LZW.java

Below is the syntax highlighted version of LZW.java from §5.5 Data Compression.

/******************************************************************************
 *  Compilation:  javac LZW.java
 *  Execution:    java LZW - < input.txt   (compress)
 *  Execution:    java LZW + < input.txt   (expand)
 *  Dependencies: BinaryIn.java BinaryOut.java
 *  Data files:   https://algs4.cs.princeton.edu/55compression/abraLZW.txt
 *                https://algs4.cs.princeton.edu/55compression/ababLZW.txt
 *
 *  Compress or expand binary input from standard input using LZW.
 *

 ******************************************************************************/

/**
 *  The {@code LZW} class provides static methods for compressing
 *  and expanding a binary input using LZW compression over the 8-bit extended
 *  ASCII alphabet with 12-bit codewords.
 *  <p>
 *  WARNING: Starting with Oracle Java 7u6, the substring method takes time and
 *  space linear in the length of the extracted substring (instead of constant
 *  time an space as in earlier versions). As a result, compression takes
 *  quadratic time. TODO: fix.
 *  See <a href = "http://java-performance.info/changes-to-string-java-1-7-0_06/">this article</a>
 *  for more details.
 *  <p>
 *  For additional documentation,
 *  see <a href="https://algs4.cs.princeton.edu/55compression">Section 5.5</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class LZW {
    private static final int R = 256;        // number of input chars
    private static final int L = 4096;       // number of codewords = 2^W
    private static final int W = 12;         // codeword width

    // Do not instantiate.
    private LZW() { }

    /**
     * Reads a sequence of 8-bit bytes from standard input; compresses
     * them using LZW compression with 12-bit codewords; and writes the results
     * to standard output.
     */
    public static void compress() {
        String input = BinaryStdIn.readString();
        TST<Integer> st = new TST<Integer>();

        // since TST is not balanced, it would be better to insert in a different order
        for (int i = 0; i < R; i++)
            st.put("" + (char) i, i);

        int code = R+1;  // R is codeword for EOF

        while (input.length() > 0) {
            String s = st.longestPrefixOf(input);  // Find max prefix match s.
            BinaryStdOut.write(st.get(s), W);      // Print s's encoding.
            int t = s.length();
            if (t < input.length() && code < L)    // Add s to symbol table.
                st.put(input.substring(0, t + 1), code++);
            input = input.substring(t);            // Scan past s in input.
        }
        BinaryStdOut.write(R, W);
        BinaryStdOut.close();
    }

    /**
     * Reads a sequence of bit encoded using LZW compression with
     * 12-bit codewords from standard input; expands them; and writes
     * the results to standard output.
     */
    public static void expand() {
        String[] st = new String[L];
        int i; // next available codeword value

        // initialize symbol table with all 1-character strings
        for (i = 0; i < R; i++)
            st[i] = "" + (char) i;
        st[i++] = "";                        // (unused) lookahead for EOF

        int codeword = BinaryStdIn.readInt(W);
        if (codeword == R) return;           // expanded message is empty string
        String val = st[codeword];

        while (true) {
            BinaryStdOut.write(val);
            codeword = BinaryStdIn.readInt(W);
            if (codeword == R) break;
            String s = st[codeword];
            if (i == codeword) s = val + val.charAt(0);   // special case hack
            if (i < L) st[i++] = val + s.charAt(0);
            val = s;
        }
        BinaryStdOut.close();
    }

    /**
     * Sample client that calls {@code compress()} if the command-line
     * argument is "-" an {@code expand()} if it is "+".
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        if      (args[0].equals("-")) compress();
        else if (args[0].equals("+")) expand();
        else throw new IllegalArgumentException("Illegal command line argument");
    }

}