Added archive function to gzip.

Sources/GzipArchive.swift

@@ -72,6 +72,7 @@ public struct GzipHeader {
 
     /// Compression method of archive. Always equals to `.deflate`.
     public let compressionMethod: CompressionMethod
+    // TODO: Make it optional.
     /// The most recent modification time of the original file.
     public let modificationTime: Date
     /// Type of file system on which compression took place.
@@ -118,6 +119,7 @@ public struct GzipHeader {
         for i in 0..<4 {
             headerBytes.append(((mtime & (0xFF << (i * 8))) >> (i * 8)).toUInt8())
         }
+        // TODO: Should be nil in case of mtime == 0.
         self.modificationTime = Date(timeIntervalSince1970: TimeInterval(mtime))
 
         let extraFlags = pointerData.alignedByte()
@@ -218,4 +220,33 @@ public final class GzipArchive: Archive {
         return Data(bytes: out)
     }
 
+    public static func archive(data: Data) throws -> Data {
+        let out: [UInt8] = [
+            0x1f, 0x8b, // 'magic' bytes.
+            8, // Compression method (DEFLATE).
+            0, // Flags; currently no flags are set.
+            0, 0, 0, 0, // Mtime; currently timestamp is not set.
+            2, // Extra flags; 2 means that DEFLATE used slowest algorithm.
+            255, // OS Type; set to default value (unknown).
+        ]
+        var outData = Data(bytes: out)
+        outData.append(try Deflate.compress(data: data))
+
+        let crc32 = CheckSums.crc32(data.toArray(type: UInt8.self))
+        var crcBytes = [UInt8]()
+        for i: UInt32 in 0..<4 {
+            crcBytes.append(UInt8((crc32 & (0xFF << (i * 8))) >> (i * 8)))
+        }
+        outData.append(Data(bytes: crcBytes))
+
+        let isize = UInt64(data.count) % UInt64(1) << 32
+        var isizeBytes = [UInt8]()
+        for i: UInt64 in 0..<4 {
+            isizeBytes.append(UInt8((isize & (0xFF << (i * 8))) >> (i * 8)))
+        }
+        outData.append(Data(bytes: isizeBytes))
+
+        return outData
+    }
+
 }