
                    @return $fallback;
                }
                @else {

                    // Traverse map for key
                    @each $map-key, $map-value in $map {
                         @if $i == $previous-index {
                            @if $return == 'key' {
                                @if $debug {
                                    @debug 'found! returning map-key: #{$map-key}';
                                }
                                @return $map-key;
                            }
                            @else {
                                @if $debug {
                                    @debug 'found! returning map-value: #{$map-value}';
                                }
                                @return $map-value;
                            }
                        }
                        
                        // Update index
                        $i: $i + 1;
                    }
                }
            }
            @else {
                @warn 'No previous map item for key #{$key}';
                @return $fallback;
            }
        }
        @else {
            @warn 'No valid key #{$key} in map';
            @return $fallback;
        }
    }
    @else {
        @warn 'No valid map';
        @return $fallback;
    }
}