Sizzle一步步实现所有功能（基本筛选）

第二步：实现:first,:last,:eq(),even,odd,:gt(),:lt(); :header,:root,:taget; :not()。

;(function( window ){
var arr = [];
var push = arr.push;
var slice = arr.slice;
var select ;
var Expr;
// 标识
var expando = "sizzle" + 1 * new Date();
// http://www.w3.org/TR/css3-selectors/#whitespace
// 各种空白待穿正则字符串
var whitespace = "[\\x20\\t\\r\\n\\f]";
// 带空格选择器正则，记忆无空格选择器
// http://www.w3.org/TR/CSS21/syndata.html#value-def-identifier
var    identifier = "(?:\\\\.|[\\w-]|[^\\x00-\\xa0])+";
var pseudos = ":(" + identifier + ")(?:\\((" +
        // To reduce the number of selectors needing tokenize in the preFilter, prefer arguments:
        // 1. quoted (capture 3; capture 4 or capture 5)
        "('((?:\\\\.|[^\\\\'])*)'|\"((?:\\\\.|[^\\\\\"])*)\")|" +
        // 2. simple (capture 6)
        "((?:\\\\.|[^\\\\()[\\]]|" + attributes + ")*)|" +
        // 3. anything else (capture 2)
        ".*" +
        ")\\)|)";
// 属性选择器: http://www.w3.org/TR/selectors/#attribute-selectors
var    attributes = "\\[" + whitespace + "*(" + identifier + ")(?:" + whitespace +
        // Operator (capture 2)
        "*([*^$|!~]?=)" + whitespace +
        // "Attribute values must be CSS identifiers [capture 5] or strings [capture 3 or capture 4]"
        "*(?:'((?:\\\\.|[^\\\\'])*)'|\"((?:\\\\.|[^\\\\\"])*)\"|(" + identifier + "))|)" + whitespace +
        "*\\]";
var rtrim = new RegExp( "^" + whitespace + "+|((?:^|[^\\\\])(?:\\\\.)*)" + whitespace + "+$", "g" );
// 快速选择器正则 ID 或者 TAG(包括*) 或者 CLASS 选择器
var rquickExpr = /^(?:#([\w-]+)|(\w+|\*)|\.([\w-]+))$/;
// 连接符号
var rcomma = new RegExp( "^" + whitespace + "*," + whitespace + "*" );
// 层级符号正则'>',' ','+','~'
var rcombinators = new RegExp( "^" + whitespace + "*([>+~]|" + whitespace + ")" + whitespace + "*" );
var matchExpr = {
        "ID": new RegExp( "^#(" + identifier + ")" ),
        "CLASS": new RegExp( "^\\.(" + identifier + ")" ),
        "TAG": new RegExp( "^(" + identifier + "|[*])" ),
        "PSEUDO": new RegExp( "^" + pseudos ),
        "needsContext": new RegExp( "^" + whitespace + "*[>+~]|:(even|odd|eq|gt|lt|nth|first|last)(?:\\(" +
            whitespace + "*((?:-\\d)?\\d*)" + whitespace + "*\\)|)(?=[^-]|$)", "i" )
};
var rheader = /^h\d$/i;
// 浏览器代码正则
var rnative = /^[^{]+\{\s*\[native \w/;
// token缓存
var tokenCache = createCache();
// 编译缓存
var compilerCache = createCache();
// 入口
function Sizzle( selector, context, results){
    // 清除空格
    selector = selector.replace( rtrim, "$1" )
    var results = results || [];
    var match;
    var matcher;
    var elem;
    var m;
    var context = context || document;

    // 是否为最简选择器
    if( match = rquickExpr.exec( selector )){
        // Sizzle('#ID)
        if ( (m = match[1]) ) {
            elem = context.getElementById( m );
            if( elem ){
                results.push( elem );
            }
            return results;

        // Sizzle("TAG")
        }else if( (m = match[2]) ){
            push.apply( results, context.getElementsByTagName( selector ) );
            return results;

