////
/// @group BEM
///
/// @access public
////

/// 
/// Generate a new BEM modifier.
/// 
/// If the parent context is block or element, the modifier will modify said block or element according
/// to the BEM naming convention.
///
/// If the parent context is a modifier or suffix, then the modifier will depend on said modifier or suffix.
/// Depending on $extend, the meaning of this dependency (and the resulting selector) varies:
/// If it's false (default), you signalize that the modifier also exists by itself, but it changes its
/// behavior when the parent modifier or suffix is set.
/// If it's true, you signalize that the modifier extends the parent modifier or suffix and can only be
/// used in conjunction with it.
///
/// @param {string | list} $name  - First element name or list with two items: 1. first element name, 2. bool indicating if the modifier is extending
/// @param {string | list} $names - More element names or lists with two items: 1. element name, 2. bool indicating if the modifier is extending
///
/// @content
///
/// @throw If the element is not preceded by a block, element, modifier or suffix.
///
/// @example scss - Modifier that modifies a block or element
///   @include iro-bem-component('block') {
///     @include iro-bem-modifier('mod') {
///       background-color: #eee;
///     }
///     
///     @include iro-bem-element('elem') {
///       @include iro-bem-modifier('mod') {
///         background-color: #222;
///       }
///     }
///   }
///   
///   // Generates:
///   
///   .c-block--mod {
///     background-color: #eee;
///   }
///   
///   .c-block__elem--mod {
///     background-color: #222;
///   }
///
/// @example scss - Modifier nested in another modifier, not extending
///   @include iro-bem-component('block') {
///     @include iro-bem-modifier('mod') {
///       background-color: #eee;
///     }
///     
///     @include iro-bem-modifier('dark') {
///       /* some definitions */
///       
///       @include iro-bem-modifier('mod') {
///         background-color: #222;
///       }
///     }
///   }
///   
///   // Generates:
///   
///   .c-block--mod {
///     background-color: #eee;
///   }
///   
///   .c-block--dark {
///     /* some definitions */
///   }
///   
///   .c-block--dark.c-block--mod {
///     background-color: #222;
///   }
///
/// @example scss - Modifier nested in another modifier, extending
///   @include iro-bem-component('block') {
///     @include iro-bem-modifier('mod') {
///       background-color: #eee;
///     }
///     
///     @include iro-bem-modifier('dark') {
///       /* some definitions */
///       
///       @include iro-bem-modifier('mod' true) {
///         background-color: #222;
///       }
///     }
///   }
///   
///   // Generates:
///   
///   .c-block--mod {
///     background-color: #eee;
///   }
///   
///   .c-block--dark {
///     /* some definitions */
///   }
///   
///   .c-block--dark--mod {
///     background-color: #222;
///   }
///
@mixin iro-bem-modifier($name, $names...) {
    $result:   iro-bem-modifier($name, $names...);
    $selector: nth($result, 1);
    $context:  nth($result, 2);

    @include iro-bem-validate(
        'modifier',
        (name: $name, names: $names),
        $selector,
        $context
    );

    @include iro-context-push($iro-bem-context-id, $context...);
    @at-root #{$selector} {
        @content;
    }
    @include iro-context-pop($iro-bem-context-id);
}

///
/// Generate a new BEM modifier. Check the respective mixin documentation for more information.
/// 
/// @return {list} A list with two items: 1. selector, 2. context or `null`
///
/// @see {mixin} iro-bem-modifier
///
@function iro-bem-modifier($name, $names...) {
    $noop: iro-context-assert-stack-count($iro-bem-context-id, $iro-bem-max-depth);
    $noop: iro-context-assert-stack-must-contain($iro-bem-context-id, 'block');

    $parent-context:  iro-context-get($iro-bem-context-id, 'block' 'element' 'modifier' 'suffix' 'state');
    $parent-selector: map-get(nth($parent-context, 2), 'selector');
    $selector:        ();
    $parts-data:      ();

    @if not iro-selector-suffix-match(&, $parent-selector) {
        //
        // The current selector doesn't match the parent selector.
        // The user manually added a selector between parent context and this modifier call.
        // This case is forbidden because any outcome semantically wouldn't make sense:
        //   - {b,e,m,s} [manual selector] {b,e,m,s}--modifier
        //   - {b,e,m,s}--modifier [manual selector]
        // The first case would make the modifier behave like an element.
        // The second case is unintuitive, the code would be more clear by nesting the manual
        // selector in the modifier instead.
        //

        @error 'A modifier must be an immediate child of the parent context';
    }

    @each $name in iro-list-prepend($names, $name) {
        $extend: false;
        @if type-of($name) == list {
            $extend: nth($name, 2);
            $name:   nth($name, 1);
        }

        @if index('block' 'element', nth($parent-context, 1)) or $extend == true {
            //
            // Either the parent context is block or element, or a modifier or suffix
            // is to be extended. The modifier part can simply be appended.
            // Possible outcomes:
            //   - {b,e,m,s}--modifier
            //

            $sel:        selector-append(&, $iro-bem-modifier-separator + $name);
            $selector:   join($selector, $sel, comma);
            $parts-data: append($parts-data, (
                    'name':     $name,
                    'selector': $sel
            ));
        } @else {
            //
            // Parent context is modifier, suffix or state and $extend is false.
            //

            $be-context: iro-context-get($iro-bem-context-id, 'block' 'element');

            @if nth($be-context, 1) == 'element' {
                //
                // Latest context is element. Since element contexts can consist of multiple single
                // elements, inspect all elements and append its selector with the suffix "--$name".
                // This has to be done with string comparison since none of Sass selector functions
                // is of use here. 
                // Possible outcomes:
                //   - {m,s}.{e}--modifier
                //

                $nsel: ();

                @each $elem-part-data in map-get(nth($be-context, 2), 'parts') {
                    $elem-part-selector: map-get($elem-part-data, 'selector');

                    $sel: ();
                    @each $s in & {
                        @each $ps in $elem-part-selector {
                            @if str-index(inspect($s), inspect($ps) + $iro-bem-modifier-separator) or str-index(inspect($s), inspect($ps) + $iro-bem-suffix-separator) {
                                $sel: join($sel, selector-unify($s, selector-append($ps, $iro-bem-modifier-separator + $name)), comma);
                            }
                        }
                    }
                    @if length($sel) == 0 {
                        @error 'Could not generate modifier selector.';
                    }

                    $nsel: join($nsel, $sel, comma);
                }

                $selector:   join($selector, $nsel, comma);
                $parts-data: append($parts-data, (
                        'name':     $name,
                        'selector': $nsel
                ));
            } @else {
                //
                // Latest context is block. Just append the modifier part.
                // Possible outcomes:
                //   - {m,s}.{b}--modifier
                //

                $block-base-selector: map-get(nth($be-context, 2), 'base-selector');

                $sel:        selector-append(&, $block-base-selector, $iro-bem-modifier-separator + $name);
                $selector:   join($selector, $sel, comma);
                $parts-data: append($parts-data, (
                        'name':     $name,
                        'selector': $sel
                ));
            }
        }
    }

    $context: 'modifier', (
        'parts':    $parts-data,
        'selector': $selector
    );

    @return $selector $context;
}