pagespeed/kernel/base/charset_util.cc

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.
 */


#include "pagespeed/kernel/base/charset_util.h"

#include "strings/stringpiece_utils.h"
#include "pagespeed/kernel/base/string.h"
#include "pagespeed/kernel/base/string_util.h"

namespace net_instaweb {

bool StripUtf8Bom(StringPiece* contents) {
  bool result = false;
  StringPiece bom;
  bom.set(kUtf8Bom, STATIC_STRLEN(kUtf8Bom));
  if (strings::StartsWith(*contents, bom)) {
    contents->remove_prefix(bom.length());
    result = true;
  }
  return result;
}

const StringPiece GetCharsetForBom(const StringPiece contents) {
  // Bad/empty data?
  if (contents.empty()) {
    return StringPiece();
  }
  // If it starts with a printable ASCII character it can't have a BOM, and
  // since that's the most common case and the comparisons below are relatively
  // expensive, test this here for early exit.
  if (contents[0] >= ' ' && contents[0] <= '~') {
    return StringPiece();
  }

  // Check for the BOMs we know about. Since some BOMs contain NUL(s) we have
  // to use STATIC_STRLEN and manual StringPiece construction.
  StringPiece bom;
  bom.set(kUtf8Bom, STATIC_STRLEN(kUtf8Bom));
  if (strings::StartsWith(contents, bom)) {
    return kUtf8Charset;
  }
  bom.set(kUtf16BigEndianBom, STATIC_STRLEN(kUtf16BigEndianBom));
  if (strings::StartsWith(contents, bom)) {
    return kUtf16BigEndianCharset;
  }
  // UTF-16LE's BOM is a leading substring of UTF-32LE's BOM, so we must
  // check the longer one first. All the others have unique prefixes.
  bom.set(kUtf32LittleEndianBom, STATIC_STRLEN(kUtf32LittleEndianBom));
  if (strings::StartsWith(contents, bom)) {
    return kUtf32LittleEndianCharset;
  }
  bom.set(kUtf16LittleEndianBom, STATIC_STRLEN(kUtf16LittleEndianBom));
  if (strings::StartsWith(contents, bom)) {
    return kUtf16LittleEndianCharset;
  }
  bom.set(kUtf32BigEndianBom, STATIC_STRLEN(kUtf32BigEndianBom));
  if (strings::StartsWith(contents, bom)) {
    return kUtf32BigEndianCharset;
  }

  return StringPiece();
}

}  // namespace net_instaweb