Agfdhyk

I am facing a tough time in decoding the LZ4 encoded data in block mode and the apple documentation is not helping me in getting the results,

here is my code on iOS 12, Swift 4.1:

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) }



// For Visibility the uintArray unsigned is [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)



                if decodedCharCount == 0 {

                    fatalError("Decoding failed.")

                }



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



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



            }

reference from : https://developer.apple.com/documentation/compression/compressing_and_decompressing_data_with_buffer_compression

Thanks in advance, It will be a big time help!

Current Output: Decode Failed

Expected Output in signed bytes:

[1, 39, 0, 19, 11, -30, 7, 10, 29, 14, 0, 0, 0, 0, 96, 9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 14, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 14, 0, 0, 0, 0, 0, 0, 0, 0, 0, -51, 0, 0, 0, 0, 0, 0, 0, 0, 0, 96, 9, 0, 0, 2, 0, 0, 0, 0, 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, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, -105, 58, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

Expected Output in unSigned bytes:

[1, 39, 0, 19, 11, 226, 7, 10, 29, 14, 0, 0, 0, 0, 96, 9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 14, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 14, 0, 0, 0, 0, 0, 0, 0, 0, 0, 205, 0, 0, 0, 0, 0, 0, 0, 0, 0, 96, 9, 0, 0, 2, 0, 0, 0, 0, 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, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 10, 0, 0, 0, 151, 58, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

OK My Apologies once again on complaining Apple's Documentation,

They clearly mentioned that,

A compressed block header consists of the octets 0x62, 0x76, 0x34, and 0x31, followed by the size in bytes of the decoded (plaintext) data represented by the block and the size (in bytes) of the encoded data stored in the block. The headers stores both size fields as (possibly unaligned) 32-bit little-endian values. The actual LZ4-encoded data stream immediately follows the compressed block header.

In my case I don't need a header,

So the best constant would be COMPRESSION_LZ4_RAW , The LZ4 compression algorithm, without frame headers.

Changing the algorithm type COMPRESSION_LZ4 to COMPRESSION_LZ4_RAW fixed the issue