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/*
 * Copyright 2010-2015 JetBrains s.r.o.
 *
 * 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 org.jetbrains.kotlin.serialization.jvm;

import org.jetbrains.annotations.NotNull;

import java.util.ArrayList;
import java.util.List;

import static org.jetbrains.kotlin.serialization.jvm.UtfEncodingKt.MAX_UTF8_INFO_LENGTH;

public class BitEncoding {
    private static final boolean FORCE_8TO7_ENCODING = "true".equals(System.getProperty("kotlin.jvm.serialization.use8to7"));

    private static final char _8TO7_MODE_MARKER = (char) -1;

    private BitEncoding() {
    }

    /**
     * Converts a byte array of serialized data to an array of {@code String} satisfying JVM annotation value argument restrictions:
     * 
    *
  1. Each string's length should be no more than 65535
  2. *
  3. UTF-8 representation of each string cannot contain bytes in the range 0xf0..0xff
  4. *
*/ @NotNull public static String[] encodeBytes(@NotNull byte[] data) { // TODO: try both encodings here and choose the best one (with the smallest size) if (!FORCE_8TO7_ENCODING) { return UtfEncodingKt.bytesToStrings(data); } byte[] bytes = encode8to7(data); // Since 0x0 byte is encoded as two bytes in the Modified UTF-8 (0xc0 0x80) and zero is rather common to byte arrays, we increment // every byte by one modulo max byte value, so that the less common value 0x7f will be represented as two bytes instead. addModuloByte(bytes, 1); return splitBytesToStringArray(bytes); } /** * Converts a byte array to another byte array, every element of which is in the range 0x0..0x7f. * * The conversion is equivalent to the following: input bytes are combined into one long bit string. This big string is then split into * groups of 7 bits. Each resulting 7-bit chunk is then converted to a byte (with a leading bit = 0). The last chunk may have less than * 7 bits, it's prepended with zeros to form a byte. The result is then the array of these bytes, each of which is obviously in the * range 0x0..0x7f. * * Suppose the input of 4 bytes is given (bytes are listed from the beginning to the end, each byte from the least significant bit to * the most significant bit, bits within each byte are numbered): * * 01234567 01234567 01234567 01234567 * * The output for this kind of input will be of the following form ('#' represents a zero bit): * * 0123456# 7012345# 6701234# 5670123# 4567#### */ @NotNull private static byte[] encode8to7(@NotNull byte[] data) { // ceil(data.length * 8 / 7) int resultLength = (data.length * 8 + 6) / 7; byte[] result = new byte[resultLength]; // We maintain a pointer to the bit in the input, which is represented by two numbers: index of the current byte in the input and // the index of a bit inside this byte (0 is least significant, 7 is most significant) int byteIndex = 0; int bit = 0; // Write all resulting bytes except the last one. To do this we need to collect exactly 7 bits, starting from the current, into a // byte. In almost all cases these 7 bits can be collected from two parts: the first is several (at least one) most significant bits // from the current byte, the second is several (maybe zero) least significant bits from the next byte. The special case is when the // current bit is the first (least significant) bit in its byte (bit == 0): then the 7 needed bits are just the 7 least significant // of the current byte. for (int i = 0; i < resultLength - 1; i++) { if (bit == 0) { result[i] = (byte) (data[byteIndex] & 0x7f); bit = 7; continue; } int firstPart = (data[byteIndex] & 0xff) >>> bit; int newBit = (bit + 7) & 7; int secondPart = (data[++byteIndex] & ((1 << newBit) - 1)) << 8 - bit; result[i] = (byte) (firstPart + secondPart); bit = newBit; } // Write the last byte, which is just several most significant bits of the last byte in the input, padded with zeros if (resultLength > 0) { assert bit != 0 : "The last chunk cannot start from the input byte since otherwise at least one bit will remain unprocessed"; assert byteIndex == data.length - 1 : "The last 7-bit chunk should be encoded from the last input byte: " + byteIndex + " != " + (data.length - 1); result[resultLength - 1] = (byte) ((data[byteIndex] & 0xff) >>> bit); } return result; } private static void addModuloByte(@NotNull byte[] data, int increment) { for (int i = 0, n = data.length; i < n; i++) { data[i] = (byte) ((data[i] + increment) & 0x7f); } } /** * Converts a big byte array into the array of strings, where each string, when written to the constant pool table in bytecode, produces * a byte array of not more than MAX_UTF8_INFO_LENGTH. Each byte, except those which are 0x0, occupies exactly one byte in the constant * pool table. Zero bytes occupy two bytes in the table each. * * When strings are constructed from the array of bytes here, they are encoded in the platform's default