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export function generate(
  ast: RootNode,
  options: CodegenOptions & {
    onContextCreated?: (context: CodegenContext) => void
  } = {}
): CodegenResult {
  // 创建代码生成上下文
  const context = createCodegenContext(ast, options)
  // 执行创建完毕勾子
  if (options.onContextCreated) options.onContextCreated(context)
    
  // 解构出结果
  const {
    mode,
    push,
    prefixIdentifiers,
    indent,
    deindent,
    newline,
    scopeId,
    ssr
  } = context

  const hasHelpers = ast.helpers.length > 0
  const useWithBlock = !prefixIdentifiers && mode !== 'module'
  const genScopeId = !__BROWSER__ && scopeId != null && mode === 'module'
  const isSetupInlined = !__BROWSER__ && !!options.inline

  // preambles
  // in setup() inline mode, the preamble is generated in a sub context
  // and returned separately.
  const preambleContext = isSetupInlined
    ? createCodegenContext(ast, options)
    : context
  if (!__BROWSER__ && mode === 'module') {
    genModulePreamble(ast, preambleContext, genScopeId, isSetupInlined)
  } else {
    genFunctionPreamble(ast, preambleContext)
  }
  // enter render function
  const functionName = ssr ? `ssrRender` : `render`
  const args = ssr ? ['_ctx', '_push', '_parent', '_attrs'] : ['_ctx', '_cache']
  if (!__BROWSER__ && options.bindingMetadata && !options.inline) {
    // binding optimization args
    args.push('$props', '$setup', '$data', '$options')
  }
  const signature =
    !__BROWSER__ && options.isTS
      ? args.map(arg => `${arg}: any`).join(',')
      : args.join(', ')

  if (isSetupInlined) {
    push(`(${signature}) => {`)
  } else {
    push(`function ${functionName}(${signature}) {`)
  }
  indent()

  if (useWithBlock) {
    push(`with (_ctx) {`)
    indent()
    // function mode const declarations should be inside with block
    // also they should be renamed to avoid collision with user properties
    if (hasHelpers) {
      push(`const { ${ast.helpers.map(aliasHelper).join(', ')} } = _Vue`)
      push(`\n`)
      newline()
    }
  }

  // generate asset resolution statements
  if (ast.components.length) {
    genAssets(ast.components, 'component', context)
    if (ast.directives.length || ast.temps > 0) {
      newline()
    }
  }
  if (ast.directives.length) {
    genAssets(ast.directives, 'directive', context)
    if (ast.temps > 0) {
      newline()
    }
  }
  if (__COMPAT__ && ast.filters && ast.filters.length) {
    newline()
    genAssets(ast.filters, 'filter', context)
    newline()
  }

  if (ast.temps > 0) {
    push(`let `)
    for (let i = 0; i < ast.temps; i++) {
      push(`${i > 0 ? `, ` : ``}_temp${i}`)
    }
  }
  if (ast.components.length || ast.directives.length || ast.temps) {
    push(`\n`)
    newline()
  }

  // generate the VNode tree expression
  if (!ssr) {
    push(`return `)
  }
  if (ast.codegenNode) {
    genNode(ast.codegenNode, context)
  } else {
    push(`null`)
  }

  if (useWithBlock) {
    deindent()
    push(`}`)
  }

  deindent()
  push(`}`)

  return {
    ast,
    code: context.code,
    preamble: isSetupInlined ? preambleContext.code : ``,
    // SourceMapGenerator does have toJSON() method but it's not in the types
    map: context.map ? (context.map as any).toJSON() : undefined
  }
}

