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      1 /*
      2  * Copyright (C) 2011 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 package com.android.settings.bluetooth;
     18 
     19 import android.text.InputFilter;
     20 import android.text.Spanned;
     21 
     22 /**
     23  * This filter will constrain edits so that the text length is not
     24  * greater than the specified number of bytes using UTF-8 encoding.
     25  * <p>The JNI method used by {@link android.server.BluetoothService}
     26  * to convert UTF-16 to UTF-8 doesn't support surrogate pairs,
     27  * therefore code points outside of the basic multilingual plane
     28  * (0000-FFFF) will be encoded as a pair of 3-byte UTF-8 characters,
     29  * rather than a single 4-byte UTF-8 encoding. Dalvik implements this
     30  * conversion in {@code convertUtf16ToUtf8()} in
     31  * {@code dalvik/vm/UtfString.c}.
     32  * <p>This JNI method is unlikely to change in the future due to
     33  * backwards compatibility requirements. It's also unclear whether
     34  * the installed base of Bluetooth devices would correctly handle the
     35  * encoding of surrogate pairs in UTF-8 as 4 bytes rather than 6.
     36  * However, this filter will still work in scenarios where surrogate
     37  * pairs are encoded as 4 bytes, with the caveat that the maximum
     38  * length will be constrained more conservatively than necessary.
     39  */
     40 class Utf8ByteLengthFilter implements InputFilter {
     41     private final int mMaxBytes;
     42 
     43     Utf8ByteLengthFilter(int maxBytes) {
     44         mMaxBytes = maxBytes;
     45     }
     46 
     47     public CharSequence filter(CharSequence source, int start, int end,
     48                                Spanned dest, int dstart, int dend) {
     49         int srcByteCount = 0;
     50         // count UTF-8 bytes in source substring
     51         for (int i = start; i < end; i++) {
     52             char c = source.charAt(i);
     53             srcByteCount += (c < (char) 0x0080) ? 1 : (c < (char) 0x0800 ? 2 : 3);
     54         }
     55         int destLen = dest.length();
     56         int destByteCount = 0;
     57         // count UTF-8 bytes in destination excluding replaced section
     58         for (int i = 0; i < destLen; i++) {
     59             if (i < dstart || i >= dend) {
     60                 char c = dest.charAt(i);
     61                 destByteCount += (c < (char) 0x0080) ? 1 : (c < (char) 0x0800 ? 2 : 3);
     62             }
     63         }
     64         int keepBytes = mMaxBytes - destByteCount;
     65         if (keepBytes <= 0) {
     66             return "";
     67         } else if (keepBytes >= srcByteCount) {
     68             return null; // use original dest string
     69         } else {
     70             // find end position of largest sequence that fits in keepBytes
     71             for (int i = start; i < end; i++) {
     72                 char c = source.charAt(i);
     73                 keepBytes -= (c < (char) 0x0080) ? 1 : (c < (char) 0x0800 ? 2 : 3);
     74                 if (keepBytes < 0) {
     75                     return source.subSequence(start, i);
     76                 }
     77             }
     78             // If the entire substring fits, we should have returned null
     79             // above, so this line should not be reached. If for some
     80             // reason it is, return null to use the original dest string.
     81             return null;
     82         }
     83     }
     84 }
     85