Preon 无法编码/解码 little endian答案

【问题标题】：Preon unable to encode/decode little endianPreon 无法编码/解码 little endian
【发布时间】：2015-07-21 17:59:50
【问题描述】：

背景

我正在尝试使用preon 库来编码/解码二进制数据。当我为大端配置字节顺序时，我的 1 字节结构工作正常，但为小端配置失败。似乎字节序会影响 preon 中的字节内数据。似乎this 问题涉及相同的问题，但没有解决小端方面的问题。

我附上了一个简单的案例来证明这个问题。

PreonLittleEndianNumber.java 中是否存在缺陷，或者在指定小端字节顺序时 preon 根本不起作用？

PreonTest.java

package me;

import org.codehaus.preon.Codec;
import org.codehaus.preon.Codecs;
import org.codehaus.preon.DecodingException;
import org.junit.Test;

import java.io.IOException;

import org.junit.Assert;

public class PreonTest {

    @Test
    public void bigEndianDecodeEncodeTest() {

        byte[] testByte = {(byte) 0xDE};
        Codec<PreonBigEndianNumber> bigCodec = Codecs.create(PreonBigEndianNumber.class);

        try {

            // Big Endian Decode/Encode Test
            PreonBigEndianNumber pben = Codecs.decode(bigCodec, testByte);
            byte[] testByte1 = Codecs.encode(pben, bigCodec);
            Assert.assertArrayEquals(testByte, testByte1);
        } catch (IOException e) {
            e.printStackTrace();
        } catch (DecodingException e) {
            e.printStackTrace();
        }
    }

    @Test
    public void bigEndianEncodeDecodeTest() {

        PreonBigEndianNumber pben = new PreonBigEndianNumber((byte)0xDE);
        Codec<PreonBigEndianNumber> bigCodec = Codecs.create(PreonBigEndianNumber.class);

        try {

            byte[] testByte1 = Codecs.encode(pben, bigCodec);
            PreonBigEndianNumber pben2 = Codecs.decode(bigCodec, testByte1);
            Assert.assertEquals(pben, pben2);

        } catch (IOException e) {
            e.printStackTrace();
        } catch (DecodingException e) {
            e.printStackTrace();
        }
    }

    @Test
    public void littleEndianDecodeEncodeTest() {

        byte[] testByte = {(byte) 0xDE};
        Codec<PreonLittleEndianNumber> littleCodec = Codecs.create(PreonLittleEndianNumber.class);

        try {

            // Big Endian Decode/Encode Test
            PreonLittleEndianNumber plen = Codecs.decode(littleCodec, testByte);
            byte[] testByte1 = Codecs.encode(plen, littleCodec);
            Assert.assertArrayEquals(testByte, testByte1);
        } catch (IOException e) {
            e.printStackTrace();
        } catch (DecodingException e) {
            e.printStackTrace();
        }
    }

    @Test
    public void littleEndianEncodeDecodeTest() {

        PreonLittleEndianNumber plen = new PreonLittleEndianNumber((byte)0xDE);
        Codec<PreonLittleEndianNumber> littleCodec = Codecs.create(PreonLittleEndianNumber.class);

        try {

            byte[] testByte1 = Codecs.encode(plen, littleCodec);
            PreonLittleEndianNumber plen2 = Codecs.decode(littleCodec, testByte1);
            Assert.assertEquals(plen, plen2);

        } catch (IOException e) {
            e.printStackTrace();
        } catch (DecodingException e) {
            e.printStackTrace();
        }
    }
}

PreonBigEndianNumber.java

package me;

import org.codehaus.preon.annotation.BoundNumber;
import org.codehaus.preon.buffer.ByteOrder;

public class PreonBigEndianNumber {
    @BoundNumber(size="1", byteOrder=ByteOrder.BigEndian) public byte bit7;
    @BoundNumber(size="1", byteOrder=ByteOrder.BigEndian) public byte bit6;
    @BoundNumber(size="1", byteOrder=ByteOrder.BigEndian) public byte bit5;
    @BoundNumber(size="1", byteOrder=ByteOrder.BigEndian) public byte bit4;
    @BoundNumber(size="1", byteOrder=ByteOrder.BigEndian) public byte bit3;
    @BoundNumber(size="1", byteOrder=ByteOrder.BigEndian) public byte bit2;
    @BoundNumber(size="1", byteOrder=ByteOrder.BigEndian) public byte bit1;
    @BoundNumber(size="1", byteOrder=ByteOrder.BigEndian) public byte bit0;

