text: /* @flow */
  
  import { decode } from 'he'
  import { parseHTML } from './html-parser'
  import { parseText } from './text-parser'
  import { parseFilters } from './filter-parser'
  import { cached, no, camelize } from 'shared/util'
  import { genAssignmentCode } from '../directives/model'
  import { isIE, isEdge, isServerRendering } from 'core/util/env'
  
  import {
    addProp,
    addAttr,
    baseWarn,
    addHandler,
    addDirective,
    getBindingAttr,
    getAndRemoveAttr,
    pluckModuleFunction
  } from '../helpers'
  
  export const onRE = /^@|^v-on:/
  export const dirRE = /^v-|^@|^:/
  export const forAliasRE = /(.*?)\s+(?:in|of)\s+(.*)/
  export const forIteratorRE = /\((\{[^}]*\}|[^,]*),([^,]*)(?:,([^,]*))?\)/
  
  const argRE = /:(.*)$/
  const bindRE = /^:|^v-bind:/
  const modifierRE = /\.[^.]+/g
  
  const decodeHTMLCached = cached(decode)
  
  // configurable state
  export let warn
  let delimiters
  let transforms
  let preTransforms
  let postTransforms
  let platformIsPreTag
  let platformMustUseProp
  let platformGetTagNamespace
  
  /**
   * Convert HTML string to AST.
   */
  export function parse (
    template: string,
    options: CompilerOptions
  ): ASTElement | void {
    warn = options.warn || baseWarn
    platformGetTagNamespace = options.getTagNamespace || no
    platformMustUseProp = options.mustUseProp || no
    platformIsPreTag = options.isPreTag || no
    preTransforms = pluckModuleFunction(options.modules, 'preTransformNode')
    transforms = pluckModuleFunction(options.modules, 'transformNode')
    postTransforms = pluckModuleFunction(options.modules, 'postTransformNode')
    delimiters = options.delimiters
  
    const stack = []
    const preserveWhitespace = options.preserveWhitespace !== false
    let root
    let currentParent
    let inVPre = false
    let inPre = false
    let warned = false
  
    function warnOnce (msg) {
      if (!warned) {
        warned = true
        warn(msg)
      }
    }
  
    function endPre (element) {
      // check pre state
      if (element.pre) {
        inVPre = false
      }
      if (platformIsPreTag(element.tag)) {
        inPre = false
      }
    }
  
    parseHTML(template, {
      warn,
      expectHTML: options.expectHTML,
      isUnaryTag: options.isUnaryTag,
      canBeLeftOpenTag: options.canBeLeftOpenTag,
      shouldDecodeNewlines: options.shouldDecodeNewlines,
      start (tag, attrs, unary) {
        // check namespace.
        // inherit parent ns if there is one
        const ns = (currentParent && currentParent.ns) || platformGetTagNamespace(tag)
  
        // handle IE svg bug
        /* istanbul ignore if */
        if (isIE && ns === 'svg') {
          attrs = guardIESVGBug(attrs)
        }
  
        const element: ASTElement = {
          type: 1,
          tag,
          attrsList: attrs,
          attrsMap: makeAttrsMap(attrs),
          parent: currentParent,
          children: []
        }
        if (ns) {
          element.ns = ns
        }
  
        if (isForbiddenTag(element) && !isServerRendering()) {
          element.forbidden = true
          process.env.NODE_ENV !== 'production' && warn(
            'Templates should only be responsible for mapping the state to the ' +
            'UI. Avoid placing tags with side-effects in your templates, such as ' +
            `<${tag}>` + ', as they will not be parsed.'
          )
        }
  
        // apply pre-transforms
        for (let i = 0; i < preTransforms.length; i++) {
          preTransforms[i](element, options)
        }
  
        if (!inVPre) {
          processPre(element)
          if (element.pre) {
            inVPre = true
          }
        }
        if (platformIsPreTag(element.tag)) {
          inPre = true
        }
        if (inVPre) {
          processRawAttrs(element)
        } else {
          processFor(element)
          processIf(element)
          processOnce(element)
          processKey(element)
  