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function genFunctionPreamble(ast: RootNode, context: CodegenContext) {
  const {
    ssr,
    prefixIdentifiers,
    push,
    newline,
    runtimeModuleName,
    runtimeGlobalName,
    ssrRuntimeModuleName
  } = context
  const VueBinding =
    !__BROWSER__ && ssr
      ? `require(${JSON.stringify(runtimeModuleName)})`
      : runtimeGlobalName
  // Generate const declaration for helpers
  // In prefix mode, we place the const declaration at top so it's done
  // only once; But if we not prefixing, we place the declaration inside the
  // with block so it doesn't incur the `in` check cost for every helper access.
  // 优化点： 
  // 前缀模式下，官方把声明放到函数内顶层作用域内，所以整个函数内只会执行一次
  // 非前缀模式下，官方把声明放到函数内with作用域下，以此减少每个helper函数执行前判断是否在上下文的消耗
  if (ast.helpers.length > 0) {
    if (!__BROWSER__ && prefixIdentifiers) {
      push(
        `const { ${ast.helpers.map(aliasHelper).join(', ')} } = ${VueBinding}\n`
      )
    } else {
      // "with" mode.
      // save Vue in a separate variable to avoid collision
      // 把vue顶层声明重新命名，防止混乱
      push(`const _Vue = ${VueBinding}\n`)
      // in "with" mode, helpers are declared inside the with block to avoid
      // has check cost, but hoists are lifted out of the function - we need
      // to provide the helper here.
      if (ast.hoists.length) {
      // 以下几个静态函数是一定会导入到作用域内
        const staticHelpers = [
          CREATE_VNODE,
          CREATE_ELEMENT_VNODE,
          CREATE_COMMENT,
          CREATE_TEXT,
          CREATE_STATIC
        ]
          .filter(helper => ast.helpers.includes(helper))
          .map(aliasHelper)
          .join(', ')
        push(`const { ${staticHelpers} } = _Vue\n`)
      }
    }
  }
  // generate variables for ssr helpers
  if (!__BROWSER__ && ast.ssrHelpers && ast.ssrHelpers.length) {
    // ssr guarantees prefixIdentifier: true
    push(
      `const { ${ast.ssrHelpers
        .map(aliasHelper)
        .join(', ')} } = require("${ssrRuntimeModuleName}")\n`
    )
  }
  // 生成静态节点
  genHoists(ast.hoists, context)
  newline()
  push(`return `)
}

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function genHoists(hoists: (JSChildNode | null)[], context: CodegenContext) {
  if (!hoists.length) {
    return
  }
  context.pure = true
  const { push, newline, helper, scopeId, mode } = context
  const genScopeId = !__BROWSER__ && scopeId != null && mode !== 'function'
  newline()

  // generate inlined withScopeId helper
  // 
  if (genScopeId) {
    push(
      `const _withScopeId = n => (${helper(
        PUSH_SCOPE_ID
      )}("${scopeId}"),n=n(),${helper(POP_SCOPE_ID)}(),n)`
    )
    newline()
  }

  for (let i = 0; i < hoists.length; i++) {
    const exp = hoists[i]
    if (exp) {
      const needScopeIdWrapper = genScopeId && exp.type === NodeTypes.VNODE_CALL
      push(
        `const _hoisted_${i + 1} = ${
          needScopeIdWrapper ? `${PURE_ANNOTATION} _withScopeId(() => ` : ``
        }`
      )
      // 生成node代码
      genNode(exp, context)
      if (needScopeIdWrapper) {
        push(`)`)
      }
      newline()
    }
  }

  context.pure = false
}

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function genNode(node: CodegenNode | symbol | string, context: CodegenContext) {
  if (isString(node)) {
    context.push(node)
    return
  }
  if (isSymbol(node)) {
    context.push(context.helper(node))
    return
  }
  switch (node.type) {
    // 元素、if、for节点
    case NodeTypes.ELEMENT:
    case NodeTypes.IF:
    case NodeTypes.FOR:
      __DEV__ &&
        assert(
          node.codegenNode != null,
          `Codegen node is missing for element/if/for node. ` +
            `Apply appropriate transforms first.`
        )
      genNode(node.codegenNode!, context)
      break
    // 文本节点
    case NodeTypes.TEXT:
      genText(node, context)
      break
          
