/*
 * Copyright 2008 Google Inc.
 * 
 * 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.gwt.resources.rg;

import com.google.gwt.core.ext.Generator;
import com.google.gwt.core.ext.TreeLogger;
import com.google.gwt.core.ext.UnableToCompleteException;
import com.google.gwt.core.ext.typeinfo.JMethod;
import com.google.gwt.resources.client.TextResource;
import com.google.gwt.resources.ext.AbstractResourceGenerator;
import com.google.gwt.resources.ext.ResourceContext;
import com.google.gwt.resources.ext.ResourceGeneratorUtil;
import com.google.gwt.resources.ext.SupportsGeneratorResultCaching;
import com.google.gwt.thirdparty.guava.common.io.CharStreams;
import com.google.gwt.user.rebind.SourceWriter;
import com.google.gwt.user.rebind.StringSourceWriter;

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;
import java.nio.charset.StandardCharsets;

/**
 * Provides implementations of TextResource.
 */
public final class TextResourceGenerator extends AbstractResourceGenerator
    implements SupportsGeneratorResultCaching {
  /**
   * Java compiler has a limit of 2^16 bytes for encoding string constants in a
   * class file. Since the max size of a character is 4 bytes, we'll limit the
   * number of characters to (2^14 - 1) to fit within one record.
   */
  private static final int MAX_STRING_CHUNK = 16383;

  @Override
  public String createAssignment(TreeLogger logger, ResourceContext context,
      JMethod method) throws UnableToCompleteException {
    URL[] resources = ResourceGeneratorUtil.findResources(logger, context,
        method);

    if (resources.length != 1) {
      logger.log(TreeLogger.ERROR, "Exactly one resource must be specified",
          null);
      throw new UnableToCompleteException();
    }

    URL resource = resources[0];

    SourceWriter sw = new StringSourceWriter();
    // Write the expression to create the subtype.
    sw.println("new " + TextResource.class.getName() + "() {");
    sw.indent();

    if (!AbstractResourceGenerator.STRIP_COMMENTS) {
      // Convenience when examining the generated code.
      sw.println("// " + resource.toExternalForm());
    }

    sw.println("public String getText() {");
    sw.indent();

    String toWrite;
    try (InputStream in = resource.openStream()) {
      toWrite = CharStreams.toString(new InputStreamReader(in, StandardCharsets.UTF_8));
    } catch (IOException e) {
      logger.log(TreeLogger.ERROR, "Unable to read resource: " + resource, e);
      throw new UnableToCompleteException();
    }

    if (toWrite.length() > MAX_STRING_CHUNK) {
      writeLongString(sw, toWrite);
    } else {
      sw.println("return \"" + Generator.escape(toWrite) + "\";");
    }
    sw.outdent();
    sw.println("}");

    sw.println("public String getName() {");
    sw.indent();
    sw.println("return \"" + method.getName() + "\";");
    sw.outdent();
    sw.println("}");

    sw.outdent();
    sw.println("}");

    return sw.toString();
  }

  /**
   * A single constant that is too long will crash the compiler with an out of
   * memory error. Break up the constant and generate code that appends using a
   * buffer.
   */
  private void writeLongString(SourceWriter sw, String toWrite) {
    sw.println("StringBuilder builder = new StringBuilder();");
    int offset = 0;
    int length = toWrite.length();
    while (offset < length - 1) {
      int subLength = Math.min(MAX_STRING_CHUNK, length - offset);
      sw.print("builder.append(\"");
      sw.print(Generator.escape(toWrite.substring(offset, offset + subLength)));
      sw.println("\");");
      offset += subLength;
    }
    sw.println("return builder.toString();");
  }
}