          // determine whether this is a plain element after
          // removing structural attributes
          element.plain = !element.key && !attrs.length
  
          processRef(element)
          processSlot(element)
          processComponent(element)
          for (let i = 0; i < transforms.length; i++) {
            transforms[i](element, options)
          }
          processAttrs(element)
        }
  
        function checkRootConstraints (el) {
          if (process.env.NODE_ENV !== 'production') {
            if (el.tag === 'slot' || el.tag === 'template') {
              warnOnce(
                `Cannot use <${el.tag}> as component root element because it may ` +
                'contain multiple nodes.'
              )
            }
            if (el.attrsMap.hasOwnProperty('v-for')) {
              warnOnce(
                'Cannot use v-for on stateful component root element because ' +
                'it renders multiple elements.'
              )
            }
          }
        }
  
        // tree management
        if (!root) {
          root = element
          checkRootConstraints(root)
        } else if (!stack.length) {
          // allow root elements with v-if, v-else-if and v-else
          if (root.if && (element.elseif || element.else)) {
            checkRootConstraints(element)
            addIfCondition(root, {
              exp: element.elseif,
              block: element
            })
          } else if (process.env.NODE_ENV !== 'production') {
            warnOnce(
              `Component template should contain exactly one root element. ` +
              `If you are using v-if on multiple elements, ` +
              `use v-else-if to chain them instead.`
            )
          }
        }
        if (currentParent && !element.forbidden) {
          if (element.elseif || element.else) {
            processIfConditions(element, currentParent)
          } else if (element.slotScope) { // scoped slot
            currentParent.plain = false
            const name = element.slotTarget || '"default"'
            ;(currentParent.scopedSlots || (currentParent.scopedSlots = {}))[name] = element
          } else {
            currentParent.children.push(element)
            element.parent = currentParent
          }
        }
        if (!unary) {
          currentParent = element
          stack.push(element)
        } else {
          endPre(element)
        }
        // apply post-transforms
        for (let i = 0; i < postTransforms.length; i++) {
          postTransforms[i](element, options)
        }
      },
  
      end () {
        // remove trailing whitespace
        const element = stack[stack.length - 1]
        const lastNode = element.children[element.children.length - 1]
        if (lastNode && lastNode.type === 3 && lastNode.text === ' ' && !inPre) {
          element.children.pop()
        }
        // pop stack
        stack.length -= 1
        currentParent = stack[stack.length - 1]
        endPre(element)
      },
  
      chars (text: string) {
        if (!currentParent) {
          if (process.env.NODE_ENV !== 'production') {
            if (text === template) {
              warnOnce(
                'Component template requires a root element, rather than just text.'
              )
            } else if ((text = text.trim())) {
              warnOnce(
                `text "${text}" outside root element will be ignored.`
              )
            }
          }
          return
        }
        // IE textarea placeholder bug
        /* istanbul ignore if */
        if (isIE &&
            currentParent.tag === 'textarea' &&
            currentParent.attrsMap.placeholder === text) {
          return
        }
        const children = currentParent.children
        text = inPre || text.trim()
          ? isTextTag(currentParent) ? text : decodeHTMLCached(text)
          // only preserve whitespace if its not right after a starting tag
          : preserveWhitespace && children.length ? ' ' : ''
        if (text) {
          let expression
          if (!inVPre && text !== ' ' && (expression = parseText(text, delimiters))) {
            children.push({
              type: 2,
              expression,
              text
            })
          } else if (text !== ' ' || !children.length || children[children.length - 1].text !== ' ') {
            children.push({
              type: 3,
              text
            })
          }
        }
      }
    })
    return root
  }
  
  function processPre (el) {
    if (getAndRemoveAttr(el, 'v-pre') != null) {
      el.pre = true
    }
  }
  
  function processRawAttrs (el) {
    const l = el.attrsList.length
    if (l) {
      const attrs = el.attrs = new Array(l)
      for (let i = 0; i < l; i++) {
        attrs[i] = {
          name: el.attrsList[i].name,
          value: JSON.stringify(el.attrsList[i].value)
        }
      }
    } else if (!el.pre) {
      // non root node in pre blocks with no attributes
      el.plain = true
    }
  }
  
