Search for a String (as an byte[]) in a binary stream

Question

Hi Team, I am trying to find a String "Henry" in a binary file and change the String to a different string. FYI the file is the output of serialisation of an object. Original Question here

I am new to searching bytes and imagined this code would search for my byte[] and exchange it. But it doesn't come close to working it doesn't even find a match.

{
    byte[] bytesHenry = new String("Henry").getBytes();
    byte[] bytesSwap = new String("Zsswd").getBytes();

    byte[] seekHenry = new byte[bytesHenry.length];

    RandomAccessFile file = new RandomAccessFile(fileString,"rw");

    long filePointer;
    while (seekHenry != null) {
       filePointer = file.getFilePointer();
       file.readFully(seekHenry);
       if (bytesHenry == seekHenry) {
           file.seek(filePointer);
           file.write(bytesSwap);
           break;
       }
     }
}

Okay I see the bytesHenry==seekHenry problem and will swap to Arrays.equals( bytesHenry , seekHenry )

I think I need to move along by -4 byte positions each time i read 5 bytes.

Bingo it finds it now

    while (seekHenry != null) {
                    filePointer = file.getFilePointer();
                    file.readFully(seekHenry);;
                    if (Arrays.equals(bytesHenry,
                                      seekHenry)) {
                        file.seek(filePointer);
                        file.write(bytesSwap);
                        break;
                    }
                    file.seek(filePointer);
                    file.read();
                }

well at least upvote my original answer if you found it useful for opening this question :) — robermann, Mar 06 '14 at 19:38
for this example its a one off "Henry", but I'm interested hearing a better approach for the future. — Another Compiler Error, Mar 06 '14 at 19:53
see my answer; you should 1) read all your file into the byte array, 2) search it for a occurrence, 3) if find it, replace it, 4) loop until all your occurrences are replaced. 5) Then write the final byte[] into disk. — robermann, Mar 06 '14 at 21:24

score 2 · Accepted Answer · answered Mar 06 '14 at 21:22

The following could work for you, see the method search(byte[] input, byte[] searchedFor) which returns the index where the first match matches, or -1.

public class SearchBuffer {

    public static void main(String[] args) throws UnsupportedEncodingException {
        String charset= "US-ASCII";
        byte[] searchedFor = "ciao".getBytes(charset);
        byte[] input = "aaaciaaaciaojjcia".getBytes(charset);

        int idx = search(input, searchedFor);
        System.out.println("index: "+idx); //should be 8
    }

    public static int search(byte[] input, byte[] searchedFor) {
        //convert byte[] to Byte[]
        Byte[] searchedForB = new Byte[searchedFor.length];
        for(int x = 0; x<searchedFor.length; x++){
            searchedForB[x] = searchedFor[x];
        }

        int idx = -1;

        //search:
        Deque<Byte> q = new ArrayDeque<Byte>(input.length);
        for(int i=0; i<input.length; i++){
            if(q.size() == searchedForB.length){
                //here I can check
                Byte[] cur = q.toArray(new Byte[]{});
                if(Arrays.equals(cur, searchedForB)){
                    //found!
                    idx = i - searchedForB.length;
                    break;
                } else {
                    //not found
                    q.pop();
                    q.addLast(input[i]);
                }
            } else {
                q.addLast(input[i]);
            }
        }

        return idx;
    }
}

score 0 · Answer 2 · edited May 23 '17 at 10:09

From Fastest way to find a string in a text file with java:

The best realization I've found in MIMEParser: https://github.com/samskivert/ikvm-openjdk/blob/master/build/linux-amd64/impsrc/com/sun/xml/internal/org/jvnet/mimepull/MIMEParser.java

/**
  * Finds the boundary in the given buffer using Boyer-Moore algo.
  * Copied from java.util.regex.Pattern.java
  *
  * @param mybuf boundary to be searched in this mybuf
  * @param off start index in mybuf
  * @param len number of bytes in mybuf
  *
  * @return -1 if there is no match or index where the match starts
  */

  private int match(byte[] mybuf, int off, int len) {

Needed also:

  private void compileBoundaryPattern();

Search for a String (as an byte[]) in a binary stream

2 Answers2

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