        // Sizzle(".CLASS")
        }else if( (m = match[3]) ){
            // 支持getElementsByClassName
            if( support.getElementsByClassName ){
                push.apply( results, context.getElementsByClassName( m ) );
                return results;
            }
        }
    }
    // 复杂选择调到select
    return select( selector, context, results);
}
// 创建缓存函数
function createCache() {
    var keys = [];

    function cache( key, value ) {
        // Use (key + " ") to avoid collision with native prototype properties (see Issue #157)
        if ( keys.push( key + " " ) > 10 ) {
            // Only keep the most recent entries
            delete cache[ keys.shift() ];
        }
        return (cache[ key + " " ] = value);
    }
    return cache;
}
// 函数标记函数
function markFunction( fn ) {
    fn[ expando ] = true;
    return fn;
}
// 错误函数
Sizzle.error = function( msg ) {
    throw new Error( "Syntax error, unrecognized expression: " + msg );
};
// 版本支持变量的对外访问入口
var support = Sizzle.support = {};

// 判断是否支持getElementsByClassName
// 支持: IE<9
support.getElementsByClassName = rnative.test( document.getElementsByClassName );
// 表达式对象
// 存放各类相对位置，各种查询函数，各种过滤函数等。
Expr = {
    relative: {
        ">": { dir: "parentNode", first: true },
        " ": { dir: "parentNode" },
        "+": { dir: "previousSibling", first: true },
        "~": { dir: "previousSibling" }
    },
    filter: {
        "TAG": function( nodeNameSelector ) {
            var nodeName = nodeNameSelector.toLowerCase();
            return nodeNameSelector === "*" ?
                function() { return true; } :
                function( elem ) {
                    return elem.nodeName.toLowerCase() === nodeName;
                };
        },
        "CLASS": function( className ) {
            var className = className.toLowerCase();
            return function( elem ) {
                    return elem.className.toLowerCase() === className;
            };
        },
        "PSEUDO": function( pseudo, argument ) {
            var fn = Expr.pseudos[ pseudo ];
            if ( fn[ expando ] ) {
                return fn( argument );
            }
            return fn;
        }
    },
    find: {
        "TAG": function( tag, context ) {
            return context.getElementsByTagName( tag );
        },
        "CLASS": support.getElementsByClassName&&function( tag, context ) {
            return context.getElementsByClassName( tag );
        },
    },
    // 筛选方法
    pseudos: {
        // not是个难点
        // superMatcher将结果存入restlts，未匹配的元素则返回
        // not两个路线一个是处理not参数中为复杂伪类筛选setMatcher
        // 一个是elementMatcher
        "not": markFunction(function( selector ){
                var results = [];
                var input = [];
                var matcher = compile( selector );
                return matcher[ expando ] ?
                markFunction(function( seed, matches, context){
                    var elem;
                    var unmatched = matcher(seed, null, [] );
                    var i = seed.length;
                    while ( i-- ) {
                        if ( (elem = unmatched[i]) ) {
                            seed[i] = !(matches[i] = elem);
                        }
                    }
                }):
                function( elem, context ) {
                    input[0] = elem;
                    matcher( input, null, results );
                    return !results.pop();
                }
            }
        ),
        "target": function( elem ) {
            var hash = window.location && window.location.hash;
            return hash && hash.slice( 1 ) === elem.id;
        },
        "header": function( elem ) {
            return rheader.test( elem.nodeName );
        },
        "root": function( elem ) {
            return elem === document.documentElement;
        },
        // 以下为位置筛选，我们采用一个函数，但其index不同。
        // 用createPositionalPseudo生成一个新函数，这个函数的index数组根据位置不同而不同
        "first": createPositionalPseudo(
            function(){
                return [0];
            }
        ),
        "last":createPositionalPseudo(
            function( matchesIndex ,length){
                return [ length-1 ];
            }
        ),
        "eq": createPositionalPseudo(
            function( matchesIndex ,length, argument){
                return [ argument < 0 ? argument + length : argument ];
            }
        ),
        "even": createPositionalPseudo(
            function( matchesIndex, length ){
                for(var i = 0; i<length; i+=2){
                    matchesIndex.push(i);
                }
                return matchesIndex;
            }
        ),
        "odd": createPositionalPseudo(
            function( matchesIndex, length ){
                for(var i = 1; i<length; i+=2){
                    matchesIndex.push(i);
                }
                return matchesIndex;
            }
        ),
        "lt": createPositionalPseudo(
            function( matchesIndex ,length, argument){
                var i = argument < 0 ? argument + length : argument;
                while(i--){
                    matchesIndex.push(i);
                }
                return matchesIndex;
            }
        ),
        "gt": createPositionalPseudo(
            function( matchesIndex, length, argument){
                var i = argument < 0 ? argument + length : argument;
                while( ++i < length){
                    matchesIndex.push(i);
                }
                return matchesIndex;
            }
        ),
    }
}
// 返回一个函数，用于所有位置筛选
function createPositionalPseudo( fn ) {
    return markFunction( function( argument ){
        argument = + argument;
        return markFunction(function( seed, matches ) {
            var j;
            var matchIndexes = fn([],seed.length,argument);
            var i = matchIndexes.length;
            while(i--){
                if ( seed[ (j = matchIndexes[i]) ] ) {
                    seed[j] = !(matches[j] = seed[j]);
                }
            }
        })
    })
}
// tokenize函数
// 将选择器字符串转化为方便使用的数组对象形式
tokenize = Sizzle.tokenize = function( selector, parseOnly ) {
    var cached = tokenCache[ selector + " " ];

