PL/I 语言数据压缩与解压缩存储文件实战
PL/I（Programming Language One）是一种高级程序设计语言，它结合了多种编程语言的特性，旨在提供一种通用、高效的编程环境。在数据存储和处理领域，数据压缩技术是一种重要的优化手段，可以减少存储空间的需求，提高数据传输效率。本文将围绕PL/I语言，探讨数据压缩与解压缩存储文件的实战方法。
数据压缩与解压缩的基本原理
数据压缩的基本原理是通过某种算法减少数据中的冗余信息，从而减小数据的大小。解压缩则是将压缩后的数据恢复到原始状态。常见的压缩算法有：
- 无损压缩：如Huffman编码、LZ77、LZ78等，压缩后的数据可以完全恢复到原始数据。
- 有损压缩：如JPEG、MP3等，压缩过程中会丢失部分信息，但可以显著减小数据大小。
PL/I语言中的数据压缩与解压缩
1. Huffman编码
Huffman编码是一种常用的无损压缩算法。以下是一个使用PL/I实现Huffman编码和解码的示例：
pl/i
IDENTIFICATION DIVISION.
PROGRAM-ID. Huffman-Compression.
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
SELECT INPUT-FILE ASSIGN TO "input.txt".
SELECT OUTPUT-FILE ASSIGN TO "output.txt".
DATA DIVISION.
FILE SECTION.
FD INPUT-FILE.
01 INPUT-RECORD.
05 DATA-VALUE PIC X(100).
FD OUTPUT-FILE.
01 OUTPUT-RECORD.
05 COMPRESSION-BITS PIC X(100).
WORKING-STORAGE SECTION.
01 TABLE.
05 FILLER PIC X(256).
01 HUFFMAN-TABLE.
05 HUFFMAN-CODE OCCURS 256 TIMES INDEXED BY INDEX-VALUE.
10 HUFFMAN-LENGTH PIC 9(2).
10 HUFFMAN-BITS PIC X(100).
PROCEDURE DIVISION.
PERFORM INITIALIZE-HUFFMAN-TABLE
PERFORM BUILD-HUFFMAN-TABLE
PERFORM COMPRESS
PERFORM DECOMPRESSION
STOP RUN.
INITIALIZE-HUFFMAN-TABLE.
PERFORM VARYING INDEX-VALUE FROM 1 BY 1 UNTIL INDEX-VALUE > 256
MOVE 0 TO HUFFMAN-LENGTH(INDEX-VALUE)
MOVE SPACES TO HUFFMAN-BITS(INDEX-VALUE)
END-PERFORM.
BUILD-HUFFMAN-TABLE.
-- 此处省略Huffman编码表构建过程，需要根据数据统计频率生成编码
COMPRESS.
OPEN INPUT INPUT-FILE
OPEN OUTPUT OUTPUT-FILE
READ INPUT-FILE INTO INPUT-RECORD
PERFORM VARYING INDEX-VALUE FROM 1 BY 1 UNTIL INDEX-VALUE > LENGTH OF INPUT-RECORD
MOVE HUFFMAN-BITS(INDEX-VALUE) TO OUTPUT-RECORD
END-PERFORM
CLOSE INPUT-FILE
CLOSE OUTPUT-FILE.
DECOMPRESSION.
OPEN INPUT OUTPUT-FILE
OPEN OUTPUT INPUT-FILE
READ OUTPUT-FILE INTO OUTPUT-RECORD
PERFORM VARYING INDEX-VALUE FROM 1 BY 1 UNTIL INDEX-VALUE > LENGTH OF OUTPUT-RECORD
-- 此处省略Huffman解码过程，需要根据编码表解码
END-PERFORM
CLOSE OUTPUT-FILE
CLOSE INPUT-FILE.
2. Run-Length Encoding (RLE)
RLE是一种简单的压缩算法，它通过记录连续相同数据的数量来减少数据大小。以下是一个使用PL/I实现RLE压缩和解压缩的示例：
pl/i
IDENTIFICATION DIVISION.
PROGRAM-ID. RLE-Compression.
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
SELECT INPUT-FILE ASSIGN TO "input.txt".
SELECT OUTPUT-FILE ASSIGN TO "output.txt".
DATA DIVISION.
FILE SECTION.
FD INPUT-FILE.
01 INPUT-RECORD.
05 DATA-VALUE PIC X(100).
FD OUTPUT-FILE.
01 OUTPUT-RECORD.
05 COMPRESSION-BITS PIC X(100).
WORKING-STORAGE SECTION.
01 COUNT PIC 9(4).
01 BUFFER PIC X(100).
PROCEDURE DIVISION.
PERFORM INITIALIZE
PERFORM COMPRESS
PERFORM DECOMPRESSION
STOP RUN.
INITIALIZE.
MOVE 0 TO COUNT
MOVE SPACES TO BUFFER.
COMPRESS.
OPEN INPUT INPUT-FILE
OPEN OUTPUT OUTPUT-FILE
READ INPUT-FILE INTO INPUT-RECORD
PERFORM VARYING INDEX-VALUE FROM 1 BY 1 UNTIL INDEX-VALUE > LENGTH OF INPUT-RECORD
IF BUFFER = INPUT-RECORD(INDEX-VALUE)
ADD 1 TO COUNT
ELSE
IF COUNT > 0
WRITE COUNT TO OUTPUT-FILE
WRITE BUFFER TO OUTPUT-FILE
MOVE 1 TO COUNT
MOVE INPUT-RECORD(INDEX-VALUE) TO BUFFER
END-IF
END-IF
END-PERFORM
IF COUNT > 0
WRITE COUNT TO OUTPUT-FILE
WRITE BUFFER TO OUTPUT-FILE
END-IF
CLOSE INPUT-FILE
CLOSE OUTPUT-FILE.
DECOMPRESSION.
OPEN INPUT OUTPUT-FILE
OPEN OUTPUT INPUT-FILE
READ OUTPUT-FILE INTO COUNT
PERFORM UNTIL COUNT = 0
READ OUTPUT-FILE INTO BUFFER
PERFORM VARYING INDEX-VALUE FROM 1 BY 1 UNTIL INDEX-VALUE > COUNT
WRITE BUFFER TO INPUT-FILE
END-PERFORM
READ OUTPUT-FILE INTO COUNT
END-PERFORM
CLOSE OUTPUT-FILE
CLOSE INPUT-FILE.
总结
本文通过PL/I语言实现了两种常见的数据压缩算法：Huffman编码和Run-Length Encoding（RLE）。这些示例展示了如何在PL/I中处理数据压缩和解压缩任务，从而优化数据存储和传输。在实际应用中，可以根据具体需求选择合适的压缩算法，并对其进行优化和扩展。
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