var Sizzle = function( selector, context, results, seed ) {

//修正results, context
results = results || [];
context = context || document;

//备份上下文
var origContext = context;

if ( context.nodeType !== 1 && context.nodeType !== 9 ) {
    return [];
}

if ( !selector || typeof selector !== "string" ) {
    return results;
}

//定义局部变量
var m, set, checkSet, extra, ret, cur, pop, i,
    prune = true,
    contextXML = Sizzle.isXML( context ),
    parts = [],
    soFar = selector;

    // Reset the position of the chunker regexp (start from head)
    do {
        chunker.exec( "" );
        m = chunker.exec( soFar );

        if ( m ) {
            soFar = m[3];

            parts.push( m[1] );

            if ( m[2] ) {
                extra = m[3];
                break;
            }
        }
    } while ( m );

//如果存在位置伪类，则从左向右查找
if ( parts.length > 1 && origPOS.exec( selector ) ) {

    if ( parts.length === 2 && Expr.relative[ parts[0] ] ) {
        set = posProcess( parts[0] + parts[1], context, seed );

    } else {
        set = Expr.relative[ parts[0] ] ?
            [ context ] :
            Sizzle( parts.shift(), context );

        while ( parts.length ) {
            selector = parts.shift();

            if ( Expr.relative[ selector ] ) {
                selector += parts.shift();
            }

            set = posProcess( selector, set, seed );
        }
    }

}

/*
* 在指定的上下文数组context下，查找与选择器表达式
* selector匹配的元素集合，并且支持位置伪类。selector
* 由一个块间关系符和一个块表达式组成。
* */
var posProcess = function( selector, context, seed ) {
    var match,
        tmpSet = [],
        later = "",
        root = context.nodeType ? [context] : context;

    // Position selectors must be done after the filter
    // And so must :not(positional) so we move all PSEUDOs to the end
    //删除selector中所以的伪类，并累积在later中。
    while ( (match = Expr.match.PSEUDO.exec( selector )) ) {
        later += match[0];
        selector = selector.replace( Expr.match.PSEUDO, "" );
    }
    //如果删除伪类的选择器表达式中只剩一个块间关系符，则追加一个通配符"*"
    selector = Expr.relative[selector] ? selector + "*" : selector;

    for ( var i = 0, l = root.length; i < l; i++ ) {
        Sizzle( selector, root[i], tmpSet, seed );
    }

    return Sizzle.filter( later, tmpSet );
};