Add logging (#492)

* Add logger implementation for mgba debugging

* Add logging details to contributors guide

* Clang-format

* update docs real quick

* Cleanup for PR

* Update for PR

* Fix bad rebase

* Fix docs

include/mgba_logger.h

@@ -0,0 +1,73 @@
+/**
+ * @file mgba_logger.h
+ *
+ * @brief Interface to interact with the mgba logger.
+ *
+ * Use with mgba in the command line.
+ *
+ * You can pass which logs you'd like to print with a flag passed to mgba
+ * with `-l` or `--log-level`.
+ *
+ * | 7 | 6 | 5 |   4   |   3  |   2  |   1   |   0   |
+ * |---|---|---|-------|------|------|-------|-------|
+ * | / | / | / | DEBUG | INFO | WARN | ERROR | FATAL |
+ *
+ * ```sh
+ * mgba -l 3 game.rom # ERROR and FATAL
+ * mgba -l 14 game.rom # INFO, WARN, and ERROR
+ * ```
+ *
+ * @note You have to fight with other logs in mgba and DEBUG can get messy.
+ *
+ * @note FATAL does kill the game. Use with care.
+ */
+#ifndef MGBA_LOGGER_H
+#define MGBA_LOGGER_H
+
+#include <stdbool.h>
+
+typedef enum
+{
+    MGBA_LOG_FATAL,
+    MGBA_LOG_ERROR,
+    MGBA_LOG_WARN,
+    MGBA_LOG_INFO,
+    MGBA_LOG_DEBUG,
+} MgbaLogLevel;
+
+/**
+ * @brief Initialize mgba logger
+ *
+ * Checks that the logger is available by checking the expected registers
+ * magic number
+ */
+bool mgba_logger_init(void);
+
+/**
+ * @brief Print to mgba log with a format string
+ *
+ * @param level
+ * @param fmt Format string
+ * @param ... variadic arguments
+ *
+ * @note for all logs, it's cutoff at the hard mgba limit of 0x100
+ */
+void mgba_printf(MgbaLogLevel level, const char* fmt, ...);
+
+// clang-format off
+#ifdef MGBA_LOGGING
+#define MGBA_FATAL(...) mgba_printf(MGBA_LOG_FATAL, __VA_ARGS__)
+#define MGBA_ERROR(...) mgba_printf(MGBA_LOG_ERROR, __VA_ARGS__)
+#define MGBA_WARN(...)  mgba_printf(MGBA_LOG_WARN,  __VA_ARGS__)
+#define MGBA_INFO(...)  mgba_printf(MGBA_LOG_INFO,  __VA_ARGS__)
+#define MGBA_DEBUG(...) mgba_printf(MGBA_LOG_DEBUG, __VA_ARGS__)
+#else
+#define MGBA_FATAL(...) ((void)0)
+#define MGBA_ERROR(...) ((void)0)
+#define MGBA_WARN(...) ((void)0)
+#define MGBA_INFO(...) ((void)0)
+#define MGBA_DEBUG(...) ((void)0)
+#endif
+// clang-format on
+
+#endif