From d5d0abeff2f2eddce197d2836d52ce12f998a500 Mon Sep 17 00:00:00 2001
From: Timofey Solomko <tsolomko@gmail.com>
Date: Sat, 2 Oct 2021 15:23:50 +0300
Subject: [PATCH] [LZ4] Implement a no-compression version of LZ4 compression

Basically, the frame format with all features has been implemented.
---
 Sources/LZ4/LZ4+Compress.swift | 119 +++++++++++++++++++++++++++++++--
 1 file changed, 114 insertions(+), 5 deletions(-)

diff --git a/Sources/LZ4/LZ4+Compress.swift b/Sources/LZ4/LZ4+Compress.swift
index 7af9ee72..28530a8c 100644
--- a/Sources/LZ4/LZ4+Compress.swift
+++ b/Sources/LZ4/LZ4+Compress.swift
@@ -8,13 +8,122 @@ import BitByteData
 
 extension LZ4: CompressionAlgorithm {
 
-    public static func compress(data: Data) throws -> Data {
-        fatalError("Not implemented yet.")
+    public static func compress(data: Data) -> Data {
+        return LZ4.compress(data: data, independentBlocks: true, blockChecksums: false, contentChecksum: true,
+                            contentSize: false, blockSize: 4 * 1024 * 1024, dictionary: nil, dictionaryID: nil)
     }
 
-    private static func compress(data: Data, independentBlocks: Bool, blockChecksums: Bool,
-                                 contentChecksum: Bool, contentSize: Bool) throws -> Data {
-        fatalError("Not implemented yet.")
+    public static func compress(data: Data, independentBlocks: Bool, blockChecksums: Bool,
+                                 contentChecksum: Bool, contentSize: Bool, blockSize: Int,
+                                 dictionary: Data?, dictionaryID: UInt32? = nil) -> Data {
+        var out = [UInt8]()
+
+        // Magic number.
+        out.append(contentsOf: [0x04, 0x22, 0x4D, 0x18])
+
+        // FLG byte.
+        out.append(0b0100_0000 |
+                   (independentBlocks ? 0x20 : 0) |
+                   (blockChecksums ? 0x10 : 0) |
+                   (contentSize ? 0x8 : 0) |
+                   (contentChecksum ? 0x4 : 0) |
+                   (dictionaryID != nil ? 0x1 : 0))
+
+        // BD byte.
+        let bd: UInt8
+        let maxBlockSize: Int
+        if blockSize <= 64 * 1024 {
+            maxBlockSize = 64 * 1024
+            bd = 0x40
+        } else if blockSize <= 256 * 1024 {
+            maxBlockSize = 256 * 1024
+            bd = 0x50
+        } else if blockSize <= 1024 * 1024 {
+            maxBlockSize = 1024 * 1024
+            bd = 0x60
+        } else {
+            // Reference implementation sets maximum block size to 4 MB even if the requested block size is bigger.
+            maxBlockSize = 4 * 1024 * 1024
+            bd = 0x70
+        }
+        out.append(bd)
+
+        if contentSize {
+            let size = UInt64(truncatingIfNeeded: data.count)
+            for i in 0..<8 {
+                out.append(UInt8(truncatingIfNeeded: (size & (0xFF << (i * 8))) >> (i * 8)))
+            }
+        }
+
+        if let dictionaryID = dictionaryID {
+            for i in 0..<4 {
+                out.append(UInt8(truncatingIfNeeded: (dictionaryID & (0xFF << (i * 8))) >> (i * 8)))
+            }
+        }
+
+        // Header checksum.
+        let headerChecksum = XxHash32.hash(data: Data(out[4...]))
+        out.append(UInt8(truncatingIfNeeded: (headerChecksum >> 8) & 0xFF))
+
+        var dict = dictionary
+        for i in stride(from: data.startIndex, to: data.endIndex, by: blockSize) {
+            // TODO: Which block size should we use here: the arbitrary requested by the user, or the maxBlockSize?
+            let blockData = data[i..<min(i + blockSize, data.endIndex)]
+            let compressedBlock = LZ4.compress(block: blockData, dict)
+            if !independentBlocks {
+                dict = blockData[max(blockData.endIndex - 64 * 1024, 0)...]
+            }
+
+            if compressedBlock.count >= blockData.count { // TODO: > or >=? (right now >= to make trivial impl for compress(block:))
+                // In this case the data is non-compressible, so we write the block as uncompressed.
+                if blockData.count > 0x7FFFFFFF {
+                    // TODO: In this case we cannot properly store uncompressed block, since either the highest bit of
+                    // TODO: 4-bytes is already taken, or the block size is to big to fit into 4-bytes.
+                    fatalError("Patalogical size of non-compressible block.")
+                }
+                let blockSize = (0x80000000 as UInt32) | UInt32(truncatingIfNeeded: blockData.count)
+                for i in 0..<4 {
+                    out.append(UInt8(truncatingIfNeeded: (blockSize & (0xFF << (i * 8))) >> (i * 8)))
+                }
+                out.append(contentsOf: blockData)
+            } else {
+                if compressedBlock.count > 0x7FFFFFFF {
+                    // TODO: In this case we cannot properly store uncompressed block, since either the highest bit of
+                    // TODO: 4-bytes is already taken, or the block size is to big to fit into 4-bytes.
+                    fatalError("Patalogical size of compressed block.")
+                }
+                let blockSize = UInt32(truncatingIfNeeded: compressedBlock.count)
+                for i in 0..<4 {
+                    out.append(UInt8(truncatingIfNeeded: (blockSize & (0xFF << (i * 8))) >> (i * 8)))
+                }
+                out.append(contentsOf: compressedBlock)
+            }
+
+            if blockChecksums {
+                let blockChecksum = XxHash32.hash(data: Data(compressedBlock))
+                for i in 0..<4 {
+                    out.append(UInt8(truncatingIfNeeded: (blockChecksum & (0xFF << (i * 8))) >> (i * 8)))
+                }
+            }
+        }
+
+        // EndMark.
+        out.append(contentsOf: [0x00, 0x00, 0x00, 0x00])
+
+        // Content checksum.
+        if contentChecksum {
+            let hash = XxHash32.hash(data: data)
+            for i in 0..<4 {
+                out.append(UInt8(truncatingIfNeeded: (hash & (0xFF << (i * 8))) >> (i * 8)))
+            }
+        }
+
+        return Data(out)
+    }
+
+    private static func compress(block: Data, _ dict: Data?) -> [UInt8] {
+        // TODO:
+        return block.withUnsafeBytes { $0.map { $0 } }
     }
 
 }