    /**
     * Default constructor
     */
    public PreonBigEndianNumber() {

    }

    public PreonBigEndianNumber(byte value) {
        bit7 = (byte) ((0b10000000 & value) >>> 7);
        bit6 = (byte) ((0b01000000 & value) >>> 6);
        bit5 = (byte) ((0b00100000 & value) >>> 5);
        bit4 = (byte) ((0b00010000 & value) >>> 4);
        bit3 = (byte) ((0b00001000 & value) >>> 3);
        bit2 = (byte) ((0b00000100 & value) >>> 2);
        bit1 = (byte) ((0b00000010 & value) >>> 1);
        bit0 = (byte) ((0b00000001 & value) >>> 0);
    }

    public byte getByte() {
        // Pack bits back into an int
        int b = 0;
        int shift = 0;
        for (int i=7;i>=0;i--) {
            b = (b << shift);
            if      (i==0) {b += bit0;}
            else if (i==1) {b += bit1;}
            else if (i==2) {b += bit2;}
            else if (i==3) {b += bit3;}
            else if (i==4) {b += bit4;}
            else if (i==5) {b += bit5;}
            else if (i==6) {b += bit6;}
            else if (i==7) {b += bit7;}
            shift = 1;
        }
        return (byte) b;
    }

    @Override
    public String toString() {
        return  bit7 +
                bit6 +
                bit5 +
                bit4 +
                " " +
                bit3 +
                bit2 +
                bit1 +
                bit0;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) {return true;}
        if (o == null || getClass() != o.getClass()) {return false;}

        PreonBigEndianNumber preonBits = (PreonBigEndianNumber) o;
        if (bit7 != preonBits.bit7) {return false;}
        if (bit6 != preonBits.bit6) {return false;}
        if (bit5 != preonBits.bit5) {return false;}
        if (bit4 != preonBits.bit4) {return false;}
        if (bit3 != preonBits.bit3) {return false;}
        if (bit2 != preonBits.bit2) {return false;}
        if (bit1 != preonBits.bit1) {return false;}
        return bit0 == preonBits.bit0;
    }

    @Override
    public int hashCode() {
        int result = (int) bit7;
        result = 31 * result + (int) bit6;
        result = 31 * result + (int) bit5;
        result = 31 * result + (int) bit4;
        result = 31 * result + (int) bit3;
        result = 31 * result + (int) bit2;
        result = 31 * result + (int) bit1;
        result = 31 * result + (int) bit0;
        return result;
    }
}

PreonLittleEndianNumber.java

package me;

import org.codehaus.preon.annotation.BoundNumber;
import org.codehaus.preon.buffer.ByteOrder;

public class PreonLittleEndianNumber {
    @BoundNumber(size="1", byteOrder=ByteOrder.LittleEndian) public byte bit7;
    @BoundNumber(size="1", byteOrder=ByteOrder.LittleEndian) public byte bit6;
    @BoundNumber(size="1", byteOrder=ByteOrder.LittleEndian) public byte bit5;
    @BoundNumber(size="1", byteOrder=ByteOrder.LittleEndian) public byte bit4;
    @BoundNumber(size="1", byteOrder=ByteOrder.LittleEndian) public byte bit3;
    @BoundNumber(size="1", byteOrder=ByteOrder.LittleEndian) public byte bit2;
    @BoundNumber(size="1", byteOrder=ByteOrder.LittleEndian) public byte bit1;
    @BoundNumber(size="1", byteOrder=ByteOrder.LittleEndian) public byte bit0;

    /**
     * Default constructor
     */
    public PreonLittleEndianNumber() {

    }

    public PreonLittleEndianNumber(byte value) {
        bit7 = (byte) ((0b10000000 & value) >>> 7);
        bit6 = (byte) ((0b01000000 & value) >>> 6);
        bit5 = (byte) ((0b00100000 & value) >>> 5);
        bit4 = (byte) ((0b00010000 & value) >>> 4);
        bit3 = (byte) ((0b00001000 & value) >>> 3);
        bit2 = (byte) ((0b00000100 & value) >>> 2);
        bit1 = (byte) ((0b00000010 & value) >>> 1);
        bit0 = (byte) ((0b00000001 & value) >>> 0);
    }

    public byte getByte() {
        // Pack bits back into an int
        int b = 0;
        int shift = 0;
        for (int i=7;i>=0;i--) {
            b = (b << shift);
            if      (i==0) {b += bit0;}
            else if (i==1) {b += bit1;}
            else if (i==2) {b += bit2;}
            else if (i==3) {b += bit3;}
            else if (i==4) {b += bit4;}
            else if (i==5) {b += bit5;}
            else if (i==6) {b += bit6;}
            else if (i==7) {b += bit7;}
            shift = 1;
        }
        return (byte) b;
    }

