Add `isGzipped` proeprty

CHANGELOG.md

@@ -9,6 +9,7 @@ Change Log
 
 - Migrate code to Swift 3.0
 - Add `level` option to `gzipped()` method.
+- Add `isGzipped` property (readonly).
 
 
 

Project/Tests/NSData_GZIPTests.swift

@@ -44,6 +44,10 @@ class NSData_GZIPTests: XCTestCase
         
         XCTAssertNotEqual(gzipped, data)
         XCTAssertEqual(uncompressedSentence, testSentence)
+        
+        XCTAssertTrue(gzipped.isGzipped)
+        XCTAssertFalse(data.isGzipped)
+        XCTAssertFalse(uncompressed.isGzipped)
     }
     
     

README.md

@@ -14,11 +14,17 @@ __NSData+GZIP.swift__ is an extension of Data written in Swift 3.0. It enables c
 
 ```swift
 // gzip
-let compressedData : Data = try! data.gzipped()
+let compressedData: Data = try! data.gzipped()
-let optimizedData : Data = try! data.gzipped(level: .bestCompression)
+let optimizedData: Data = try! data.gzipped(level: .bestCompression)
 
 // gunzip
-let decompressedData : Data = try! compressedData.gunzipped()
+let decompressedData: Data
+if data.isGzipped {
+    decompressedData = try! data.gunzipped()
+} else {
+    decompressedData = data
+}
+
 ```
 
 
@@ -30,11 +36,11 @@ let decompressedData : Data = try! compressedData.gunzipped()
     ![screenshot](Documentation/binary_link@2x.png)
 4. In *Build Settings* > *Swift Compiler - Search Paths*, Add path to `zlib/` to Import Paths (`SWIFT_INCLUDE_PATHS`).
     ![screenshot](Documentation/search_paths@2x.png)
-4. Invoke from your Swift/ObjC files.
+5. Invoke from your Swift/ObjC files.
 
 
 ## Lisence
 
 © 2014-2016 1024jp
 
-NSData+GZIP.swift is distributed under the terms of the __MIT License__. See [LISENCE](LISENCE) for details.
+NSData+GZIP.swift is distributed under the terms of the __MIT License__. See [LISENCE](LISENCE) for details.

Sources/NSData+GZIP.swift

@@ -132,8 +132,22 @@ public enum GzipError: ErrorProtocol {
 
 public extension Data
 {
+    
     /**
-    Create a new `Data` object by compressing the reciver using zlib.
+     Check if the reciever is already gzipped.
+     
+     - returns: Whether the data is compressed.
+     */
+    public var isGzipped: Bool {
+        
+        guard self.count >= 2 else { return false }
+        
+        return self[0] == 0x1f && self[1] == 0x8b  // check magic number
+    }
+    
+    
+    /**
+    Create a new `Data` object by compressing the receiver using zlib.
     Throws an error if compression failed.
      
     - parameters:
@@ -184,7 +198,7 @@ public extension Data
     
     
     /**
-    Create a new `Data` object by decompressing the reciver using zlib.
+    Create a new `Data` object by decompressing the receiver using zlib.
     Throws an error if decompression failed.
     
     - throws: `GzipError`