jQuery.fn.find = function (selector) {
 var i, ret, len = this.length,
 self = this;
 // 这段话真不知道是个什么的
 if (typeof selector !== "string") {
 // fn.pushStack 和 jquery.merge 很像，但是返回一个 jquery 对象，且
 // jquery 有个 prevObject 属性指向自己
 return this.pushStack(jQuery(selector).filter(function () {
  for (i = 0; i < len; i++) {
  // jQuery.contains(a, b) 判断 a 是否是 b 的父代
  if (jQuery.contains(self[i], this)) {
   return true;
  }
  }
 }));
 }

 ret = this.pushStack([]);

 for (i = 0; i < len; i++) {
 // 在这里引用到 jQuery.find 函数
 jQuery.find(selector, self[i], ret);
 }
 // uniqueSort 去重函数
 return len > 1 ? jQuery.uniqueSort(ret) : ret;
}

var rquickExpr = /^(?:#([\w-]+)|(\w+)|\.([\w-]+))$/;
rquickExpr.exec('#id') //["#id", "id", undefined, undefined]
rquickExpr.exec('div') //["div", undefined, "div", undefined]
rquickExpr.exec('.test') //[".test", undefined, undefined, "test"]
rquickExpr.exec('div p')// null

jQuery.find = Sizzle;

function Sizzle(selector, context, results, seed) {
 var m, i, elem, nid, match, groups, newSelector, newContext = context && context.ownerDocument,

 // nodeType defaults to 9, since context defaults to document
 nodeType = context ? context.nodeType : 9;

 results = results || [];

 // Return early from calls with invalid selector or context
 if (typeof selector !== "string" || !selector || nodeType !== 1 && nodeType !== 9 && nodeType !== 11) {

 return results;
 }

 // Try to shortcut find operations (as opposed to filters) in HTML documents
 if (!seed) {

 if ((context ? context.ownerDocument || context : preferredDoc) !== document) {
  // setDocument 函数其实是用来将 context 设置成 document，考虑到浏览器的兼容性
  setDocument(context);
 }
 context = context || document;
 // true
 if (documentIsHTML) {

  // match 就是那个有规律的数组
  if (nodeType !== 11 && (match = rquickExpr.exec(selector))) {

  // selector 是 id 的情况
  if ((m = match[1])) {

   // Document context
   if (nodeType === 9) {
   if ((elem = context.getElementById(m))) {

    if (elem.id === m) {
     results.push(elem);
     return results;
    }
   } else {
    return results;
   }

   // 非 document 的情况
   } else {

   if (newContext && (elem = newContext.getElementById(m)) && contains(context, elem) && elem.id === m) {

    results.push(elem);
    return results;
   }
   }

  // selector 是 tagName 情况
  } else if (match[2]) {
   // 这里的 push：var push = arr.push
   push.apply(results, context.getElementsByTagName(selector));
   return results;

  // selector 是 class 情况
  } else if ((m = match[3]) && support.getElementsByClassName && context.getElementsByClassName) {

   push.apply(results, context.getElementsByClassName(m));
   return results;
  }
  }

  // 如果浏览器支持 querySelectorAll
  if (support.qsa && !compilerCache[selector + " "] && (!rbuggyQSA || !rbuggyQSA.test(selector))) {

  if (nodeType !== 1) {
   newContext = context;
   newSelector = selector;

   // qSA looks outside Element context, which is not what we want
   // Support: IE <=8,还是要考虑兼容性
  } else if (context.nodeName.toLowerCase() !== "object") {

   // Capture the context ID, setting it first if necessary
   if ((nid = context.getAttribute("id"))) {
   nid = nid.replace(rcssescape, fcssescape);
   } else {
   context.setAttribute("id", (nid = expando));
   }

   // Sizzle 词法分析的部分
   groups = tokenize(selector);
   i = groups.length;
   while (i--) {
   groups[i] = "#" + nid + " " + toSelector(groups[i]);
   }
   newSelector = groups.join(",");

   // Expand context for sibling selectors
   newContext = rsibling.test(selector) && testContext(context.parentNode) || context;
  }

  if (newSelector) {
   try {
   push.apply(results, newContext.querySelectorAll(newSelector));
   return results;
   } catch(qsaError) {} finally {
   if (nid === expando) {
    context.removeAttribute("id");
   }
   }
  }
  }
 }
 }

 // All others，select 函数和 tokenize 函数后文再谈
 return select(selector.replace(rtrim, "$1"), context, results, seed);
}

jQuery.fn.pushStack = function (elems) {

 // Build a new jQuery matched element set
 var ret = jQuery.merge(this.constructor(), elems);

 // Add the old object onto the stack (as a reference)
 ret.prevObject = this;

 // Return the newly-formed element set
 return ret;
}

jQuery.contains = function (context, elem) {
 // 考虑到兼容性，设置 context 的值
 if ((context.ownerDocument || context) !== document) {
 setDocument(context);
 }
 return contains(context, elem);
}

// contains 是内部函数，判断 DOM_a 是否是 DOM_b 的
var contains = function (a, b) {
 var adown = a.nodeType === 9 ? a.documentElement : a,
 bup = b && b.parentNode;
 return a === bup || !!(bup && bup.nodeType === 1 && (
 adown.contains ? adown.contains(bup) : a.compareDocumentPosition && a.compareDocumentPosition(bup) & 16));
}

jQuery.uniqueSort = function (results) {
 var elem, duplicates = [],
 j = 0,
 i = 0;

 // hasDuplicate 是一个判断是否有相同元素的 flag，全局
 hasDuplicate = !support.detectDuplicates;
 sortInput = !support.sortStable && results.slice(0);
 results.sort(sortOrder);

 if (hasDuplicate) {
 while ((elem = results[i++])) {
  if (elem === results[i]) {
   j = duplicates.push(i);
  }
 }
 while (j--) {
  // splice 用于将重复的元素删除
  results.splice(duplicates[j], 1);
 }
 }

 // Clear input after sorting to release objects
 // See https://github.com/jquery/sizzle/pull/225
 sortInput = null;

 return results;
}

var sortOrder = function (a, b) {

 // 表示有相同的元素，设置 flag 为 true
 if (a === b) {
 hasDuplicate = true;
 return 0;
 }

 // Sort on method existence if only one input has compareDocumentPosition
 var compare = !a.compareDocumentPosition - !b.compareDocumentPosition;
 if (compare) {
 return compare;
 }

 // Calculate position if both inputs belong to the same document
 compare = (a.ownerDocument || a) === (b.ownerDocument || b) ? a.compareDocumentPosition(b) :

 // Otherwise we know they are disconnected
 1;

 // Disconnected nodes
 if (compare & 1 || (!support.sortDetached && b.compareDocumentPosition(a) === compare)) {

 // Choose the first element that is related to our preferred document
 if (a === document || a.ownerDocument === preferredDoc && contains(preferredDoc, a)) {
  return -1;
 }
 if (b === document || b.ownerDocument === preferredDoc && contains(preferredDoc, b)) {
  return 1;
 }

 // Maintain original order
 return sortInput ? (indexOf(sortInput, a) - indexOf(sortInput, b)) : 0;
 }

 return compare & 4 ? -1 : 1;
}