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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 2014 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 static com.google.common.base.Preconditions.checkArgument;

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.AbstractPostOrderCallback;
import com.google.javascript.jscomp.NodeUtil;
import com.google.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.JSTypeExpression;
import com.google.javascript.rhino.Node;
import java.util.HashSet;
import java.util.Set;

/**
 * Checks the following:
 *
 * 
 *   Whether there are duplicate values in enums.
 *   
Whether enum type (and its initializer values) are string/number or not.
 *   
Whether string enum values are statically initialized or not.
 * 
 */
public final class CheckEnums extends AbstractPostOrderCallback implements CompilerPass {
  public static final DiagnosticType DUPLICATE_ENUM_VALUE = DiagnosticType.disabled(
      "JSC_DUPLICATE_ENUM_VALUE",
      "The value {0} is duplicated in this enum.");
  public static final DiagnosticType COMPUTED_PROP_NAME_IN_ENUM = DiagnosticType.disabled(
      "JSC_COMPUTED_PROP_NAME_IN_ENUM",
      "Computed property name used in enum.");
  public static final DiagnosticType SHORTHAND_ASSIGNMENT_IN_ENUM = DiagnosticType.disabled(
      "JSC_SHORTHAND_ASSIGNMENT_IN_ENUM",
      "Shorthand assignment used in enum.");
  public static final DiagnosticType ENUM_PROP_NOT_CONSTANT = DiagnosticType.disabled(
      "JSC_ENUM_PROP_NOT_CONSTANT",
      "enum key {0} must be in ALL_CAPS.");

  public static final DiagnosticType ENUM_TYPE_NOT_STRING_OR_NUMBER =
      DiagnosticType.disabled(
          "JSC_ENUM_VALUE_NOT_STRING_OR_NUMBER",
          "enum type must be either string or number."
          );

  public static final DiagnosticType NON_STATIC_INITIALIZER_STRING_VALUE_IN_ENUM =
      DiagnosticType.disabled(
          "JSC_NON_STATIC_INITIALIZER_STRING_VALUE_IN_ENUM",
          "Enum string values must be statically initialized as per the style guide."
          );
  private final AbstractCompiler compiler;

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

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

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    if (n.isObjectLit()) {
      JSDocInfo jsdoc = NodeUtil.getBestJSDocInfo(n);
      if (jsdoc != null && jsdoc.hasEnumParameterType()) {
        checkNamingAndAssignmentUsage(t, n);
        checkDuplicateEnumValues(t, n);
        checkEnumTypeAndInitializerValues(t, n, jsdoc);
      }
    }
  }

  private static void checkEnumTypeAndInitializerValues(
      NodeTraversal t, Node n, JSDocInfo jsDocInfo) {
    checkArgument(n.isObjectLit(), n);
    JSTypeExpression enumTypeExpr = jsDocInfo.getEnumParameterType();

    Node enumType = enumTypeExpr.getRoot();
    boolean isStringEnum = enumType.isStringLit() && enumType.getString().equals("string");
    boolean isNumberEnum = enumType.isStringLit() && enumType.getString().equals("number");
    if (!isStringEnum && !isNumberEnum) {
      // warn on `@enum {?}`, `@enum {boolean}`, `@enum {Some|Another}`, `@enum {SomeName}` etc`
      t.report(n, ENUM_TYPE_NOT_STRING_OR_NUMBER);
    }
    if (isStringEnum) {
      checkStringEnumInitializerValues(t, n);
    }
  }

  // Reports a warning if the string enum value is not statically initialized
  private static void checkStringEnumInitializerValues(NodeTraversal t, Node enumNode) {
    checkArgument(enumNode.isObjectLit(), enumNode);
    for (Node prop = enumNode.getFirstChild(); prop != null; prop = prop.getNext()) {
      // valueNode is guaranteed to exist by this time, as shorthand `{A}`s are converted to `{A:A}`
      Node valueNode = prop.getLastChild();
      if (!valueNode.isStringLit() && !(valueNode.isTemplateLit() && valueNode.hasOneChild())) {
        // neither string nor substitution-free template literal; report finding.
        t.report(valueNode, NON_STATIC_INITIALIZER_STRING_VALUE_IN_ENUM);
      }
    }
  }

  private void checkNamingAndAssignmentUsage(NodeTraversal t, Node objLit) {
    for (Node child = objLit.getFirstChild(); child != null; child = child.getNext()) {
      checkName(t, child);
    }
  }

  private void checkName(NodeTraversal t, Node prop) {
    if (prop.isComputedProp()) {
      t.report(prop, COMPUTED_PROP_NAME_IN_ENUM);
      return;
    }

    if (prop.isStringKey() && prop.isShorthandProperty()) {
      t.report(prop, SHORTHAND_ASSIGNMENT_IN_ENUM);
    }

    if (!compiler.getCodingConvention().isValidEnumKey(prop.getString())) {
      t.report(prop, ENUM_PROP_NOT_CONSTANT);
    }
  }

  private static void checkDuplicateEnumValues(NodeTraversal t, Node enumNode) {
    Set values = new HashSet<>();
    for (Node prop = enumNode.getFirstChild(); prop != null; prop = prop.getNext()) {
      Node valueNode = prop.getLastChild();
      String value;
      if (valueNode == null) {
        return;
      } else if (valueNode.isStringLit()) {
        value = valueNode.getString();
      } else if (valueNode.isNumber()) {
        value = Double.toString(valueNode.getDouble());
      } else {
        return;
      }

      if (!values.add(value)) {
        t.report(valueNode, DUPLICATE_ENUM_VALUE, value);
      }
    }
  }
}