    case NodeTypes.SIMPLE_EXPRESSION:
      genExpression(node, context)
      break
    // 插值表达式 {{}}
    case NodeTypes.INTERPOLATION:
      genInterpolation(node, context)
      break
    // 文本函数
    case NodeTypes.TEXT_CALL:
      genNode(node.codegenNode, context)
      break
    case NodeTypes.COMPOUND_EXPRESSION:
      genCompoundExpression(node, context)
      break
    // 注释
    case NodeTypes.COMMENT:
      genComment(node, context)
      break
    // 
    case NodeTypes.VNODE_CALL:
      genVNodeCall(node, context)
      break
    // js执行表达式
    case NodeTypes.JS_CALL_EXPRESSION:
      genCallExpression(node, context)
      break
    // js对象表达式
    case NodeTypes.JS_OBJECT_EXPRESSION:
      genObjectExpression(node, context)
      break
    // js数组表达式
    case NodeTypes.JS_ARRAY_EXPRESSION:
      genArrayExpression(node, context)
      break
    // js函数表达式
    case NodeTypes.JS_FUNCTION_EXPRESSION:
      genFunctionExpression(node, context)
      break
    // js条件表达式
    case NodeTypes.JS_CONDITIONAL_EXPRESSION:
      genConditionalExpression(node, context)
      break
    // js缓存表达式
    case NodeTypes.JS_CACHE_EXPRESSION:
      genCacheExpression(node, context)
      break
    // js块状语句
    case NodeTypes.JS_BLOCK_STATEMENT:
      genNodeList(node.body, context, true, false)
      break

    // SSR only types
   	// ...ssr 相关的条件

    /* istanbul ignore next */
    case NodeTypes.IF_BRANCH:
      // noop
      break
    default:
      if (__DEV__) {
        assert(false, `unhandled codegen node type: ${(node as any).type}`)
        // make sure we exhaust all possible types
        const exhaustiveCheck: never = node
        return exhaustiveCheck
      }
  }
}

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function genText(
  node: TextNode | SimpleExpressionNode,
  context: CodegenContext
) {
  context.push(JSON.stringify(node.content), node)
}

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function genExpression(node: SimpleExpressionNode, context: CodegenContext) {
  const { content, isStatic } = node
  context.push(isStatic ? JSON.stringify(content) : content, node)
}

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function genInterpolation(node: InterpolationNode, context: CodegenContext) {
  const { push, helper, pure } = context
  if (pure) push(PURE_ANNOTATION)
  push(`${helper(TO_DISPLAY_STRING)}(`)
  genNode(node.content, context)
  push(`)`)
}

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function genCompoundExpression(
  node: CompoundExpressionNode,
  context: CodegenContext
) {
  for (let i = 0; i < node.children!.length; i++) {
    const child = node.children![i]
    if (isString(child)) {
      context.push(child)
    } else {
      genNode(child, context)
    }
  }
}

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function genComment(node: CommentNode, context: CodegenContext) {
  const { push, helper, pure } = context
  if (pure) {
    push(PURE_ANNOTATION)
  }
  push(`${helper(CREATE_COMMENT)}(${JSON.stringify(node.content)})`, node)
}

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function genVNodeCall(node: VNodeCall, context: CodegenContext) {
  const { push, helper, pure } = context
  const {
    tag,
    props,
    children,
    patchFlag,
    dynamicProps,
    directives,
    isBlock,
    disableTracking,
    isComponent
  } = node
  // 有指令函数，首先生成指令代码
  if (directives) {
    push(helper(WITH_DIRECTIVES) + `(`)
  }
  if (isBlock) {
    push(`(${helper(OPEN_BLOCK)}(${disableTracking ? `true` : ``}), `)
  }
  // 静态节点声明
  if (pure) {
    push(PURE_ANNOTATION)
  }
  const callHelper: symbol = isBlock
    ? getVNodeBlockHelper(context.inSSR, isComponent)
    : getVNodeHelper(context.inSSR, isComponent)
  push(helper(callHelper) + `(`, node)
  // nodelist 代码生成
  genNodeList(
    // 这里把数值中不存在的变量替换成null
    genNullableArgs([tag, props, children, patchFlag, dynamicProps]),
    context
  )
  push(`)`)
  // 结尾括号生成
  if (isBlock) {
    push(`)`)
  }
  if (directives) {
    push(`, `)
    // 生成指令代码
    genNode(directives, context)
    push(`)`)
  }
}

