submitted issue: kevin-wayne/algs4#114

src/main/java/edu/princeton/cs/algs4/LempelZivWelch.java

@@ -0,0 +1,156 @@
+/******************************************************************************
+ *  Compilation:  javac LempelZivWelch.java
+ *  Execution:    java LempelZivWelch - < input.txt   (compress)
+ *  Execution:    java LempelZivWelch + < 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.
+ *
+ *
+ ******************************************************************************/
+
+package edu.princeton.cs.algs4;
+
+import edu.princeton.cs.algs4.TernarySearchTrie;
+import edu.princeton.cs.algs4.BinaryStdIn;
+import edu.princeton.cs.algs4.BinaryStdOut;
+
+/**
+ * The {@code LempelZivWelch} class provides static methods for compressing and expanding a binary
+ * input using LempelZivWelch compression over the 8-bit extended ASCII alphabet with 12-bit
+ * codewords.
+ *
+ * <p>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 in the original {@code LZW} class. See <a href =
+ * "http://java-performance.info/changes-to-string-java-1-7-0_06/">this article</a> for more
+ * details.
+ *
+ * <p>This class, along with {@code TernarySearchTrie} - the enhanced version of {@code TST}, fixes
+ * the above issue.The key to the fix is to use the new method {@code
+ * TernarySearchTrie.longestPrefixOf(String query, int startIndex)} in {@code compress()}.
+ *
+ * <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 LempelZivWelch {
+  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 LempelZivWelch() {}
+
+  /**
+   * Reads a sequence of 8-bit bytes from standard input; compresses them using LempelZivWelch
+   * compression with 12-bit codewords; and writes the results to standard output.
+   */
+  public static void compress() {
+    String input = BinaryStdIn.readString();
+    TernarySearchTrie<Integer> st = new TernarySearchTrie<Integer>();
+
+    // since TernarySearchTrie 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
+    int index = 0;
+    while (index < input.length()) {
+      String s = st.longestPrefixOf(input, index); // 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(index, index + t + 1), code++);
+      }
+      index += t; // Scan past s in input.
+    }
+    BinaryStdOut.write(R, W);
+    BinaryStdOut.close();
+  }
+
+  /**
+   * Reads a sequence of bit encoded using LempelZivWelch 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) { // special case hack
+        s = val + val.charAt(0);
+      }
+      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");
+    }
+  }
+}
+
+/******************************************************************************
+ *  Copyright 2002-2020, Robert Sedgewick and Kevin Wayne.
+ *
+ *  This file is part of algs4.jar, which accompanies the textbook
+ *
+ *      Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne,
+ *      Addison-Wesley Professional, 2011, ISBN 0-321-57351-X.
+ *      http://algs4.cs.princeton.edu
+ *
+ *
+ *  algs4.jar is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  algs4.jar is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with algs4.jar.  If not, see http://www.gnu.org/licenses.
+ ******************************************************************************/