    @Override
    public String toString() {
        return  bit7 +
                bit6 +
                bit5 +
                bit4 +
                " " +
                bit3 +
                bit2 +
                bit1 +
                bit0;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) {return true;}
        if (o == null || getClass() != o.getClass()) {return false;}

        PreonLittleEndianNumber preonBits = (PreonLittleEndianNumber) o;
        if (bit7 != preonBits.bit7) {return false;}
        if (bit6 != preonBits.bit6) {return false;}
        if (bit5 != preonBits.bit5) {return false;}
        if (bit4 != preonBits.bit4) {return false;}
        if (bit3 != preonBits.bit3) {return false;}
        if (bit2 != preonBits.bit2) {return false;}
        if (bit1 != preonBits.bit1) {return false;}
        return bit0 == preonBits.bit0;
    }

    @Override
    public int hashCode() {
        int result = (int) bit7;
        result = 31 * result + (int) bit6;
        result = 31 * result + (int) bit5;
        result = 31 * result + (int) bit4;
        result = 31 * result + (int) bit3;
        result = 31 * result + (int) bit2;
        result = 31 * result + (int) bit1;
        result = 31 * result + (int) bit0;
        return result;
    }
}

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">

<modelVersion>4.0.0</modelVersion>

<groupId>me</groupId>
<artifactId>parser</artifactId>
<version>1.0</version>

<repositories>
    <repository>
        <id>atlassian-repo</id>
        <name>Atlassian Repository</name>
        <url>https://maven.atlassian.com/repository/public</url>
    </repository>
    <repository>
        <id>limbo-repository</id>
        <url>http://limbo.sourceforge.net/repository</url>
        <snapshots>
            <enabled>false</enabled>
        </snapshots>
        <releases>
            <enabled>true</enabled>
        </releases>
    </repository>
    <repository>
        <id>pecia-repository</id>
        <url>http://pecia.sourceforge.net/repository</url>
        <snapshots>
            <enabled>false</enabled>
        </snapshots>
        <releases>
            <enabled>true</enabled>
        </releases>
    </repository>
</repositories>

<dependencies>
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>4.12</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.codehaus.preon</groupId>
        <artifactId>preon-binding</artifactId>
        <version>1.1-SNAPSHOT</version>
    </dependency>
    <dependency>
        <groupId>org.codehaus.preon</groupId>
        <artifactId>preon-el</artifactId>
        <version>1.1-SNAPSHOT</version>
    </dependency>
    <dependency>
        <groupId>org.codehaus.preon</groupId>
        <artifactId>preon-emitter</artifactId>
        <version>1.1-SNAPSHOT</version>
    </dependency>
    <dependency>
        <groupId>org.codehaus.preon</groupId>
        <artifactId>preon-io</artifactId>
        <version>1.1-SNAPSHOT</version>
    </dependency>
</dependencies>

【问题讨论】：

标签： java endianness preon

【解决方案1】：

我遇到了同样的问题。看来ByteOrder.BigEndian 和ByteOrder.LittleEndian 的解码是不同的。但是，编码是相同的。这意味着当您使用ByteOrder.LittleEndian 时，编码和解码之间的往返将不正确。

ByteOrder.BigEndian 的解码使用 MSB 0 位编号，ByteOrder.LittleEndian 的解码使用 LSB 0 位编号解码。考虑到这一点，我认为ByteOrder.LittleEndian 的编码器已损坏。

我分叉了当前的 preon 存储库 (https://github.com/jdl17/preon/) 并进行了必要的更改。随着这些更改，我发现我需要将最初由 mrumpf 提交的刷新修复合并到拉取请求中 (https://github.com/preon/preon/pull/25)。我进一步扩展它以允许在刷新时指定字节顺序。

除了上述更改之外，分支还包括实现 ListCodecFactory 的编码和解码方法以及 SelectFromCode 的编码方法的更改。最后，我将 ANTLR 从 3.3 更新到 3.5.2，以便它可以与 Java 8 一起使用。

所有这些都已作为拉取请求提交回 preon (https://github.com/preon/preon/pull/35)。

我还应该评论说，我添加了例程以正确编码小端和大端的布尔值。但是，Bound 的当前实现不允许 byteOrder 注释当前被硬编码为ByteOrder.BigEndian。这超出了我的工作范围，我不需要使用布尔值进行解码和编码。但是，如果您打算在 ByteOrder.LittleEndian 编码中使用布尔值，请注意这一点。

希望这能解决您的问题。

除此之外，拉取请求实现 ListCodecFactory 的编码和解码方法以及 SelectFromCode 的编码方法。

【讨论】：