From 91cb18dd8f6686d064270a81aca036c702b3a23f Mon Sep 17 00:00:00 2001
From: 1024jp <1024jp@wolfrosch.com>
Date: Mon, 18 Jul 2016 00:55:34 +0900
Subject: [PATCH] Add `isGzipped` proeprty

---
 CHANGELOG.md                         |  1 +
 Project/Tests/NSData_GZIPTests.swift |  4 ++++
 README.md                            | 16 +++++++++++-----
 Sources/NSData+GZIP.swift            | 18 ++++++++++++++++--
 4 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1d58932..a57ecf0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ Change Log
 
 - Migrate code to Swift 3.0
 - Add `level` option to `gzipped()` method.
+- Add `isGzipped` property (readonly).
 
 
 
diff --git a/Project/Tests/NSData_GZIPTests.swift b/Project/Tests/NSData_GZIPTests.swift
index 9a263c5..51321d8 100644
--- a/Project/Tests/NSData_GZIPTests.swift
+++ b/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)
     }
     
     
diff --git a/README.md b/README.md
index 1199925..0beac06 100644
--- a/README.md
+++ b/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 optimizedData : Data = try! data.gzipped(level: .bestCompression)
+let compressedData: Data = try! data.gzipped()
+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.
\ No newline at end of file
+NSData+GZIP.swift is distributed under the terms of the __MIT License__. See [LISENCE](LISENCE) for details.
diff --git a/Sources/NSData+GZIP.swift b/Sources/NSData+GZIP.swift
index 6c52f6d..d3c573b 100644
--- a/Sources/NSData+GZIP.swift
+++ b/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`