it starts

.gitignore

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+# Created by https://www.gitignore.io/api/xcode
+
+### Xcode ###
+# Xcode
+#
+# gitignore contributors: remember to update Global/Xcode.gitignore, Objective-C.gitignore & Swift.gitignore
+
+## Build generated
+.build/
+build/
+DerivedData
+Carthage/
+Packages/
+
+## Various settings
+*.pbxuser
+!default.pbxuser
+*.mode1v3
+!default.mode1v3
+*.mode2v3
+!default.mode2v3
+*.perspectivev3
+!default.perspectivev3
+xcuserdata
+
+## Other
+*.xccheckout
+*.moved-aside
+*.xcuserstate
+*.xcscmblueprint

LICENSE

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+The MIT License (MIT)
+
+Copyright (c) 2015 Zewo
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

Package.swift

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+import PackageDescription
+
+let package = Package(
+    name: "Log",
+    dependencies: [
+        .Package(url: "https://github.com/Zewo/File.git", majorVersion: 0, minor: 2),
+        .Package(url: "https://github.com/Zewo/CLibvenice.git", majorVersion: 0, minor: 2)
+    ]
+)

Source/Log.swift

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+// Log.swift
+//
+// The MIT License (MIT)
+//
+// Copyright (c) 2015 Zewo
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+@_exported import File
+
+public final class Log {
+    public struct Level: OptionSetType {
+        public let rawValue: Int32
+
+        public init(rawValue: Int32) {
+            self.rawValue = rawValue
+        }
+
+        public static let Trace   = Level(rawValue: 1 << 0)
+        public static let Debug   = Level(rawValue: 1 << 1)
+        public static let Info    = Level(rawValue: 1 << 2)
+        public static let Warning = Level(rawValue: 1 << 3)
+        public static let Error   = Level(rawValue: 1 << 4)
+        public static let Fatal   = Level(rawValue: 1 << 5)
+        public static let All     = Level(rawValue: ~0)
+    }
+
+    public var stream: File
+    public var levels: Level
+    private let messageChannel = Channel<String>()
+    private let once = Once()
+
+    public init(stream: File = standardErrorStream, levels: Level = .All) {
+        self.stream = stream
+        self.levels = levels
+    }
+
+    deinit {
+        messageChannel.close()
+    }
+
+    public func log(level: Level, item: Any, terminator: String = "\n", flush: Bool = true) {
+        if levels.contains(level) {
+            once.runInBackground {
+                for message in self.messageChannel {
+                    do {
+                        try self.stream.write(message)
+                    } catch {
+                        print("Log error: \(error)")
+                        print("Log message: \(message)")
+                    }
+                }
+            }
+            messageChannel.send(String(item) + terminator)
+        }
+    }
+
+    public func trace(item: Any, terminator: String = "\n", flush: Bool = true) {
+        log(.Trace, item: item, terminator: terminator, flush: flush)
+    }
+
+    public func debug(item: Any, terminator: String = "\n", flush: Bool = true) {
+        log(.Debug, item: item, terminator: terminator, flush: flush)
+    }
+
+    public func info(item: Any, terminator: String = "\n", flush: Bool = true) {
+        log(.Info, item: item, terminator: terminator, flush: flush)
+    }
+
+    public func warning(item: Any, terminator: String = "\n", flush: Bool = true) {
+        log(.Warning, item: item, terminator: terminator, flush: flush)
+    }
+
+    public func error(item: Any, terminator: String = "\n", flush: Bool = true) {
+        log(.Error, item: item, terminator: terminator, flush: flush)
+    }
+
+    public func fatal(item: Any, terminator: String = "\n", flush: Bool = true) {
+        log(.Fatal, item: item, terminator: terminator, flush: flush)
+    }
+}