  function processKey (el) {
    const exp = getBindingAttr(el, 'key')
    if (exp) {
      if (process.env.NODE_ENV !== 'production' && el.tag === 'template') {
        warn(`<template> cannot be keyed. Place the key on real elements instead.`)
      }
      el.key = exp
    }
  }
  
  function processRef (el) {
    const ref = getBindingAttr(el, 'ref')
    if (ref) {
      el.ref = ref
      el.refInFor = checkInFor(el)
    }
  }
  
  function processFor (el) {
    let exp
    if ((exp = getAndRemoveAttr(el, 'v-for'))) {
      const inMatch = exp.match(forAliasRE)
      if (!inMatch) {
        process.env.NODE_ENV !== 'production' && warn(
          `Invalid v-for expression: ${exp}`
        )
        return
      }
      el.for = inMatch[2].trim()
      const alias = inMatch[1].trim()
      const iteratorMatch = alias.match(forIteratorRE)
      if (iteratorMatch) {
        el.alias = iteratorMatch[1].trim()
        el.iterator1 = iteratorMatch[2].trim()
        if (iteratorMatch[3]) {
          el.iterator2 = iteratorMatch[3].trim()
        }
      } else {
        el.alias = alias
      }
    }
  }
  
  function processIf (el) {
    const exp = getAndRemoveAttr(el, 'v-if')
    if (exp) {
      el.if = exp
      addIfCondition(el, {
        exp: exp,
        block: el
      })
    } else {
      if (getAndRemoveAttr(el, 'v-else') != null) {
        el.else = true
      }
      const elseif = getAndRemoveAttr(el, 'v-else-if')
      if (elseif) {
        el.elseif = elseif
      }
    }
  }
  
  function processIfConditions (el, parent) {
    const prev = findPrevElement(parent.children)
    if (prev && prev.if) {
      addIfCondition(prev, {
        exp: el.elseif,
        block: el
      })
    } else if (process.env.NODE_ENV !== 'production') {
      warn(
        `v-${el.elseif ? ('else-if="' + el.elseif + '"') : 'else'} ` +
        `used on element <${el.tag}> without corresponding v-if.`
      )
    }
  }
  
  function findPrevElement (children: Array<any>): ASTElement | void {
    let i = children.length
    while (i--) {
      if (children[i].type === 1) {
        return children[i]
      } else {
        if (process.env.NODE_ENV !== 'production' && children[i].text !== ' ') {
          warn(
            `text "${children[i].text.trim()}" between v-if and v-else(-if) ` +
            `will be ignored.`
          )
        }
        children.pop()
      }
    }
  }
  
  function addIfCondition (el, condition) {
    if (!el.ifConditions) {
      el.ifConditions = []
    }
    el.ifConditions.push(condition)
  }
  
  function processOnce (el) {
    const once = getAndRemoveAttr(el, 'v-once')
    if (once != null) {
      el.once = true
    }
  }
  
  function processSlot (el) {
    if (el.tag === 'slot') {
      el.slotName = getBindingAttr(el, 'name')
      if (process.env.NODE_ENV !== 'production' && el.key) {
        warn(
          `\`key\` does not work on <slot> because slots are abstract outlets ` +
          `and can possibly expand into multiple elements. ` +
          `Use the key on a wrapping element instead.`
        )
      }
    } else {
      const slotTarget = getBindingAttr(el, 'slot')
      if (slotTarget) {
        el.slotTarget = slotTarget === '""' ? '"default"' : slotTarget
      }
      if (el.tag === 'template') {
        el.slotScope = getAndRemoveAttr(el, 'scope')
      }
    }
  }
  
  function processComponent (el) {
    let binding
    if ((binding = getBindingAttr(el, 'is'))) {
      el.component = binding
    }
    if (getAndRemoveAttr(el, 'inline-template') != null) {
      el.inlineTemplate = true
    }
  }
  