    // cached.slice生成新的数组，对其修改不会修改其引用缓存
    if ( cached ) {
        return cached.slice( 0 );
    }
    // 循环条件
    var soFar = selector;
    // 结果数组
    var groups = [];
    // 匹配参数
    var matched;
    // 一个独立的tokens
    var tokens;
    // 辅助变量
    var match;

    while ( soFar ) {

        //首次默认创建一个tokens
        //之后每碰到一个逗号新增一个新的tokens
        if ( !matched || (match = rcomma.exec( soFar )) ) {
            if ( match ) {
                // Don't consume trailing commas as valid
                soFar = soFar.slice( match[0].length ) || soFar;
            }
            groups.push( (tokens = []) );
        }

        matched = false;

        // 关系token
        if ( (match = rcombinators.exec( soFar )) ) {
            matched = match.shift();
            tokens.push({
                value: matched,
                // Cast descendant combinators to space
                type: match[0].replace( rtrim, " " )
            });
            soFar = soFar.slice( matched.length );
        }
        // TAG，CLASS，ID token
        for ( type in Expr.filter ) {
            if ((match = matchExpr[ type ].exec( soFar )) ) {
                matched = match.shift();
                tokens.push({
                    value: matched,
                    type: type,
                    matches: match
                });
                soFar = soFar.slice( matched.length );
            }
        }
        // 一次循环到这里三个条件都不符合没有匹配结果时，跳出。
        if ( !matched ) {
            break;
        }
    }
    // 意外跳出，soFar存在，报错。
    return soFar ?
            Sizzle.error( selector ) :
            // 缓存后转成新数组返回（预防修改缓存内容）
            tokenCache( selector, groups ).slice( 0 );
};
// 将tokens转化为selector字符串形式。
function toSelector( tokens ) {
    var i = 0,
        len = tokens.length,
        selector = "";
    for ( ; i < len; i++ ) {
        selector += tokens[i].value;
    }
    return selector;
}
// 压缩数组
// condense([undefined,1,2,3]) = [123];
function condense(matcherOut) {
    var newMatcherOut = [];
    var elem;
    for(var i =0; i< matcherOut.length;i++){
        if((elem = matcherOut[i])){
            newMatcherOut.push(elem);
        }
    }
    return newMatcherOut;
}
// 对多重上下文执行元素选择
// 用于上下文是数组的情况
// 服务于setMatcher返回函数的某些情况
function multipleContexts(selector,context,results){
    for(var i = 0;i<context.length;i++){
        Sizzle(selector, context[i],results);
    }
    return results;
}
// setMatcher筛选
// setMatcher是不同于elementMatcher的另一个方向，用来处理各种伪类筛选
function setMatcher( preFilter, selector, matcher, postFinder, postSelector){
    if ( postFinder && !postFinder[ expando ] ) {
        postFinder = setMatcher( postFinder, postSelector );
    }
    return markFunction(function( seed, results, context ) {
        var elems =  seed ||  multipleContexts( selector || "*" , context.nodeType ? [ context ] : context ,[]);
        var matcherIn = elems;
        var matcherOut = [];
        if ( matcher ) {
            matcher( matcherIn, matcherOut);
        }
        matcherOut = condense(matcherOut);
        if ( postFinder ) {
            postFinder( null, results, matcherOut );
        } else {
            push.apply( results, matcherOut );
        }
    })
}
// !addCombinator
// 增加关系处理函数
// 返回关系函数，主要功能是，遍历种子节点的关系节点。
// 比如li>a,传入无数个种子节点a，a.parentNode，再执行matcher,matcher里再判断这个父亲节点是不是li
function addCombinator( matcher, combinator ) {
    var dir = combinator.dir;
    return combinator.first ?
        function( elem, context ) {
            while( (elem = elem[ dir ]) ){
                if ( elem.nodeType === 1 ) {
                    return matcher( elem, context );
                }
            }
        }:
        function( elem, context ) {
            while ( (elem = elem[ dir ]) ) {
                if ( elem.nodeType === 1 ) {
                    if(matcher( elem, context )) {
                        return true;
                    }
                }
            }
            return false;
        }
}

