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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;

import com.google.common.base.Joiner;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Ordering;
import com.google.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.JSDocInfo.Visibility;
import com.google.javascript.rhino.JSTypeExpression;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Set;

/**
 * Prints a JSDocInfo, used for preserving type annotations in ES6 transpilation.
 *
 */
public final class JSDocInfoPrinter {
  public static String print(JSDocInfo info) {
    boolean multiline = false;

    List parts = new ArrayList<>();

    // order:
    //   export|public|private|package|protected
    //   const
    //   dict|struct|unrestricted
    //   constructor|interface|record
    //   extends
    //   implements
    //   this
    //   param
    //   return
    //   throws
    //   template
    //   override
    //   type|define|typedef|enum
    //   suppress
    //   deprecated
    parts.add("/**");

    if (info.isExport()) {
      parts.add("@export");
    } else if (info.getVisibility() != null
        && info.getVisibility() != Visibility.INHERITED) {
      parts.add("@" + info.getVisibility().toString().toLowerCase());
    }

    if (info.isConstant() && !info.isDefine()) {
      parts.add("@const");
    }

    if (info.makesDicts()) {
      parts.add("@dict");
    }

    if (info.makesStructs()) {
      parts.add("@struct");
    }

    if (info.makesUnrestricted()) {
      parts.add("@unrestricted ");
    }

    if (info.isConstructor()) {
      parts.add("@constructor");
    }

    if (info.isInterface() && !info.usesImplicitMatch()) {
      parts.add("@interface");
    }

    if (info.isInterface() && info.usesImplicitMatch()) {
      parts.add("@record");
    }

    if (info.hasBaseType()) {
      multiline = true;
      Node typeNode = stripBang(info.getBaseType().getRoot());
      parts.add(buildAnnotationWithType("extends", typeNode));
    }

    for (JSTypeExpression type : info.getExtendedInterfaces()) {
      multiline = true;
      Node typeNode = stripBang(type.getRoot());
      parts.add(buildAnnotationWithType("extends", typeNode));
    }

    for (JSTypeExpression type : info.getImplementedInterfaces()) {
      multiline = true;
      Node typeNode = stripBang(type.getRoot());
      parts.add(buildAnnotationWithType("implements", typeNode));
    }

    if (info.hasThisType()) {
      multiline = true;
      Node typeNode = stripBang(info.getThisType().getRoot());
      parts.add(buildAnnotationWithType("this", typeNode));
    }

    if (info.getParameterCount() > 0) {
      multiline = true;
      for (String name : info.getParameterNames()) {
        parts.add("@param " + buildParamType(name, info.getParameterType(name)));
      }
    }

    if (info.hasReturnType()) {
      multiline = true;
      parts.add(buildAnnotationWithType("return", info.getReturnType()));
    }

    if (!info.getThrownTypes().isEmpty()) {
      parts.add(buildAnnotationWithType("throws", info.getThrownTypes().get(0)));
    }

    ImmutableList names = info.getTemplateTypeNames();
    if (!names.isEmpty()) {
      parts.add("@template " + Joiner.on(',').join(names));
      multiline = true;
    }

    if (info.isOverride()) {
      parts.add("@override");
    }

   if (info.hasType() && !info.isDefine()) {
      if (info.isInlineType()) {
        parts.add(typeNode(info.getType().getRoot()));
      } else {
        parts.add(buildAnnotationWithType("type", info.getType()));
      }
    }

    if (info.isDefine()) {
      parts.add(buildAnnotationWithType("define", info.getType()));
    }

    if (info.hasTypedefType()) {
      parts.add(buildAnnotationWithType("typedef", info.getTypedefType()));
    }

    if (info.hasEnumParameterType()) {
      parts.add(buildAnnotationWithType("enum", info.getEnumParameterType()));
    }

    Set suppressions = info.getSuppressions();
    if (!suppressions.isEmpty()) {
      // Print suppressions in sorted order to avoid non-deterministic output.
      String[] arr = suppressions.toArray(new String[0]);
      Arrays.sort(arr, Ordering.natural());
      parts.add("@suppress {" + Joiner.on(',').join(arr) + "}");
      multiline = true;
    }

    if (info.isDeprecated()) {
      parts.add("@deprecated " + info.getDeprecationReason());
      multiline = true;
    }

    parts.add("*/");

