Question

I’m currently writing my own Brainfuck interpreter in Java and because I want it to be able to read code from files I wrote a BFInputStream that filters unnecessary symbols out. It looks like this:

import java.io.FilterInputStream;
import java.io.InputStream;
import java.io.IOException;

public class BFInputStream extends FilterInputStream {
    public BFInputStream(InputStream in) {
        super(in);
    }
    public int read() throws IOException {
        while(true) {
            int i = super.read();
            // keep LF for line number checking.
            if(i == -1 || i == 10 ||( i >= 43 && i <= 46) || i == 60 || i == 62 || i == 91 || i == 93)
                return i;
        }
    }
    public int read(byte[] b, int off, int len) throws IOException {
        if(off < 0 || len < 0 || len > b.length - off) throw new IndexOutOfBoundsException();
        for(int i=0; i<len; i++) {
            int j = read();
            if(j < 1) return i;
            b[off+i] = (byte)j;
        }
        return len;
    }
}

My Interpreter uses LineNumberReader<-InputStreamReader<-BFInputStream<-FileInputStream to read the file. But everytime it reaches the end of the file it throws:

java.io.IOException: Underlying input stream returned zero bytes

  at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:268)
  at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:306)
  at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:158)
  at java.io.InputStreamReader.read(InputStreamReader.java:167)
  at java.io.BufferedReader.fill(BufferedReader.java:136)
  at java.io.BufferedReader.read(BufferedReader.java:157)
  at java.io.LineNumberReader.read(LineNumberReader.java:108)
  at Interpreter.run(Interpreter.java:101)
  at Interpreter.main(Interpreter.java:180)

Interpreter.java:101 contains a call to BFInputStream.read().

I’m not sure if I understand the exception correctly. I think the stream returned no bytes at all (because of a timeout). I guess if there was a huge chunk of non-BF characters then that could be possible but at the end of the file? FileInputStream and therefore FilterInputStream and BFInputStream should return -1.

What is wrong with my code?

Answer 1

if (j < 1) should be if (j < 0), because EOF is indicated by -1. Also, your method never returns -1 at EOF.

The correct implementation would look like this:

public int read(byte[] b, int off, int len) throws IOException {
    if(off < 0 || len <= 0 || len > b.length - off) throw new IndexOutOfBoundsException();         
    int i = 0;
    while (i < len) {
        int j = read();
        if (j < 0) break; // Stop reading at EOF
        b[off + i] = (byte) j;
        i++;
    }
    if (i == 0) return -1; // If we get EOF with no data, return it to the caller
    else return i;
}