// !elementMatcher
// 生成matchers遍历器
// matchers数组存放我要过滤的函数，这个函数遍历所有过滤函数，一个不符合就返回false。
function elementMatcher( matchers ) {
    return function( elem, context ) {
        var i = matchers.length;
        while ( i-- ) {
            if ( !matchers[i]( elem, context ) ) {
                return false;
            }
        }
        return true;
    };
}
// !matcherFromTokens
// 根据tokens,生成过滤一组函数matchers，供elementMatcher使用
// 返回的是一个执行所有过滤函数的函数
function matcherFromTokens( tokens ){

    var matchers = [];
    var matcher;
    var type;
    var i = 0;
    var j ;
    var len = tokens.length;
    for ( ; i < len; i++ ) {
        if ( (matcher = Expr.relative[ tokens[i].type ]) ) {
            matchers = [ addCombinator(elementMatcher( matchers ), matcher) ];
        } else {
            matcher = Expr.filter[ (type = tokens[i].type) ].apply( null, tokens[i].matches );
            if ( matcher[ expando ] ) {
                j = i+1;
                return setMatcher(
                    i > 0 && elementMatcher( matchers ),
                    i > 0 && toSelector( tokens.slice( 0, i ) ),
                    matcher,
                    j < len && matcherFromTokens( (tokens = tokens.slice( j )) ),
                    j < len && toSelector( tokens )
                );
            }
            matchers.push( matcher );
        }
    }
    return elementMatcher( matchers );
}
// !matcherFromGroupMatchers
// 返回超级匹配器，
function matcherFromGroupMatchers( elementMatchers, setMatchers ){
    var bySet = setMatchers.length > 0,
        byElement = elementMatchers.length > 0;
    // !!最重要superMatcher，也是最核心的函数，其它的函数为它服务。
    // 获取种子元素，遍历所有种子元素。
    // 遍历elementMatchers
    // 符合的推入结果数组
    // 一个选择器（逗号隔开的）生成一个elementMatcher，elementMatchers是存放所有elementMatcher的数组
    var superMatcher = function( seed, context, results) {
        var elems = seed || Expr.find["TAG"]( "*", document );
        var len = elems.length
        var i = 0;
        var j;
        var unmatched = seed ? slice.call(seed, 0) : undefined ;
        var setMatched = [];
        for ( ; i !== len && (elem = elems[i]) != null; i++ ) {
            if( byElement ){
                j = 0;
                while ( (matcher = elementMatchers[j++]) ) {
                    if ( matcher( elem, context) ) {
                        results.push( elem );
                        break;
                    }
                }

