Improve trailing newline performance

This sidesteps the previous method of reversing the entire string from
every file by only checking the minimum number of trailing characters
for each file.

Source/SwiftLintFramework/Rules/TrailingNewlineRule.swift

@@ -14,16 +14,20 @@ public struct TrailingNewlineRule: Rule {
     public let identifier = "trailing_newline"
 
     public func validateFile(file: File) -> [StyleViolation] {
-        let countOfTrailingNewlines = file.contents.countOfTailingCharactersInSet(
-            NSCharacterSet.newlineCharacterSet()
-        )
-        if countOfTrailingNewlines != 1 {
+        let string = file.contents
+        let start = advance(string.endIndex, -2, string.startIndex)
+        let range = Range(start: start, end: string.endIndex)
+        let substring = string[range].utf16
+        let newLineSet = NSCharacterSet.newlineCharacterSet()
+        let slices = split(substring, allowEmptySlices: true) { !newLineSet.characterIsMember($0) }
+
+        if let slice = slices.last where count(slice) != 1 {
             return [StyleViolation(type: .TrailingNewline,
                 location: Location(file: file.path, line: file.contents.lines().count + 1),
                 severity: .Medium,
-                reason: "File should have a single trailing newline: " +
-                "currently has \(countOfTrailingNewlines)")]
+                reason: "File should have a single trailing newline")]
         }
+
         return []
     }
 

Source/SwiftLintFrameworkTests/StringRuleTests.swift

@@ -34,11 +34,11 @@ class StringRuleTests: XCTestCase {
         XCTAssertEqual(violations(""), [StyleViolation(type: .TrailingNewline,
             location: Location(file: nil, line: 1),
             severity: .Medium,
-            reason: "File should have a single trailing newline: currently has 0")])
+            reason: "File should have a single trailing newline")])
         XCTAssertEqual(violations("//\n\n"), [StyleViolation(type: .TrailingNewline,
             location: Location(file: nil, line: 3),
             severity: .Medium,
-            reason: "File should have a single trailing newline: currently has 2")])
+            reason: "File should have a single trailing newline")])
     }
 
     func testFileLengths() {