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function genNodeList(
  nodes: (string | symbol | CodegenNode | TemplateChildNode[])[],
  context: CodegenContext,
  multilines: boolean = false,
  comma: boolean = true
) {
  const { push, newline } = context
  // 遍历
  for (let i = 0; i < nodes.length; i++) {
    const node = nodes[i]
    // 根据不同的类型生成不同的代码
    if (isString(node)) {
      push(node)
    } else if (isArray(node)) {
      genNodeListAsArray(node, context)
    } else {
      genNode(node, context)
    }
    // 格式化代码
    if (i < nodes.length - 1) {
      if (multilines) {
        comma && push(',')
        newline()
      } else {
        comma && push(', ')
      }
    }
  }
}

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function genNodeListAsArray(
  nodes: (string | CodegenNode | TemplateChildNode[])[],
  context: CodegenContext
) {
  const multilines =
    nodes.length > 3 ||
    ((!__BROWSER__ || __DEV__) && nodes.some(n => isArray(n) || !isText(n)))
  context.push(`[`)
  // 缩进
  multilines && context.indent()
  // 生成数组节点代码
  genNodeList(nodes, context, multilines)
  multilines && context.deindent()
  context.push(`]`)
}

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function genCallExpression(node: CallExpression, context: CodegenContext) {
  const { push, helper, pure } = context
  // 获取调用函数名
  const callee = isString(node.callee) ? node.callee : helper(node.callee)
  if (pure) {
    push(PURE_ANNOTATION)
  }
  // 组装，括号调用形式
  push(callee + `(`, node)
  // 参数节点代码
  genNodeList(node.arguments, context)
  push(`)`)
}

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function genObjectExpression(node: ObjectExpression, context: CodegenContext) {
  const { push, indent, deindent, newline } = context
  const { properties } = node
  // properties长度为0，返回一个空对象
  if (!properties.length) {
    push(`{}`, node)
    return
  }
  // 是否是多行
  const multilines =
    properties.length > 1 ||
    ((!__BROWSER__ || __DEV__) &&
      properties.some(p => p.value.type !== NodeTypes.SIMPLE_EXPRESSION))
  // 
  push(multilines ? `{` : `{ `)
  // 缩进
  multilines && indent()
  for (let i = 0; i < properties.length; i++) {
    const { key, value } = properties[i]
    // key
    // 生成key值插值形式 { [`${key}`]: value }
    genExpressionAsPropertyKey(key, context)
    push(`: `)
    // value
    genNode(value, context)
    if (i < properties.length - 1) {
      // will only reach this if it's multilines
      push(`,`)
      newline()
    }
  }
  multilines && deindent()
  push(multilines ? `}` : ` }`)
}

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function genFunctionExpression(
  node: FunctionExpression,
  context: CodegenContext
) {
  const { push, indent, deindent } = context
  const { params, returns, body, newline, isSlot } = node
  if (isSlot) {
    // wrap slot functions with owner context
    // slot外面包含一层函数
    push(`_${helperNameMap[WITH_CTX]}(`)
  }
  push(`(`, node)
  // 插入参数列表
  if (isArray(params)) {
    genNodeList(params, context)
  } else if (params) {
    genNode(params, context)
  }
  // 箭头函数
  push(`) => `)
  // 函数主体
  if (newline || body) {
    push(`{`)
    indent()
  }
  // 函数返回
  if (returns) {
    if (newline) {
      push(`return `)
    }
    // 返回结果
    if (isArray(returns)) {
      genNodeListAsArray(returns, context)
    } else {
      genNode(returns, context)
    }
  } else if (body) {
    // 函数主体代码
    genNode(body, context)
  }
  if (newline || body) {
    deindent()
    push(`}`)
  }
  if (isSlot) {
    // slot的后续返回包裹
    if (__COMPAT__ && node.isNonScopedSlot) {
      push(`, undefined, true`)
    }
    push(`)`)
  }
}