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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs.

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/*
 * Copyright 2018 The Closure Compiler Authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.javascript.jscomp.lint;

import com.google.common.base.Joiner;
import com.google.javascript.jscomp.AbstractCompiler;
import com.google.javascript.jscomp.CompilerPass;
import com.google.javascript.jscomp.DiagnosticType;
import com.google.javascript.jscomp.NodeTraversal;
import com.google.javascript.jscomp.NodeTraversal.AbstractPreOrderCallback;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.QualifiedName;
import java.util.Set;
import java.util.TreeSet;

/** Checks the file structure of ES6 modules. */
public final class CheckEs6ModuleFileStructure extends AbstractPreOrderCallback
    implements CompilerPass {

  public static final DiagnosticType MUST_COME_BEFORE =
      DiagnosticType.warning(
          "JSC_MUST_COME_BEFORE_IN_ES6_MODULE", "In ES6 modules, {0} should come before {1}.");

  /** Statements that must appear in a certain order within ES6 modules (for the sake of style). */
  private enum OrderedStatement {
    IMPORT("import statements"),
    DECLARE_MODULE_ID("a call to goog.declareModuleId()"),
    GOOG_REQUIRE("calls to goog.require()"),
    OTHER("other statements");

    final String description;

    OrderedStatement(String description) {
      this.description = description;
    }

    @Override
    public String toString() {
      return description;
    }
  }

  private final AbstractCompiler compiler;
  private final TreeSet orderedStatements = new TreeSet<>();

  public CheckEs6ModuleFileStructure(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    switch (n.getToken()) {
      case ROOT:
        return true;
      case SCRIPT:
        return n.getBooleanProp(Node.ES6_MODULE);
      case MODULE_BODY:
        orderedStatements.clear();
        return true;
      case IMPORT:
        visitImport(t, n);
        return false;
      case EXPR_RESULT:
        return visitExprResult(t, n, parent);
      case VAR:
      case LET:
      case CONST:
        return visitDeclaration(t, n, parent);
      default:
        if (parent != null && parent.isModuleBody()) {
          checkOrder(t, n, OrderedStatement.OTHER);
        }
        return false;
    }
  }

  private void checkOrder(NodeTraversal t, Node n, OrderedStatement statement) {
    orderedStatements.add(statement);
    Set outOfOrder = orderedStatements.tailSet(statement, /* inclusive= */ false);
    if (!outOfOrder.isEmpty()) {
      t.report(n, MUST_COME_BEFORE, statement.toString(), Joiner.on(", ").join(outOfOrder));
    }
  }

  private static final QualifiedName GOOG_DECLAREMODULEID =
      QualifiedName.of("goog.declareModuleId");
  private static final QualifiedName GOOG_REQUIRE = QualifiedName.of("goog.require");

  private boolean visitExprResult(NodeTraversal t, Node exprResult, Node parent) {
    if (parent.isModuleBody() && exprResult.getFirstChild().isCall()) {
      Node call = exprResult.getFirstChild();
      if (GOOG_DECLAREMODULEID.matches(call.getFirstChild())) {
        checkOrder(t, call, OrderedStatement.DECLARE_MODULE_ID);
        return false;
      } else if (GOOG_REQUIRE.matches(call.getFirstChild())) {
        checkOrder(t, call, OrderedStatement.GOOG_REQUIRE);
        return false;
      }
    }

    checkOrder(t, exprResult, OrderedStatement.OTHER);
    return true;
  }

  private boolean visitDeclaration(NodeTraversal t, Node declaration, Node parent) {
    if (parent.isModuleBody()
        && declaration.hasOneChild()
        && declaration.getFirstChild().hasOneChild()
        && declaration.getFirstFirstChild().isCall()) {
      Node call = declaration.getFirstFirstChild();
      if (GOOG_REQUIRE.matches(call.getFirstChild())) {
        checkOrder(t, call, OrderedStatement.GOOG_REQUIRE);
        return false;
      }
    }

    checkOrder(t, declaration, OrderedStatement.OTHER);
    return true;
  }

  private void visitImport(NodeTraversal t, Node importNode) {
    checkOrder(t, importNode, OrderedStatement.IMPORT);
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverse(compiler, root, this);
  }
}