log: add minimal stderr logger

1 files changed, 176 insertions, 0 deletions

diff --git a/log.h b/log.h
new file mode 100644
index 0000000..c0880fb
--- /dev/null
+++ b/log.h
@@ -0,0 +1,176 @@
+#ifndef UTILS_LOG_H
+#define UTILS_LOG_H
+
+#include <bits/chrono.h>
+#include <cassert>
+#include <chrono>
+#include <cstdio>
+#include <type_traits>
+
+// -- LOGGER MACROS ------------------------------------------------------------
+
+#define FAIL(log, fmt, ...) LOG(log, kFail, fmt, ##__VA_ARGS__)
+#define WARN(log, fmt, ...) LOG(log, kWarn, fmt, ##__VA_ARGS__)
+#define INFO(log, fmt, ...) LOG(log, kInfo, fmt, ##__VA_ARGS__)
+#define DBG0(log, fmt, ...) LOG(log, kDbg0, fmt, ##__VA_ARGS__)
+
+// -- LOGGER MACROS DETAILS ----------------------------------------------------
+
+#define LOG(log_, lvl, fmt, ...)                                           \
+  do {                                                                     \
+    struct fmt_str_must_be_str_literal {                                   \
+      constexpr fmt_str_must_be_str_literal(const char* ptr) : ptr{ptr} {} \
+      const char* ptr;                                                     \
+    };                                                                     \
+    /* Check if FMT is a string literal, construction fails otherwise.  */ \
+    constexpr fmt_str_must_be_str_literal _{fmt};                          \
+    (void)_;                                                               \
+                                                                           \
+    if (logging::log_level::lvl <= (log_)->get_level())                    \
+      (log_)->template log<logging::log_level::lvl>(fmt, ##__VA_ARGS__);   \
+  } while (0)
+
+namespace logging {
+
+// -- LOGGING LEVELS -----------------------------------------------------------
+
+#define LOG_LEVELS(M) \
+  M(kFail, "FAIL")    \
+  M(kWarn, "WARN")    \
+  M(kInfo, "INFO")    \
+  M(kDbg0, "DBG0")
+
+#define M(val, ignore) val,
+enum log_level { LOG_LEVELS(M) };
+#undef M
+
+#define M(ignore, val) val,
+constexpr const char* kLogPrefix[] = {LOG_LEVELS(M)};
+#undef M
+
+namespace detail {
+
+// -- SANITIZE FMT ARG HELPER (META FN) ----------------------------------------
+
+template <typename T>
+constexpr inline bool is_one_of() {
+  return false;
+}
+
+template <typename T, typename U, typename... Args>
+constexpr inline bool is_one_of() {
+  return std::is_same<T, U>::value || is_one_of<T, Args...>();
+}
+
+template <typename Arg>
+constexpr inline Arg sanitize_fmt_args(Arg arg) {
+  static_assert(
+      is_one_of<Arg, char, unsigned char, int, unsigned, float, double>() ||
+          std::is_pointer<Arg>::value,
+      "Invalid FMT arg type!");
+  return arg;
+}
+
+// -- TIME STAMP HELPER --------------------------------------------------------
+
+template <int UtcOffset = 0>
+struct time_stamp {
+  using clock = std::chrono::system_clock;
+  using repr = clock::rep;
+  using time_point = clock::time_point;
+
+  repr us() const {
+    return to_duration<std::chrono::microseconds>() % 1000;
+  }
+  repr ms() const {
+    return to_duration<std::chrono::milliseconds>() % 1000;
+  }
+  repr s() const {
+    return to_duration<std::chrono::seconds>() % 60;
+  }
+  repr m() const {
+    return to_duration<std::chrono::minutes>() % 60;
+  }
+  repr h() const {
+    return to_duration<std::chrono::hours>() % 24 + UtcOffset;
+  }
+
+ private:
+  template <typename ToDuration>
+  repr to_duration() const {
+    return std::chrono::duration_cast<ToDuration>(m_time.time_since_epoch())
+        .count();
+  }
+
+  time_point m_time{clock::now()};
+};
+}  // namespace detail
+
+// -- LOGGER -------------------------------------------------------------------
+
+template <bool WithTimestamp = true, size_t BufSize = 128>
+struct logger {
+  constexpr logger() = default;
+  constexpr logger(log_level lvl) : m_lvl{lvl} {}
+
+  log_level get_level() const {
+    return m_lvl;
+  }
+  void set_level(log_level lvl) {
+    m_lvl = lvl;
+  }
+
+  template <log_level L, typename... Args>
+  constexpr void log(const char* fmt, Args... args)
+      __attribute__((format(printf, 2, 0)));
+
+ private:
+  log_level m_lvl{kInfo};
+  char m_buf[BufSize];
+};
+
+// -- LOGGER IMPLEMENTATION ----------------------------------------------------
+
+template <bool WithTimestamp, size_t BufSize>
+template <log_level L, typename... Args>
+constexpr void logger<WithTimestamp, BufSize>::log(const char* fmt,
+                                                   Args... args) {
+  size_t pos{0};
+
+  // Add timestamp if enabled.
+  if (WithTimestamp) {
+    detail::time_stamp<2> ts;
+    pos += std::snprintf(m_buf + pos, BufSize - pos,
+                         "[%02ld:%02ld:%02ld:%03ld%03ld] ", ts.h(), ts.m(),
+                         ts.s(), ts.ms(), ts.us());
+    assert(pos > 0);
+  }
+
+  // Add log level prefix.
+  pos += std::snprintf(m_buf + pos, BufSize - pos, "%s: ", kLogPrefix[L]);
+  assert(pos < BufSize);
+
+  // Add log message using user specified fmt string.
+  //
+  // SAFETY: User of this function is responsible to provide a "safe" fmt
+  // string. When using the provided macros we check that the user specifies a
+  // string literal as fmt string and hence the user controls the fmt string.
+  // Additionally, we sanitize the arguments to only allow explicitly specified
+  // argument types.
+  //
+  // NOLINTNEXTLINE
+  pos += std::snprintf(m_buf + pos, BufSize - pos, fmt,
+                       detail::sanitize_fmt_args(args)...);
+  assert(pos < BufSize);
+
+  // Ensure terminated with new line and null terminator.
+  assert(pos < BufSize - 1);
+  m_buf[pos++] = '\0';
+
+  // Write out log message.
+  std::fprintf(stderr, "%s\n", m_buf);
+}
+
+}  // namespace logging
+
+#endif