Added basic implementation for NSString contentsOfURL:usedEncoding: (#893)

* Added basic implementation for NSString contentsOfURL:usedEncoding:

In NSString, for contentsOfURL:usedEncoding: checking the BOM of the received data and determinig whether encoding to be used should be UTF 16 BE

Fixed guard statement to handle nil case

Fixed indentation in test case

Updated build.py file to include test input file

Fixed Indentation in build.py

* Ensuring file size is atleasy 2 bytes and added test case for UTF 16 LE

Author: mohitathwani

Foundation/NSString.swift

@@ -1298,7 +1298,33 @@ extension NSString {
     }
 
     public convenience init(contentsOf url: URL, usedEncoding enc: UnsafeMutablePointer<UInt>?) throws {
-        NSUnimplemented()    
+        let readResult = try NSData(contentsOf: url, options:[])
+
+        let bytePtr = readResult.bytes.bindMemory(to: UInt8.self, capacity:readResult.length)
+        if readResult.length >= 2 && bytePtr[0] == 254 && bytePtr[1] == 255 {
+          enc?.pointee = String.Encoding.utf16BigEndian.rawValue
+        }
+        else if readResult.length >= 2 && bytePtr[0] == 255 && bytePtr[1] == 254 {
+          enc?.pointee = String.Encoding.utf16LittleEndian.rawValue
+        }
+        else {
+          //Need to work on more conditions. This should be the default
+          enc?.pointee = String.Encoding.utf8.rawValue
+        }
+
+        guard let enc = enc, let cf = CFStringCreateWithBytes(kCFAllocatorDefault, bytePtr, readResult.length, CFStringConvertNSStringEncodingToEncoding(enc.pointee), true) else {
+            throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.fileReadInapplicableStringEncoding.rawValue, userInfo: [
+                "NSDebugDescription" : "Unable to create a string using the specified encoding."
+                ])
+        }
+        var str: String?
+        if String._conditionallyBridgeFromObjectiveC(cf._nsObject, result: &str) {
+            self.init(str!)
+        } else {
+            throw NSError(domain: NSCocoaErrorDomain, code: CocoaError.fileReadInapplicableStringEncoding.rawValue, userInfo: [
+                "NSDebugDescription" : "Unable to bridge CFString to String."
+                ])
+        }    
     }
 
     public convenience init(contentsOfFile path: String, usedEncoding enc: UnsafeMutablePointer<UInt>?) throws {

TestFoundation/Resources/NSString-UTF16-BE-data.txt

@@ -66,6 +66,8 @@ class TestNSString : XCTestCase {
             ("test_rangeOfCharacterFromSet", test_rangeOfCharacterFromSet ),
             ("test_CFStringCreateMutableCopy", test_CFStringCreateMutableCopy),
             ("test_FromContentsOfURL",test_FromContentsOfURL),
+            ("test_FromContentsOfURLUsedEncodingUTF16BE", test_FromContentsOfURLUsedEncodingUTF16BE),
+            ("test_FromContentsOfURLUsedEncodingUTF16LE", test_FromContentsOfURLUsedEncodingUTF16LE),
             ("test_FromContentOfFile",test_FromContentOfFile),
             ("test_swiftStringUTF16", test_swiftStringUTF16),
             // This test takes forever on build servers; it has been seen up to 1852.084 seconds
@@ -300,6 +302,38 @@ class TestNSString : XCTestCase {
         }
     }
 
+    func test_FromContentsOfURLUsedEncodingUTF16BE() {
+      guard let testFileURL = testBundle().url(forResource: "NSString-UTF16-BE-data", withExtension: "txt") else {
+        XCTFail("URL for NSString-UTF16-BE-data.txt is nil")
+        return
+      }
+
+      do {
+          var encoding: UInt = 0
+          let string = try NSString(contentsOf: testFileURL, usedEncoding: &encoding)
+          XCTAssertEqual(string, "NSString fromURL usedEncoding test with UTF16 BE file", "Wrong result when reading UTF16BE file")
+          XCTAssertEqual(encoding, String.Encoding.utf16BigEndian.rawValue, "Wrong encoding detected from UTF16BE file")
+      } catch {
+          XCTFail("Unable to init NSString from contentsOf:usedEncoding:")
+      }
+    }
+
+    func test_FromContentsOfURLUsedEncodingUTF16LE() {
+      guard let testFileURL = testBundle().url(forResource: "NSString-UTF16-LE-data", withExtension: "txt") else {
+        XCTFail("URL for NSString-UTF16-LE-data.txt is nil")
+        return
+      }
+
+      do {
+          var encoding: UInt = 0
+          let string = try NSString(contentsOf: testFileURL, usedEncoding: &encoding)
+          XCTAssertEqual(string, "NSString fromURL usedEncoding test with UTF16 LE file", "Wrong result when reading UTF16LE file")
+          XCTAssertEqual(encoding, String.Encoding.utf16LittleEndian.rawValue, "Wrong encoding detected from UTF16LE file")
+      } catch {
+          XCTFail("Unable to init NSString from contentOf:usedEncoding:")
+      }
+    }
+
     func test_FromContentOfFile() {
         let testFilePath = testBundle().path(forResource: "NSStringTestData", ofType: "txt")
         XCTAssertNotNil(testFilePath)

TestFoundation/Resources/NSString-UTF16-LE-data.txt

@@ -460,6 +460,8 @@
     'TestFoundation/Resources/NSURLTestData.plist',
     'TestFoundation/Resources/Test.plist',
     'TestFoundation/Resources/NSStringTestData.txt',
+    'TestFoundation/Resources/NSString-UTF16-BE-data.txt',
+    'TestFoundation/Resources/NSString-UTF16-LE-data.txt',
     'TestFoundation/Resources/NSXMLDocumentTestData.xml',
     'TestFoundation/Resources/PropertyList-1.0.dtd',
     'TestFoundation/Resources/NSXMLDTDTestData.xml',