            }
        }
        if ( bySet) {
            j = 0;
            while ( (matcher = setMatchers[j++]) ) {
                matcher( unmatched, setMatched, context);
            }
            push.apply( results, setMatched );
        }
        return unmatched;
    }
    return bySet ?
        markFunction( superMatcher ) :
        superMatcher;;
}
// compile
// 最初的编译器，存放elementMatchers，缓存超级匹配函数并返回
compile = Sizzle.compile = function( selector, match ) {
    var i;
    var elementMatchers = [];
    var setMatchers = [];
    var    cached = compilerCache[ selector + " "];
    if ( !cached ) {
        if ( !match ) {
            match = tokenize( selector );
        }
        i = match.length;
        while ( i-- ) {
            cached = matcherFromTokens( match[i] );
            if ( cached[ expando ] ) {
                setMatchers.push( cached );
            } else {
                elementMatchers.push( cached );
            }
        }
        cached = compilerCache( selector, matcherFromGroupMatchers( elementMatchers, setMatchers ));
    }
    return cached;
}
// select
// 兼容的自写的选择器
select = Sizzle.select = function( selector, context, results){
    var token;
    var seed;
    var tokens;
    var find;
    var match = tokenize( selector )
    if ( match.length === 1 ) {
        // tokens
        var tokens = match[0].slice( 0 );
        // 如果tokens的首项是ID，将其设置为上下文
        if ( (token = tokens[0]).type === 'ID' ){
            context = document.getElementById(token.matches[0]);
            selector = selector.slice( tokens.shift().value.length );
        }
        // 生成种子seed
        // 如"div ul li"，所谓种子就是所有的li
        // 后面编译函数需要过滤出符合祖先是ul，ul的祖先是div的节点
        // 在筛选选择中，如li:eq(0),不可以从左边直接取种子。
        i = matchExpr["needsContext"].test( selector ) ? 0 : tokens.length;;
        while ( i-- ){
            token = tokens[i];
            if ( Expr.relative[ (type = token.type) ] ) {
                break;
            }
            if((find =  Expr.find[ type ]))
                if( seed = find( token.matches[0],context ) ) {
                    tokens.splice( i, 1 );
                    selector = toSelector( tokens )
                    break;
                }
        }
    };
    // 根据selector,tokens(match防止对原tokens修改)生成superMatcher并调用
    compile( selector, match )( seed, context, results );
    return results;
}

// 对外入口
window.MSizzle = Sizzle;

})(window)
// 测试/*
console.log(MSizzle("a:not(.b)"))
console.log(Sizzle("a:not(.b)"))

1.整体的思路是，comple生成超级选择函数，这个函数主要用两种情况，一种是以前的elementMatcher,一种是setMatcher,用函数expando属性来区别执行哪条。本章的内容主要分析伪类筛选的基本筛选，大多数都是setMatcher路线，当然少数如:header，:root,：target则不是。

我们继续以一条路径来分析setMatcher路线，div:first，在setMatcher函数里，先根据前面的选择器执行Sizzle(’div‘),在这些div里进行基本筛选的first筛选。

2.markFunction，对函数进行标记并返回函数。Sizzle中根据选择器获取函数，如果有标记属性，则存入setMatchers。这是伪类路线，否则存入elementMatchers,一般路线。示例：

 var expando = "sizzle" + 1 * new Date();
 var filter = {
     'pseudos': markFunction(function(){

     }),
     'TAG': function(){

     }
 }
 function markFunction(fn){
     fn[expando] = true;
     return fn;
 }
 function devider(selectorType){
     if( filter[type][expando] ){
         console.log('伪类路线')
     }else{
         console.log('基本路线')
     }
 }
 devider('pseudos')

3.只要选择器中有走setMatchers路线的方法