-
Notifications
You must be signed in to change notification settings - Fork 18
Expand file tree
/
Copy pathLZ77.java
More file actions
161 lines (139 loc) · 5.45 KB
/
LZ77.java
File metadata and controls
161 lines (139 loc) · 5.45 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
/**
* Copyright © https://github.com/microwind All rights reserved.
*
* @author: jarryli@gmail.com
* @version: 1.0
*/
import java.util.*;
public class LZ77 {
static class Token {
int offset, length;
char character;
/**
* LZ77压缩令牌构造函数
* @param offset 偏移量
* @param length 匹配长度
* @param character 下一个字符
*/
Token(int offset, int length, char character) {
this.offset = offset;
this.length = length;
this.character = character;
}
@Override
public String toString() {
if (length > 0) {
return String.format("(%d,%d)", offset, length);
} else {
return String.format("(%c)", character);
}
}
}
/**
* LZ77压缩算法
* @param input 输入字符串
* @return 压缩后的令牌列表
*/
public static List<Token> compress(String input) {
System.out.println("LZ77压缩开始...");
List<Token> tokens = new ArrayList<>();
int windowSize = 256;
int position = 0;
while (position < input.length()) {
int maxLength = Math.min(windowSize, position);
String window = input.substring(Math.max(0, position - maxLength), position);
int bestLength = 0;
int bestOffset = 0;
// 在窗口中搜索最长匹配
for (int i = 0; i < window.length(); i++) {
int matchLength = 0;
while (position + matchLength < input.length() &&
i + matchLength < window.length() &&
input.charAt(position + matchLength) == window.charAt(i + matchLength)) {
matchLength++;
}
if (matchLength > bestLength) {
bestLength = matchLength;
bestOffset = window.length() - i;
}
}
if (bestLength >= 3) { // 最小匹配长度
char nextChar = position + bestLength < input.length() ?
input.charAt(position + bestLength) : '\0';
tokens.add(new Token(bestOffset, bestLength, nextChar));
System.out.println("找到匹配: 偏移=" + bestOffset + ", 长度=" + bestLength);
position += bestLength + 1;
} else {
tokens.add(new Token(0, 0, input.charAt(position)));
System.out.println("字面量: " + input.charAt(position));
position++;
}
}
System.out.println("LZ77压缩完成，令牌数: " + tokens.size());
return tokens;
}
/**
* LZ77解压算法
* @param tokens 压缩令牌列表
* @return 解压后的字符串
*/
public static String decompress(List<Token> tokens) {
System.out.println("LZ77解压开始...");
StringBuilder output = new StringBuilder();
for (int i = 0; i < tokens.size(); i++) {
Token token = tokens.get(i);
System.out.println("处理令牌 " + i + ": " + token);
if (token.length > 0) {
int start = output.length() - token.offset;
for (int j = 0; j < token.length; j++) {
output.append(output.charAt(start + j));
}
if (token.character != '\0') {
output.append(token.character);
}
} else {
output.append(token.character);
}
}
System.out.println("LZ77解压完成，长度: " + output.length());
return output.toString();
}
/**
* 计算压缩比
* @param original 原始字符串
* @param compressed 压缩令牌列表
* @return 压缩比
*/
public static double compressionRatio(String original, List<Token> compressed) {
int originalSize = original.length() * 8; // 假设每个字符8位
int compressedSize = 0;
for (Token token : compressed) {
compressedSize += 16; // 偏移和长度各8位
if (token.length > 0) compressedSize += 8; // 字符8位
}
return (double) compressedSize / originalSize;
}
/**
* 主函数，演示LZ77压缩和解压
*/
public static void main(String[] args) {
String input = "ABABABABABAABABABABA";
System.out.println("=== LZ77算法演示 ===");
System.out.println("原始文本: " + input);
System.out.println("原始长度: " + input.length());
// 压缩
List<Token> compressed = compress(input);
System.out.println("压缩结果: " + compressed);
// 计算压缩比
double ratio = compressionRatio(input, compressed);
System.out.println("压缩比: " + String.format("%.2f", ratio));
// 解压
String decompressed = decompress(compressed);
System.out.println("解压结果: " + decompressed);
System.out.println("解压长度: " + decompressed.length());
// 验证
boolean isValid = input.equals(decompressed);
System.out.println("验证结果: " + (isValid ? "✓ 成功" : "✗ 失败"));
System.out.println("=== 演示结束 ===");
}
}