100 lines
2.7 KiB
Java
100 lines
2.7 KiB
Java
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import java.io.FileInputStream;
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import java.util.*;
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public class Lzw {
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static final char END = (char) 0xFFFF;
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public static int[] lzwEncoding(String s) {
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List<Integer> result = new ArrayList<>();
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Map<String, Integer> dict = new HashMap<>();
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int cnt = 0;
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for (int i = 0; i < 256; i++)
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dict.put(String.valueOf((char) i), cnt++);
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dict.put(String.valueOf(END), cnt++);
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String cur = "";
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for (char ch : s.toCharArray()) {
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cur += ch;
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if (!dict.containsKey(cur)) {
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Integer e = dict.get(cur.substring(0, cur.length() - 1));
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result.add(e);
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dict.put(cur, cnt++);
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cur = String.valueOf(ch);
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}
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}
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result.add(dict.get(cur));
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result.add(dict.get(String.valueOf(END)));
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int[] res = new int[result.size()];
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for (int i = 0; i < res.length; i++)
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res[i] = result.get(i);
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System.out.printf("Dict items: %,d\n", dict.size());
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// System.out.printf("Dict size: %,d\n", SizingAgent.deepSizeOf(dict));
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return res;
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}
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public static String lzwDecoding(int[] s) {
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StringBuilder sb = new StringBuilder();
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Map<Integer, String> dict = new HashMap<>();
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int cnt = 0;
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for (int i = 0; i < 256; i++)
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dict.put(cnt++, String.valueOf((char) i));
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dict.put(cnt++, String.valueOf(END));
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String cur = dict.get(s[0]);
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sb.append(cur);
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char a = cur.charAt(0);
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for (int i = 1; i < s.length; i++) {
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int code = s[i];
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if (code < cnt) {
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String str = dict.get(code);
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a = str.charAt(0);
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if (a == END)
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break;
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dict.put(cnt++, cur + a);
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sb.append(str);
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cur = str;
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} else {
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cur += a;
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dict.put(cnt++, cur);
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sb.append(cur);
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}
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}
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return sb.toString();
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}
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// Usage example
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public static void main(String[] args) throws Exception {
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FileInputStream fs = new FileInputStream("src/Lzw.java");
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byte[] data = new byte[1_000_000];
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int len = fs.read(data, 0, data.length);
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data = Arrays.copyOf(data, len);
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char[] buffer = new char[len];
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for (int i = 0; i < len; i++)
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buffer[i] = (char) (Byte.toUnsignedInt(data[i]));
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String s = new String(buffer, 0, len);
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int[] encoded = lzwEncoding(s);
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String s2 = lzwDecoding(encoded);
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byte[] data2 = new byte[len];
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for (int i = 0; i < len; i++)
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data2[i] = (byte) s2.charAt(i);
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System.out.println(Arrays.equals(data, data2));
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Locale.setDefault(Locale.US);
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System.out.printf("%d -> %.0f\n", s.length(), optimalCompressedLength(encoded));
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}
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static double optimalCompressedLength(int[] a) {
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int max = 0;
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for (int x : a)
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max = Math.max(max, x);
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int[] freq = new int[max + 1];
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for (int x : a)
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++freq[x];
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double optimalLength = 0;
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for (int f : freq)
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if (f > 0)
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optimalLength += f * Math.log((double) a.length / f) / Math.log(2) / 8;
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return optimalLength;
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}
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}
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