    StringBuilder sb = new StringBuilder();
    if (multiline) {
      Joiner.on("\n ").appendTo(sb, parts);
    } else {
      Joiner.on(" ").appendTo(sb, parts);
    }
    sb.append((multiline) ? "\n" : " ");
    return sb.toString();
  }

  private static Node stripBang(Node typeNode) {
    if (typeNode.getType() == Token.BANG) {
      typeNode = typeNode.getFirstChild();
    }
    return typeNode;
  }

  private static String buildAnnotationWithType(String annotation, JSTypeExpression type) {
    return buildAnnotationWithType(annotation, type.getRoot());
  }

  private static String buildAnnotationWithType(String annotation, Node type) {
    StringBuilder sb = new StringBuilder();
    sb.append("@");
    sb.append(annotation);
    sb.append(" {");
    appendTypeNode(sb, type);
    sb.append("}");
    return sb.toString();
  }

  private static String buildParamType(String name, JSTypeExpression type) {
    if (type != null) {
      return "{" + typeNode(type.getRoot()) + "} " + name;
    } else {
      return name;
    }
  }

  private static String typeNode(Node typeNode) {
    StringBuilder sb = new StringBuilder();
    appendTypeNode(sb, typeNode);
    return sb.toString();
  }

  private static void appendTypeNode(StringBuilder sb, Node typeNode) {
    if (typeNode.getType() == Token.BANG) {
      sb.append("!");
      appendTypeNode(sb, typeNode.getFirstChild());
    } else if (typeNode.getType() == Token.EQUALS) {
      appendTypeNode(sb, typeNode.getFirstChild());
      sb.append("=");
    } else if (typeNode.getType() == Token.PIPE) {
      sb.append("(");
      for (int i = 0; i < typeNode.getChildCount() - 1; i++) {
        appendTypeNode(sb, typeNode.getChildAtIndex(i));
        sb.append("|");
      }
      appendTypeNode(sb, typeNode.getLastChild());
      sb.append(")");
    } else if (typeNode.getType() == Token.ELLIPSIS) {
      sb.append("...");
      if (typeNode.hasChildren()) {
        appendTypeNode(sb, typeNode.getFirstChild());
      }
    } else if (typeNode.getType() == Token.STAR) {
      sb.append("*");
    } else if (typeNode.getType() == Token.QMARK) {
      sb.append("?");
      if (typeNode.hasChildren()) {
        appendTypeNode(sb, typeNode.getFirstChild());
      }
    } else if (typeNode.isFunction()) {
      appendFunctionNode(sb, typeNode);
    } else if (typeNode.getType() == Token.LC) {
      sb.append("{");
      Node lb = typeNode.getFirstChild();
      for (int i = 0; i < lb.getChildCount() - 1; i++) {
        Node colon = lb.getChildAtIndex(i);
        if (colon.hasChildren()) {
          sb.append(colon.getFirstChild().getString()).append(":");
          appendTypeNode(sb, colon.getLastChild());
        } else {
          sb.append(colon.getString());
        }
        sb.append(",");
      }
      Node lastColon = lb.getLastChild();
      if (lastColon.hasChildren()) {
        sb.append(lastColon.getFirstChild().getString()).append(":");
        appendTypeNode(sb, lastColon.getLastChild());
      } else {
        sb.append(lastColon.getString());
      }
      sb.append("}");
    } else if (typeNode.getType() == Token.VOID) {
      sb.append("void");
    } else {
      if (typeNode.hasChildren()) {
        sb.append(typeNode.getString())
            .append("<");
        Node child = typeNode.getFirstChild();
        appendTypeNode(sb, child.getFirstChild());
        for (int i = 1; i < child.getChildCount(); i++) {
          sb.append(",");
          appendTypeNode(sb, child.getChildAtIndex(i));
        }
        sb.append(">");
      } else {
        sb.append(typeNode.getString());
      }
    }
  }

  private static void appendFunctionNode(StringBuilder sb, Node function) {
    boolean hasNewOrThis = false;
    sb.append("function(");
    Node first = function.getFirstChild();
    if (first.isNew()) {
      sb.append("new:");
      appendTypeNode(sb, first.getFirstChild());
      hasNewOrThis = true;
    } else if (first.isThis()) {
      sb.append("this:");
      appendTypeNode(sb, first.getFirstChild());
      hasNewOrThis = true;
    } else if (first.isEmpty()) {
      sb.append(")");
      return;
    } else if (!first.isParamList()) {
      sb.append("):");
      appendTypeNode(sb, first);
      return;
    }
    Node paramList = null;
    if (first.isParamList()) {
      paramList = first;
    } else if (first.getNext().isParamList()) {
      paramList = first.getNext();
    }
    if (paramList != null) {
      boolean firstParam = true;
      for (Node param : paramList.children()) {
        if (!firstParam || hasNewOrThis) {
          sb.append(",");
        }
        appendTypeNode(sb, param);
        firstParam = false;
      }
    }
    sb.append(")");

    Node returnType = function.getLastChild();
    if (!returnType.isEmpty()) {
      sb.append(":");
      appendTypeNode(sb, returnType);
    }
  }
}