Swift Compression：不解压 LZ4 图像

Question

我正在尝试使用 Swift 压缩包解压 lz4 压缩的 png 图像，但代码以零大小退出。\n我的代码如下，解压后的文件预计为 240Kb。

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[更新2] \n我在 Apple 的文档中偶然发现了这一点：

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此处记录了该帧，以便您可以轻松包装另一个 LZ4\n 编码器/解码器以在必要时生成/使用相同的数据流。\n LZ4 编码缓冲区是一系列块，每个块\n 以标头开头。共有三种可能的标头：

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压缩块头由八位位组 0x62、0x76、0x34 和 0x31 组成，后跟块表示的解码（明文）数据的字节大小以及编码数据的大小（以字节为单位）。 n 存储在块中。两个大小字段都存储为（可能\n 未对齐的）32 位小端值。压缩块头后面紧跟着实际的 LZ4 编码数据流。

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流标头结束由八位字节 0x62、0x76、0x34 和 0x24 组成，标记 LZ4 帧的结束。除此标头之外，不得写入或读取更多数据。

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所以我相应地添加了页眉和页脚并且它有效。但是 \xe2\x80\x94 总是有一个 \'but\'\xe2\x80\x94，在解压缩之前我如何知道“解码（明文）数据的字节大小”？

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[更新1]

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我提供了一个 LZ4 文件供您测试，以及从主包中解压该文件的代码用法。

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使用 sw“LZ4 命令行界面 64 位 v1.8.0，由 Yann Collet”压缩的 LZ4 文件在互联网上广泛使用

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

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LZ4减压使用

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

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LZ4解压数据扩展代码：

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import Foundation\nimport Compression\n\nextension Data {\n\n    func lz4Decompress() -> Data? {\n        let destinationBufferSize: size_t = 480000\n        let destinationBuffer = UnsafeMutablePointer<UInt8>.allocate(capacity: destinationBufferSize)\n\n        let sourceBufferSize = count\n\n        let decompressed = withUnsafeBytes { (sourceBuffer: UnsafePointer<UInt8>) -> Data? in\n\n            let size = compression_decode_buffer(\n                destinationBuffer, destinationBufferSize,\n                sourceBuffer, sourceBufferSize,\n                nil,    // scratch buffer automatically handled\n                COMPRESSION_LZ4\n            )\n\n            if size == 0 {\n                print("Error ")\n                return nil\n            }\n\n            print("Original compressed size: \\(sourceBufferSize) | Decompressed size: \\(size)")\n\n            return Data(bytesNoCopy: destinationBuffer, count: size, deallocator: .free)\n        }\n        return decompressed\n     }\n}\n

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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) }

// 对于可见性，无符号 uintArray 为 [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)

        }

如果您没有在压缩中定义标头，则应该使用 COMPRESSION_LZ4_RAW