transcendental-lisp/scanner/LispFilterStream.java

/*
 * Name: Mike Cifelli
 * Course: CIS 443 - Programming Languages
 * Assignment: Lisp Interpreter Phase 1 - Lexical Analysis
 */

package scanner;

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

/**
 * A <code>LispFilterStream</code> is an input stream that returns the bytes
 * from an underlying input stream with all Lisp comments removed and replaced
 * with newlines.
 */
public class LispFilterStream extends FilterInputStream {

    private boolean inQuote;

    /**
     * Creates a <code>LispFilterStream</code> with the specified underlying
     * <code>InputStream</code>.
     *
     * @param in
     *  the underlying input stream (must not be <code>null</code>)
     */
    public LispFilterStream(InputStream in) {
        super(in);

        inQuote = false;
    }

    /**
     * Reads the next byte of data from this input stream. The value byte is
     * returned as an <code>int</code> in the range <code>0</code> to
     * <code>255</code>. If no byte is available because the end of the stream
     * has been reached, the value <code>-1</code> is returned. 
     *
     * @return
     *  the next byte of data, or <code>-1</code> if the end of the stream has
     *  been reached.
     * @throws IOException
     *  Indicates that an I/O error has occurred.
     */
    @Override
    public int read() throws IOException {
        int next = super.read();

        if ((next == ';') && (! inQuote)) {
            // we have entered a comment, consume all the bytes from the
            // underlying input stream until we reach a newline character or
            // the end of the stream
            while ((next != '\n') && (next != -1)) {
                next = super.read();
            }
        } else if (next == '\n') {
            inQuote = false;
        } else if (next == '\"') {  // we have entered or left a quoted string
            inQuote = (! inQuote);
        }

        return next;
    }

    /**
     * Reads up to the specified number of data bytes from this input stream
     * into an array of bytes starting at the specified offset. If no bytes are
     * available because the end of this stream has been reached then
     * <code>-1</code> is returned. Also, If the specified number of bytes is
     * more than the number of remaining data bytes in this input stream, then
     * only the number of remaining data bytes are copied into the byte array.
     *
     * @param b
     *  the buffer into which the data bytes are read (must not be
     *  <code>null</code>)
     * @param off
     *  the start offset in <code>b</code> at which the data is written (must
     *  be <code>&gt;= 0</code> and <code>&lt; b.length</code>)
     * @param len
     *  the maximum number of bytes to read into <code>b</code> (<code>len +
     *  off</code> must be <code>&lt; b.length</code>)
     * @return
     *  the total number of bytes read into the buffer, or <code>-1</code> if
     *  there is no more data because the end of the stream has been reached
     * @throws IOException
     *  Indicates that the first byte could not be read into <code>b</code>
     *  for some reason other than reaching the end of the stream.
     * @throws IndexOutOfBoundsException
     *  Indicates that this method attempted to access an index in
     *  <code>b</code> that was either negative or greater than or equal to
     *  <code>b.length</code> as a result of the given parameters.
     * @throws NullPointerException
     *  Indicates that <code>b</code> is <code>null</code>.
     */
    @Override
    public int read(byte[] b, int off, int len) throws IOException {
        int bytesRead = 0;

        // make sure we are supposed to read at least one byte into 'b'
        if (len > 0) {
            int next = read();

            if (next == -1) {
                // there are no more bytes to read from this input stream

                return -1;
            }

            int i = off;

            while (next != -1) {
                ++bytesRead;

                b[i++] = (byte) next;

                if (i >= (off + len)) {  // we have read 'len' bytes into 'b'
                    break;
                }

                try {
                    next = read();
                } catch (IOException e) {
                    // treat this exception like an end of stream

                    break;
                }
            }
        }

        return bytesRead;
    }

    /**
     * Skip over and discard the specified number of bytes from this input
     * stream. This method may, for a variety of reasons, end up skipping some
     * smaller number of bytes, possibly <code>0</code>. The actual number of
     * bytes skipped is returned.
     *
     * @param n
     *  the number of bytes to be skipped
     * @return
     *  the actual number of bytes skipped
     * @throws IOException
     *  Indicates that an I/O error has occurred.
     */
    @Override
    public long skip (long n) throws IOException {
        long bytesSkipped = 0;

        while ((n > 0) && (read() != -1)) {
            ++bytesSkipped;
            --n;
        }

        return bytesSkipped;
    }

}