Swift压缩：无法解压LZ4图像

Question

我正在尝试使用Swift压缩包解压缩一个经过lz4压缩的png图像，但代码退出且大小为零。 我的代码如下，期望解压后的文件大小为240Kb。 [更新2] 我在苹果文档中偶然发现了这个内容:

框架在此处进行了记录，以便您可以轻松地包装另一个LZ4编码器/解码器，以产生/消耗相同的数据流（如果需要）。 LZ4编码缓冲区是一系列块，每个块都以标题开头。有三种可能的标题：

压缩块标题由八位字节0x62、0x76、0x34和0x31组成，后跟表示该块所代表的解码（明文）数据的字节数以及存储在该块中的编码数据的大小（以字节为单位）。两个大小字段都存储为（可能不对齐的）32位小端值。紧随压缩块标题的是实际的LZ4编码数据流。

流结束块标题由八位字节0x62、0x76、0x34和0x24组成，并标志着LZ4帧的结束。在此标题之后不能写入或读取任何其他数据。

因此，我相应地添加了标题和页脚，它可以工作。但是——总会有一个“但是”——在解压缩之前如何知道“解码（明文）数据的字节数”？

[更新 1]

我给你一个LZ4文件用于测试，并提供从主包中解压缩它的代码用法。

LZ4文件是使用互联网上广泛可用的“LZ4命令行界面64位v1.8.0，由Yann Collet”进行压缩的。

https://drive.google.com/file/d/1eQ204LJs_xsHRwJ_jUcl1Up9NFo8monO/view?usp=sharing

LZ4解压缩用法

    if let url = Bundle.main.url(forResource: "TestImage300.png", withExtension: "lz4") {
        if let compressed = try? Data(contentsOf: url) {
            if let decompressed = compressed.lz4Decompress() {
                if let image = UIImage(data: decompressed) {
                    self.sourceImages.append(image)
                    print("Unittest: Decompressed image as \(image)")
                }
            }
        }
    }

LZ4数据扩展代码用于解压缩：

import Foundation
import Compression

extension Data {

    func lz4Decompress() -> Data? {
        let destinationBufferSize: size_t = 480000
        let destinationBuffer = UnsafeMutablePointer<UInt8>.allocate(capacity: destinationBufferSize)

        let sourceBufferSize = count

        let decompressed = withUnsafeBytes { (sourceBuffer: UnsafePointer<UInt8>) -> Data? in

            let size = compression_decode_buffer(
                destinationBuffer, destinationBufferSize,
                sourceBuffer, sourceBufferSize,
                nil,    // scratch buffer automatically handled
                COMPRESSION_LZ4
            )

            if size == 0 {
                print("Error ")
                return nil
            }

            print("Original compressed size: \(sourceBufferSize) | Decompressed size: \(size)")

            return Data(bytesNoCopy: destinationBuffer, count: size, deallocator: .free)
        }
        return decompressed
     }
}

- Stéphane de Luca

Answer 1

let intArray: [Int8] =  [-16, 1, 1, 39, 0, 19, 11, -30, 7, 10, 29, 14, 0, 0, 0, 0, 96, 9, 6, 0, 1, 2, 0, 17, 14, 6, 0, 2, 2, 0, 18, 14, 7, 0, 65, 0, 0, 0, -51, 6, 0, 0, 2, 0, 0, 43, 0, 16, 2, 9, 0, -1, 13, 0, 0, 0, 1, 0, 0, 0, 3, 0, 0, 0, 5, 0, 0, 0, 6, 0, 0, 0, 7, 0, 0, 0, 8, 0, 0, 0, 10, 4, 0, 64, 33, -105, 58, 115, 0, 12, 2, 0, 80, 0, 0, 0, 0, 0]

        let uintArray = intArray.map { UInt8(bitPattern: $0) }

//为了使uintArray unsigned可见，它是[240, 1, 1, 39, 0, 19, 11, 226, 7, 10, 29, 14, 0, 0, 0, 0, 96, 9, 6, 0, 1, 2, 0, 17, 14, 6, 0, 2, 2, 0, 18, 14, 7, 0, 65, 0, 0, 0, 205, 6, 0, 0, 2, 0, 0, 43, 0, 16, 2, 9, 0, 255, 13, 0, 0, 0, 1, 0, 0, 0, 3, 0, 0, 0, 5, 0, 0, 0, 6, 0, 0, 0, 7, 0, 0, 0, 8, 0, 0, 0, 10, 4, 0, 64, 33, 151, 58, 115, 0, 12, 2, 0, 80, 0, 0, 0, 0, 0]

        var encodedData = Data.init(bytes:uintArray)

        let decodedCapacity = 205
        let decodedDestinationBuffer = UnsafeMutablePointer<UInt8>.allocate(capacity: decodedCapacity)

        let decodedData = encodedData.withUnsafeBytes {
            (encodedSourceBuffer: UnsafePointer<UInt8>) -> Data? in

            let decodedCharCount = compression_decode_buffer(decodedDestinationBuffer,
                                                             decodedCapacity,
                                                             encodedSourceBuffer,
                                                             encodedData.count,
                                                             nil,
                                                             COMPRESSION_LZ4_RAW)

            if decodedCharCount == 0 {
                fatalError("Decoding failed.")
            }

            print("Before: \(encodedSourceBuffer) | After: \(decodedCharCount)")

            return Data(bytesNoCopy: decodedDestinationBuffer, count: decodedCharCount, deallocator: .free)

        }

如果你在压缩中没有定义头部，那么应该使用COMPRESSION_LZ4_RAW。

- Nicson