  function processAttrs (el) {
    const list = el.attrsList
    let i, l, name, rawName, value, modifiers, isProp
    for (i = 0, l = list.length; i < l; i++) {
      name = rawName = list[i].name
      value = list[i].value
      if (dirRE.test(name)) {
        // mark element as dynamic
        el.hasBindings = true
        // modifiers
        modifiers = parseModifiers(name)
        if (modifiers) {
          name = name.replace(modifierRE, '')
        }
        if (bindRE.test(name)) { // v-bind
          name = name.replace(bindRE, '')
          value = parseFilters(value)
          isProp = false
          if (modifiers) {
            if (modifiers.prop) {
              isProp = true
              name = camelize(name)
              if (name === 'innerHtml') name = 'innerHTML'
            }
            if (modifiers.camel) {
              name = camelize(name)
            }
            if (modifiers.sync) {
              addHandler(
                el,
                `update:${camelize(name)}`,
                genAssignmentCode(value, `$event`)
              )
            }
          }
          if (isProp || platformMustUseProp(el.tag, el.attrsMap.type, name)) {
            addProp(el, name, value)
          } else {
            addAttr(el, name, value)
          }
        } else if (onRE.test(name)) { // v-on
          name = name.replace(onRE, '')
          addHandler(el, name, value, modifiers, false, warn)
        } else { // normal directives
          name = name.replace(dirRE, '')
          // parse arg
          const argMatch = name.match(argRE)
          const arg = argMatch && argMatch[1]
          if (arg) {
            name = name.slice(0, -(arg.length + 1))
          }
          addDirective(el, name, rawName, value, arg, modifiers)
          if (process.env.NODE_ENV !== 'production' && name === 'model') {
            checkForAliasModel(el, value)
          }
        }
      } else {
        // literal attribute
        if (process.env.NODE_ENV !== 'production') {
          const expression = parseText(value, delimiters)
          if (expression) {
            warn(
              `${name}="${value}": ` +
              'Interpolation inside attributes has been removed. ' +
              'Use v-bind or the colon shorthand instead. For example, ' +
              'instead of <div id="{{ val }}">, use <div :id="val">.'
            )
          }
        }
        addAttr(el, name, JSON.stringify(value))
      }
    }
  }
  
  function checkInFor (el: ASTElement): boolean {
    let parent = el
    while (parent) {
      if (parent.for !== undefined) {
        return true
      }
      parent = parent.parent
    }
    return false
  }
  
  function parseModifiers (name: string): Object | void {
    const match = name.match(modifierRE)
    if (match) {
      const ret = {}
      match.forEach(m => { ret[m.slice(1)] = true })
      return ret
    }
  }
  
  function makeAttrsMap (attrs: Array<Object>): Object {
    const map = {}
    for (let i = 0, l = attrs.length; i < l; i++) {
      if (
        process.env.NODE_ENV !== 'production' &&
        map[attrs[i].name] && !isIE && !isEdge
      ) {
        warn('duplicate attribute: ' + attrs[i].name)
      }
      map[attrs[i].name] = attrs[i].value
    }
    return map
  }
  
  // for script (e.g. type="x/template") or style, do not decode content
  function isTextTag (el): boolean {
    return el.tag === 'script' || el.tag === 'style'
  }
  
  function isForbiddenTag (el): boolean {
    return (
      el.tag === 'style' ||
      (el.tag === 'script' && (
        !el.attrsMap.type ||
        el.attrsMap.type === 'text/javascript'
      ))
    )
  }
  
  const ieNSBug = /^xmlns:NS\d+/
  const ieNSPrefix = /^NS\d+:/
  
  /* istanbul ignore next */
  function guardIESVGBug (attrs) {
    const res = []
    for (let i = 0; i < attrs.length; i++) {
      const attr = attrs[i]
      if (!ieNSBug.test(attr.name)) {
        attr.name = attr.name.replace(ieNSPrefix, '')
        res.push(attr)
      }
    }
    return res
  }
  
  function checkForAliasModel (el, value) {
    let _el = el
    while (_el) {
      if (_el.for && _el.alias === value) {
        warn(
          `<${el.tag} v-model="${value}">: ` +
          `You are binding v-model directly to a v-for iteration alias. ` +
          `This will not be able to modify the v-for source array because ` +
          `writing to the alias is like modifying a function local variable. ` +
          `Consider using an array of objects and use v-model on an object property instead.`
        )
      }
      _el = _el.parent
    }
  }
  