是否有实用的方法来压缩NSData？

Question

我没有看到关于该主题的任何文档，但这并不意味着它不存在。

Answer 1

是的，使用zlib压缩数据。

@Brad Larson发布了这个：see here并添加了代码。

有一个CocoaPod使用Objective-Zip flydolphinstudio。

Answer 2

关注@Zaph和@Brad Larson的帖子，下面是两种方法gzipInflate和gzipDeflate，可以很好地压缩/解压缩NSData。 （代码从cocoadev.com/wiki/NSDataCategory重新格式化

#import "zlib.h"
// don't forget to add libz.1.2.x.dylib into your project

- (NSData *)gzipInflate:(NSData*)data
{
    if ([data length] == 0) return data;

    unsigned full_length = [data length];
    unsigned half_length = [data length] / 2;

    NSMutableData *decompressed = [NSMutableData dataWithLength: full_length + half_length];
    BOOL done = NO;
    int status;

    z_stream strm;
    strm.next_in = (Bytef *)[data bytes];
    strm.avail_in = [data length];
    strm.total_out = 0;
    strm.zalloc = Z_NULL;
    strm.zfree = Z_NULL;

    if (inflateInit2(&strm, (15+32)) != Z_OK) return nil;
    while (!done)
    {
        // Make sure we have enough room and reset the lengths.
        if (strm.total_out >= [decompressed length])
            [decompressed increaseLengthBy: half_length];
        strm.next_out = [decompressed mutableBytes] + strm.total_out;
        strm.avail_out = [decompressed length] - strm.total_out;

        // Inflate another chunk.
        status = inflate (&strm, Z_SYNC_FLUSH);
        if (status == Z_STREAM_END) done = YES;
        else if (status != Z_OK) break;
    }
    if (inflateEnd (&strm) != Z_OK) return nil;

    // Set real length.
    if (done)
    {
        [decompressed setLength: strm.total_out];
        return [NSData dataWithData: decompressed];
    }
    else return nil;
}

- (NSData *)gzipDeflate:(NSData*)data
{
    if ([data length] == 0) return data;

    z_stream strm;

    strm.zalloc = Z_NULL;
    strm.zfree = Z_NULL;
    strm.opaque = Z_NULL;
    strm.total_out = 0;
    strm.next_in=(Bytef *)[data bytes];
    strm.avail_in = [data length];

    // Compresssion Levels:
    //   Z_NO_COMPRESSION
    //   Z_BEST_SPEED
    //   Z_BEST_COMPRESSION
    //   Z_DEFAULT_COMPRESSION

    if (deflateInit2(&strm, Z_DEFAULT_COMPRESSION, Z_DEFLATED, (15+16), 8, Z_DEFAULT_STRATEGY) != Z_OK) return nil;

    NSMutableData *compressed = [NSMutableData dataWithLength:16384];  // 16K chunks for expansion

    do {

        if (strm.total_out >= [compressed length])
            [compressed increaseLengthBy: 16384];

        strm.next_out = [compressed mutableBytes] + strm.total_out;
        strm.avail_out = [compressed length] - strm.total_out;

        deflate(&strm, Z_FINISH);  

    } while (strm.avail_out == 0);

    deflateEnd(&strm);

    [compressed setLength: strm.total_out];
    return [NSData dataWithData:compressed];
}

从日志中：

[data length] (orig):989631
[data length] (gz):  102757
[data length] (ungz):989631

Answer 3

从iOS 9.0开始，内置支持一些压缩算法。该库称为libcompression，支持LZ4，LZMA，ZLIB和LZFSE。

这是一个使用libcompression解压缩LZMA的Swift示例。它很冗长，但避免了外部依赖，可以隐藏在NSData的扩展中。

import Compression

let streamPtr = UnsafeMutablePointer<compression_stream>.alloc(1)
var stream = streamPtr.memory
var status: compression_status

status = compression_stream_init(&stream, COMPRESSION_STREAM_DECODE, COMPRESSION_LZMA)
stream.src_ptr = UnsafePointer<UInt8>(compressedData.bytes)
stream.src_size = compressedData.length

let dstBufferSize: size_t = 4096
let dstBufferPtr = UnsafeMutablePointer<UInt8>.alloc(dstBufferSize)
stream.dst_ptr = dstBufferPtr
stream.dst_size = dstBufferSize

let decompressedData = NSMutableData()

repeat {
    status = compression_stream_process(&stream, 0)
    switch status {
    case COMPRESSION_STATUS_OK:
        if stream.dst_size == 0 {
            decompressedData.appendBytes(dstBufferPtr, length: dstBufferSize)
            stream.dst_ptr = dstBufferPtr
            stream.dst_size = dstBufferSize
        }
    case COMPRESSION_STATUS_END:
        if stream.dst_ptr > dstBufferPtr {
            decompressedData.appendBytes(dstBufferPtr, length: stream.dst_ptr - dstBufferPtr)
        }
    default:
        break
    }
}
while status == COMPRESSION_STATUS_OK

compression_stream_destroy(&stream)

if status == COMPRESSION_STATUS_END {
    // Decompression succeeded, do something with decompressedData
}
else {
    // Decompression failed
}

Answer 4

我已经制作了一个很好的Objective-C BZip2压缩接口，可用作CocoaPod：https://github.com/blakewatters/BZipCompression

Answer 5

围绕libcompression的Swift 3 ready wrapper。 https://github.com/mw99/DataCompression

Swift libcompression包装器作为数据类型的扩展（ZLIB，LZFSE，LZMA，LZ4，deflate，RFC-1950，RFC-1951）

有趣的在操场上玩：playground compression rates

Answer 6

试试这个：https://github.com/mattt/Godzippa这对我有帮助。