Added new class for getting bits from data in efficient way. And performance tests for it.

SWCompression.xcodeproj/project.pbxproj

@@ -59,6 +59,7 @@
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 		06E961CA1DBD5FA1008C47F2 /* HuffmanTable.swift in Sources */ = {isa = PBXBuildFile; fileRef = 06E961C91DBD5FA1008C47F2 /* HuffmanTable.swift */; };
+		06F27A6D1DCD1F8800A98168 /* DataWithPointerTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 06F27A6C1DCD1F8800A98168 /* DataWithPointerTests.swift */; };
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@@ -106,6 +107,7 @@
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 		06E961C91DBD5FA1008C47F2 /* HuffmanTable.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = HuffmanTable.swift; sourceTree = "<group>"; };
+		06F27A6C1DCD1F8800A98168 /* DataWithPointerTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = DataWithPointerTests.swift; sourceTree = "<group>"; };
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 /* Begin PBXFrameworksBuildPhase section */
@@ -196,9 +198,9 @@
 				064492581DC606D400F10981 /* ZlibArchive.swift */,
 				061FCE251DBCC4BE0052F7BE /* Deflate.swift */,
 				0661F0721DC92F98006C3D35 /* DataWithPointer.swift */,
+				061FCE301DBCC8BF0052F7BE /* Extensions.swift */,
 				06E961C91DBD5FA1008C47F2 /* HuffmanTable.swift */,
 				06AC9B0C1DC1027900A5066B /* HuffmanLength.swift */,
-				061FCE301DBCC8BF0052F7BE /* Extensions.swift */,
 				06BE1ACC1DB410F100EE0F59 /* Info.plist */,
 				065569611DC65B1B00A47E66 /* Info-iOS.plist */,
 				065569621DC65B1B00A47E66 /* Info-tvOS.plist */,
@@ -213,6 +215,7 @@
 			children = (
 				061FCE271DBCC6070052F7BE /* Test Files */,
 				06BE1AD61DB410F100EE0F59 /* SWCompressionTests.swift */,
+				06F27A6C1DCD1F8800A98168 /* DataWithPointerTests.swift */,
 				0661F06E1DC92391006C3D35 /* PerformanceTests.swift */,
 				061FCE2C1DBCC6A30052F7BE /* Constants.swift */,
 				06BE1AD81DB410F100EE0F59 /* Info.plist */,
@@ -525,6 +528,7 @@
 				06BE1AD71DB410F100EE0F59 /* SWCompressionTests.swift in Sources */,
 				061FCE2D1DBCC6A30052F7BE /* Constants.swift in Sources */,
 				0661F06F1DC92391006C3D35 /* PerformanceTests.swift in Sources */,
+				06F27A6D1DCD1F8800A98168 /* DataWithPointerTests.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};

Sources/DataWithPointer.swift

@@ -0,0 +1,55 @@
+//
+//  DataWithPointer.swift
+//  SWCompression
+//
+//  Created by Timofey Solomko on 01.11.16.
+//  Copyright © 2016 Timofey Solomko. All rights reserved.
+//
+
+import Foundation
+import CoreFoundation
+
+class DataWithPointer {
+
+    /// Only needed for creation of bitVector in initData()
+    let data: Data
+    
+    private(set) var bitVector: CFBitVector?
+    private(set) var index: Int
+    private(set) var bitShift: Int
+
+    init(data: Data) {
+        self.data = data
+        self.index = 0
+        self.bitShift = 0
+        self.bitVector = nil
+        initData()
+    }
+
+    private func initData() {
+        data.withUnsafeBytes { (bytes: UnsafePointer<UInt8>) in
+            self.bitVector = CFBitVectorCreate(kCFAllocatorDefault, bytes, data.count * 8)
+        }
+    }
+
+    func bits(count: Int) -> [UInt8] {
+        guard count > 0 else { return [] }
+
+        /// Start point of bits to return
+        let startIndex = self.index * 8 + self.bitShift
+        /// Range of bits to return in bitVector
+        let range = CFRangeMake(startIndex, count)
+
+        var array: [UInt8] = Array(repeating: 0, count: count)
+        CFBitVectorGetBits(self.bitVector!, range, &array)
+
+        // Update index and bitShift
+        let amountOfBytes = (count - self.bitShift) / 8
+        self.index += amountOfBytes
+        self.bitShift += count - 8 * amountOfBytes
+
+
+        return array
+    }
+
+}

Tests/DataWithPointerTests.swift

@@ -0,0 +1,39 @@
+//
+//  DataWithPointerTests.swift
+//  SWCompression
+//
+//  Created by Timofey Solomko on 04.11.16.
+//  Copyright © 2016 tsolomko. All rights reserved.
+//
+
+import XCTest
+@testable import SWCompression
+
+class DataWithPointerTests: XCTestCase {
+
+    func testDataWithPointer() {
+        self.measure {
+            let testData = try? Data(contentsOf: Constants.relativelyBigFilePath)
+            XCTAssertNotNil(testData, "Failed to load test archive")
+            let testDataWithPointer = DataWithPointer(data: testData!)
+            for _ in 0..<testData!.count {
+                let _ = testDataWithPointer.bits(count: 8)
+            }
+        }
+    }
+
+    func testData() {
+        self.measure {
+            let testData = try? Data(contentsOf: Constants.relativelyBigFilePath)
+            XCTAssertNotNil(testData, "Failed to load test archive")
+            var index = 0
+            for _ in 0..<testData!.count  {
+                let start = (index, 0)
+                let end = (index, 7)
+                index += 1
+                let _ = testData!.bits(from: start, to: end)
+            }
+        }
+    }
+
+}