Add logging framework.

DEBUG messages are completely optimized out if NDEBUG is defined.

src/Log.h

@@ -0,0 +1,83 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) FIRST 2015. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#ifndef NT_LOG_H_
+#define NT_LOG_H_
+
+#include <functional>
+#include <sstream>
+#include <string>
+
+#include "atomic_static.h"
+#include "ntcore_c.h"
+
+namespace nt {
+
+class Logger {
+ public:
+  static Logger& GetInstance() {
+    ATOMIC_STATIC(Logger, instance);
+    return instance;
+  }
+  ~Logger();
+
+  typedef std::function<void(unsigned int level, const char* file,
+                             unsigned int line, const char* msg)> LogFunc;
+
+  void SetLogger(LogFunc func) { m_func = func; }
+
+  void set_min_level(unsigned int level) { m_min_level = level; }
+  unsigned int min_level() const { return m_min_level; }
+
+  void Log(unsigned int level, const char* file, unsigned int line,
+           const char* msg) {
+    if (!m_func || level < m_min_level) return;
+    m_func(level, file, line, msg);
+  }
+
+  bool HasLogger() const { return m_func != nullptr; }
+
+ private:
+  Logger();
+
+  LogFunc m_func;
+  unsigned int m_min_level;
+
+  ATOMIC_STATIC_DECL(Logger)
+};
+
+#define LOG(level, x)                                           \
+  do {                                                          \
+    Logger& logger = Logger::GetInstance();                     \
+    if (logger.min_level() <= level && logger.HasLogger()) {    \
+      std::ostringstream oss;                                   \
+      oss << x;                                                 \
+      logger.Log(level, __FILE__, __LINE__, oss.str().c_str()); \
+    }                                                           \
+  } while (0)
+
+#define ERROR(x) LOG(NT_LOG_ERROR, x)
+#define WARNING(x) LOG(NT_LOG_WARNING, x)
+#define INFO(x) LOG(NT_LOG_INFO, x)
+
+#ifdef NDEBUG
+#define DEBUG(x)
+#define DEBUG1(x)
+#define DEBUG2(x)
+#define DEBUG3(x)
+#define DEBUG4(x)
+#else
+#define DEBUG(x) LOG(NT_LOG_DEBUG, x)
+#define DEBUG1(x) LOG(NT_LOG_DEBUG1, x)
+#define DEBUG2(x) LOG(NT_LOG_DEBUG2, x)
+#define DEBUG3(x) LOG(NT_LOG_DEBUG3, x)
+#define DEBUG4(x) LOG(NT_LOG_DEBUG4, x)
+#endif
+
+} // namespace nt
+
+#endif  // NT_LOG_H_