/*
* Copyright (C) 2011 The Android Open Source Project
*
* 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.android.settings.bluetooth;
import android.text.InputFilter;
import android.text.Spanned;
/**
* This filter will constrain edits so that the text length is not
* greater than the specified number of bytes using UTF-8 encoding.
* <p>The JNI method used by {@link android.server.BluetoothService}
* to convert UTF-16 to UTF-8 doesn't support surrogate pairs,
* therefore code points outside of the basic multilingual plane
* (0000-FFFF) will be encoded as a pair of 3-byte UTF-8 characters,
* rather than a single 4-byte UTF-8 encoding. Dalvik implements this
* conversion in {@code convertUtf16ToUtf8()} in
* {@code dalvik/vm/UtfString.c}.
* <p>This JNI method is unlikely to change in the future due to
* backwards compatibility requirements. It's also unclear whether
* the installed base of Bluetooth devices would correctly handle the
* encoding of surrogate pairs in UTF-8 as 4 bytes rather than 6.
* However, this filter will still work in scenarios where surrogate
* pairs are encoded as 4 bytes, with the caveat that the maximum
* length will be constrained more conservatively than necessary.
*/
class Utf8ByteLengthFilter implements InputFilter {
private final int mMaxBytes;
Utf8ByteLengthFilter(int maxBytes) {
mMaxBytes = maxBytes;
}
public CharSequence filter(CharSequence source, int start, int end,
Spanned dest, int dstart, int dend) {
int srcByteCount = 0;
// count UTF-8 bytes in source substring
for (int i = start; i < end; i++) {
char c = source.charAt(i);
srcByteCount += (c < (char) 0x0080) ? 1 : (c < (char) 0x0800 ? 2 : 3);
}
int destLen = dest.length();
int destByteCount = 0;
// count UTF-8 bytes in destination excluding replaced section
for (int i = 0; i < destLen; i++) {
if (i < dstart || i >= dend) {
char c = dest.charAt(i);
destByteCount += (c < (char) 0x0080) ? 1 : (c < (char) 0x0800 ? 2 : 3);
}
}
int keepBytes = mMaxBytes - destByteCount;
if (keepBytes <= 0) {
return "";
} else if (keepBytes >= srcByteCount) {
return null; // use original dest string
} else {
// find end position of largest sequence that fits in keepBytes
for (int i = start; i < end; i++) {
char c = source.charAt(i);
keepBytes -= (c < (char) 0x0080) ? 1 : (c < (char) 0x0800 ? 2 : 3);
if (keepBytes < 0) {
return source.subSequence(start, i);
}
}
// If the entire substring fits, we should have returned null
// above, so this line should not be reached. If for some
// reason it is, return null to use the original dest string.
return null;
}
}
}