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rOACCT/node_modules/csso/lib/restructure/7-mergeRuleset.js
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7-mergeRuleset.js
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			var walk = require('css-tree').walk;
			var utils = require('./utils');

			/*
			At this step all rules has single simple selector. We try to join by equal
			declaration blocks to first rule, e.g.

			.a { color: red }
			b { ... }
			.b { color: red }
			->
			.a, .b { color: red }
			b { ... }
			*/

			function processRule(node, item, list) {
			var selectors = node.prelude.children;
			var declarations = node.block.children;
			var nodeCompareMarker = selectors.first().compareMarker;
			var skippedCompareMarkers = {};

			list.nextUntil(item.next, function(next, nextItem) {
			// skip non-ruleset node if safe
			if (next.type !== 'Rule') {
			return utils.unsafeToSkipNode.call(selectors, next);
			}

			if (node.pseudoSignature !== next.pseudoSignature) {
			return true;
			}

			var nextFirstSelector = next.prelude.children.head;
			var nextDeclarations = next.block.children;
			var nextCompareMarker = nextFirstSelector.data.compareMarker;

			// if next ruleset has same marked as one of skipped then stop joining
			if (nextCompareMarker in skippedCompareMarkers) {
			return true;
			}

			// try to join by selectors
			if (selectors.head === selectors.tail) {
			if (selectors.first().id === nextFirstSelector.data.id) {
			declarations.appendList(nextDeclarations);
			list.remove(nextItem);
			return;
			}
			}

			// try to join by properties
			if (utils.isEqualDeclarations(declarations, nextDeclarations)) {
			var nextStr = nextFirstSelector.data.id;

			selectors.some(function(data, item) {
			var curStr = data.id;

			if (nextStr < curStr) {
			selectors.insert(nextFirstSelector, item);
			return true;
			}

			if (!item.next) {
			selectors.insert(nextFirstSelector);
			return true;
			}
			});

			list.remove(nextItem);
			return;
			}

			// go to next ruleset if current one can be skipped (has no equal specificity nor element selector)
			if (nextCompareMarker === nodeCompareMarker) {
			return true;
			}

			skippedCompareMarkers[nextCompareMarker] = true;
			});
			}

			module.exports = function mergeRule(ast) {
			walk(ast, {
			visit: 'Rule',
			enter: processRule
			});
			};