encoding. This is fine: the * conversion to the Modified UTF-8 (which here would be equivalent to replacing each 0x0 with 0xc0 0x80) will happen later by ASM, when * it writes these strings to the bytecode */ @NotNull private static String[] splitBytesToStringArray(@NotNull byte[] data) { List result = new ArrayList(); // The offset where the currently processed string starts int off = 0; // The effective length the bytes of the current string would occupy in the constant pool table. // 2 because the first char is -1 which denotes the encoding mode and occupies two bytes in Modified UTF-8 int len = 2; boolean encodingModeAdded = false; for (int i = 0, n = data.length; i < n; i++) { // When the effective length reaches at least MAX - 1, we add the current string to the result. Note that the effective length // is at most MAX here: non-zero bytes occupy 1 byte and zero bytes occupy 2 bytes, so we couldn't jump over more than one byte if (len >= MAX_UTF8_INFO_LENGTH - 1) { assert len <= MAX_UTF8_INFO_LENGTH : "Produced strings cannot contain more than " + MAX_UTF8_INFO_LENGTH + " bytes: " + len; String string = new String(data, off, i - off); if (!encodingModeAdded) { encodingModeAdded = true; result.add(_8TO7_MODE_MARKER + string); } else { result.add(string); } off = i; len = 0; } if (data[i] == 0) { len += 2; } else { len++; } } if (len >= 0) { result.add(new String(data, off, data.length - off)); } return result.toArray(new String[result.size()]); } /** * Converts encoded array of {@code String} obtained by {@link BitEncoding#encodeBytes(byte[])} back to a byte array. */ @NotNull public static byte[] decodeBytes(@NotNull String[] data) { if (data.length > 0 && !data[0].isEmpty()) { char possibleMarker = data[0].charAt(0); if (possibleMarker == UtfEncodingKt.UTF8_MODE_MARKER) { return UtfEncodingKt.stringsToBytes(dropMarker(data)); } if (possibleMarker == _8TO7_MODE_MARKER) { data = dropMarker(data); } } byte[] bytes = combineStringArrayIntoBytes(data); // Adding 0x7f modulo max byte value is equivalent to subtracting 1 the same modulo, which is inverse to what happens in encodeBytes addModuloByte(bytes, 0x7f); return decode7to8(bytes); } @NotNull private static String[] dropMarker(@NotNull String[] data) { // Clone because the clients should be able to use the passed array for their own purposes. // This is cheap because the size of the array is 1 or 2 almost always. String[] result = data.clone(); result[0] = result[0].substring(1); return result; } /** * Combines the array of strings resulted from encodeBytes() into one long byte array */ @NotNull private static byte[] combineStringArrayIntoBytes(@NotNull String[] data) { int resultLength = 0; for (String s : data) { assert s.length() <= MAX_UTF8_INFO_LENGTH : "String is too long: " + s.length(); resultLength += s.length(); } byte[] result = new byte[resultLength]; int p = 0; for (String s : data) { for (int i = 0, n = s.length(); i < n; i++) { result[p++] = (byte) s.charAt(i); } } return result; } /** * Decodes the byte array resulted from encode8to7(). * * Each byte of the input array has at most 7 valuable bits of information. So the decoding is equivalent to the following: least * significant 7 bits of all input bytes are combined into one long bit string. This bit string is then split into groups of 8 bits, * each of which forms a byte in the output. If there are any leftovers, they are ignored, since they were added just as a padding and * do not comprise a full byte. * * Suppose the following encoded byte array is given (bits are numbered the same way as in encode8to7() doc): * * 01234567 01234567 01234567 01234567 * * The output of the following form would be produced: * * 01234560 12345601 23456012 * * Note how all most significant bits and leftovers are dropped, since they don't contain any useful information */ @NotNull private static byte[] decode7to8(@NotNull byte[] data) { // floor(7 * data.length / 8) int resultLength = 7 * data.length / 8; byte[] result = new byte[resultLength]; // We maintain a pointer to an input bit in the same fashion as in encode8to7(): it's represented as two numbers: index of the // current byte in the input and index of the bit in the byte int byteIndex = 0; int bit = 0; // A resulting byte is comprised of 8 bits, starting from the current bit. Since each input byte only "contains 7 bytes", a // resulting byte always consists of two parts: several most significant bits of the current byte and several least significant bits // of the next byte for (int i = 0; i < resultLength; i++) { int firstPart = (data[byteIndex] & 0xff) >>> bit; byteIndex++; int secondPart = (data[byteIndex] & ((1 << (bit + 1)) - 1)) << 7 - bit; result[i] = (byte) (firstPart + secondPart); if (bit == 6) { byteIndex++; bit = 0; } else { bit++; } } return result; } }




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