node_modules/csso/lib/restructure/prepare/processSelector.js

var generate = require('css-tree').generate;
var specificity = require('./specificity');

var nonFreezePseudoElements = {
    'first-letter': true,
    'first-line': true,
    'after': true,
    'before': true
};
var nonFreezePseudoClasses = {
    'link': true,
    'visited': true,
    'hover': true,
    'active': true,
    'first-letter': true,
    'first-line': true,
    'after': true,
    'before': true
};

module.exports = function freeze(node, usageData) {
    var pseudos = Object.create(null);
    var hasPseudo = false;

    node.prelude.children.each(function(simpleSelector) {
        var tagName = '*';
        var scope = 0;

        simpleSelector.children.each(function(node) {
            switch (node.type) {
                case 'ClassSelector':
                    if (usageData && usageData.scopes) {
                        var classScope = usageData.scopes[node.name] || 0;

                        if (scope !== 0 && classScope !== scope) {
                            throw new Error('Selector can\'t has classes from different scopes: ' + generate(simpleSelector));
                        }

                        scope = classScope;
                    }
                    break;

                case 'PseudoClassSelector':
                    var name = node.name.toLowerCase();

                    if (!nonFreezePseudoClasses.hasOwnProperty(name)) {
                        pseudos[':' + name] = true;
                        hasPseudo = true;
                    }
                    break;

                case 'PseudoElementSelector':
                    var name = node.name.toLowerCase();

                    if (!nonFreezePseudoElements.hasOwnProperty(name)) {
                        pseudos['::' + name] = true;
                        hasPseudo = true;
                    }
                    break;

                case 'TypeSelector':
                    tagName = node.name.toLowerCase();
                    break;

                case 'AttributeSelector':
                    if (node.flags) {
                        pseudos['[' + node.flags.toLowerCase() + ']'] = true;
                        hasPseudo = true;
                    }
                    break;

                case 'WhiteSpace':
                case 'Combinator':
                    tagName = '*';
                    break;
            }
        });

        simpleSelector.compareMarker = specificity(simpleSelector).toString();
        simpleSelector.id = null; // pre-init property to avoid multiple hidden class
        simpleSelector.id = generate(simpleSelector);

        if (scope) {
            simpleSelector.compareMarker += ':' + scope;
        }

        if (tagName !== '*') {
            simpleSelector.compareMarker += ',' + tagName;
        }
    });

    // add property to all rule nodes to avoid multiple hidden class
    node.pseudoSignature = hasPseudo && Object.keys(pseudos).sort().join(',');
};