[Layer] Remove the 'context' argument from 'Layer.applied(to:in:)'. (#87)

* [Layer] Remove the 'context' argument to `Layer.applied(to:in:)`.

* Add tests.

Author: rxwei

Sources/DeepLearning/Context.swift

@@ -0,0 +1,148 @@
+// Copyright 2018 The TensorFlow Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#if !COMPILING_TENSORFLOW_MODULE
+import TensorFlow
+#endif
+
+#if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
+import Darwin
+#else
+import Glibc
+#endif
+
+/// A value that indicates the phase of using a machine learning model.
+public enum LearningPhase {
+    case training
+    case inference
+}
+
+/// A context that stores thread-local contextual information used by deep learning APIs such as
+/// layers.
+///
+/// Use `Context.local` to retrieve the current thread-local context.
+///
+/// Examples:
+///
+/// * Set the current learning phase to training so that layers like `BatchNorm` will
+///   compute mean and variance when applied to inputs.
+///
+///   ```swift
+///   Context.local.learningPhase = .training
+///   ```
+/// * Set the current learning phase to inference so that layers like `Dropout` will not drop out
+///   units when applied to inputs.
+///
+///   ```swift
+///   Context.local.learningPhase = .inference
+///   ```
+public struct Context {
+    /// The learning phase.
+    public var learningPhase: LearningPhase = .inference
+
+    /// Creates a context with default properties.
+    public init() {}
+
+    /// The current thread-local context.
+    ///
+    /// - Note: Accessing this property is thread-safe.
+    public static var local: Context {
+        _read { yield ContextManager.local.currentContext }
+        _modify { yield &ContextManager.local.currentContext }
+    }
+}
+
+/// Calls the given closure within a context that has everything identical to the current context
+/// except for the given learning phase.
+///
+/// - Parameters:
+///   - context: A context that will be set before the closure gets called and restored after the
+///     closure returns.
+///   - body: A nullary closure. If the closure has a return value, that value is also used as the
+///     return value of the `withContext(_:_:)` function.
+/// - Returns: The return value, if any, of the `body` closure.
+public func withContext<R>(_ context: Context, _ body: () throws -> R) rethrows -> R {
+    ContextManager.local.push(context)
+    defer { ContextManager.local.popContext() }
+    return try body()
+}
+
+/// Calls the given closure within a context that has everything identical to the current context
+/// except for the given learning phase.
+///
+/// - Parameters:
+///   - learningPhase: A learning phase that will be set before the closure gets called and restored
+///     after the closure returns.
+///   - body: A nullary closure. If the closure has a return value, that value is also used as the
+///     return value of the `withLearningPhase(_:_:)` function.
+/// - Returns: The return value, if any, of the `body` closure.
+public func withLearningPhase<R>(_ learningPhase: LearningPhase,
+                                 _ body: () throws -> R) rethrows -> R {
+    var context = ContextManager.local.currentContext
+    context.learningPhase = learningPhase
+    return try withContext(context, body)
+}
+
+/// A manager that maintains and provides safe access to thread-local `Context` values.
+private final class ContextManager {
+    var contextStack: [Context] = [Context()]
+
+    /// The data key for the singleton `Context` in the current thread.
+    static let key: pthread_key_t = {
+        var key = pthread_key_t()
+        pthread_key_create(&key) { obj in
+#if !(os(macOS) || os(iOS) || os(watchOS) || os(tvOS))
+            let obj = obj!
+#endif
+            Unmanaged<ContextManager>.fromOpaque(obj).release()
+        }
+        return key
+    }()
+
+    /// The thread-local singleton.
+    static var local: ContextManager {
+        if let address = pthread_getspecific(key) {
+            return Unmanaged<ContextManager>.fromOpaque(address).takeUnretainedValue()
+        }
+        let context = ContextManager()
+        pthread_setspecific(key, Unmanaged.passRetained(context).toOpaque())
+        return context
+    }
+
+    /// Pushes the given context to the context stack.
+    func push(_ context: Context) {
+        contextStack.append(context)
+    }
+
+    /// Pops a context out of a stack.
+    ///
+    /// - Precondition: The context stack must contain more than `1` contexts.
+    func popContext() {
+        assert(contextStack.count > 1,
+               "Internal error: Only 1 context is available. Popping is not allowed.")
+        contextStack.removeLast()
+    }
+
+    /// The most recent context.
+    var currentContext: Context {
+        _read {
+            assert(!contextStack.isEmpty, "Internal error: No contexts exist.")
+            yield contextStack[contextStack.endIndex - 1]
+        }
+        _modify {
+            assert(!contextStack.isEmpty, "Internal error: No contexts exist.")
+            yield &contextStack[contextStack.endIndex - 1]
+        }
+    }
+}