include/ruby/internal/core/rhash.h: add doxygen

Must not be a bad idea to improve documents. [ci skip]

include/ruby/internal/core/rhash.h

@@ -18,19 +18,8 @@
  *             Do not  expect for  instance `__VA_ARGS__` is  always available.
  *             We assume C99  for ruby itself but we don't  assume languages of
  *             extension libraries.  They could be written in C++98.
- * @brief      Routines to manipulate struct ::RHash.
- *
- * Shyouhei really suffered agnish over placement of macros in this file.  They
- * are half-broken.  The situation (as of writing) is:
- *
- * - #RHASH_TBL: works.
- * - #RHASH_ITER_LEV: compile-time error.
- * - #RHASH_IFNONE: compile-time error.
- * - #RHASH_SIZE: works.
- * - #RHASH_EMPTY_P: works.
- * - #RHASH_SET_IFNONE: works (why... given you cannot query).
- *
- * Shyouhei stopped thinking.  Let them be as is.
+ * @brief      Routines to manipulate struct RHash.
+ * @note       The struct RHash itself is opaque.
  */
 #include "ruby/internal/config.h"
 
@@ -44,18 +33,111 @@
 # include "ruby/backward.h"
 #endif
 
+/**
+ * Retrieves the internal table.
+ *
+ * @param[in]  h  An instance of RHash.
+ * @pre        `h` must be of ::RUBY_T_HASH.
+ * @return     A struct st_table which has the contents of this hash.
+ * @note       Nowadays as Ruby  evolved over ages, RHash  has multiple backend
+ *             storage  engines.   `h`'s backend  is  not  guaranteed to  be  a
+ *             st_table.  This function creates one when necessary.
+ */
 #define RHASH_TBL(h)                rb_hash_tbl(h, __FILE__, __LINE__)
+
+/**
+ * @private
+ *
+ * @deprecated  This macro once was a thing in the old days, but makes no sense
+ *              any  longer today.   Exists  here  for backwards  compatibility
+ *              only.  You can safely forget about it.
+ *
+ * @internal
+ *
+ * Declaration of rb_hash_iter_lev() is at include/ruby/backward.h.
+ */
 #define RHASH_ITER_LEV(h)           rb_hash_iter_lev(h)
+
+/**
+ * @private
+ *
+ * @deprecated  This macro once was a thing in the old days, but makes no sense
+ *              any  longer today.   Exists  here  for backwards  compatibility
+ *              only.  You can safely forget about it.
+ *
+ * @internal
+ *
+ * Declaration of rb_hash_ifnone() is at include/ruby/backward.h.
+ */
 #define RHASH_IFNONE(h)             rb_hash_ifnone(h)
+
+/**
+ * Queries the size of  the hash.  Size here means the number  of keys that the
+ * hash stores.
+ *
+ * @param[in]  h  An instance of RHash.
+ * @pre        `h` must be of ::RUBY_T_HASH.
+ * @return     The size of the hash.
+ */
 #define RHASH_SIZE(h)               rb_hash_size_num(h)
+
+/**
+ * Checks if the hash is empty.
+ *
+ * @param[in]  h      An instance of RHash.
+ * @pre        `h` must be of ::RUBY_T_HASH.
+ * @retval     true   It is.
+ * @retval     false  It isn't.
+ */
 #define RHASH_EMPTY_P(h)            (RHASH_SIZE(h) == 0)
+
+/**
+ * Destructively updates the default value of the hash.
+ *
+ * @param[out]  h       An instance of RHash.
+ * @param[in]   ifnone  Arbitrary default value.
+ * @pre        `h` must be of ::RUBY_T_HASH.
+ *
+ * @internal
+ *
+ * But why you can set this, given rb_hash_ifnone() doesn't exist?
+ */
 #define RHASH_SET_IFNONE(h, ifnone) rb_hash_set_ifnone((VALUE)h, ifnone)
 
 struct st_table;  /* in ruby/st.h */
 
 RBIMPL_SYMBOL_EXPORT_BEGIN()
+
+/**
+ * This is  the implementation detail  of #RHASH_SIZE.  People don't  call this
+ * directly.
+ *
+ * @param[in]  hash  An instance of RHash.
+ * @pre        `hash` must be of ::RUBY_T_HASH.
+ * @return     The size of the hash.
+ */
 size_t rb_hash_size_num(VALUE hash);
-struct st_table *rb_hash_tbl(VALUE, const char *file, int line);
+
+/**
+ * This is  the implementation  detail of #RHASH_TBL.   People don't  call this
+ * directly.
+ *
+ * @param[in]  hash  An instance of RHash.
+ * @param[in]  file  The `__FILE__`.
+ * @param[in]  line  The `__LINE__`.
+ * @pre        `hash` must be of ::RUBY_T_HASH.
+ * @return     Table that has the contents of the hash.
+ */
+struct st_table *rb_hash_tbl(VALUE hash, const char *file, int line);
+
+/**
+ * This is the  implementation detail of #RHASH_SET_IFNONE.   People don't call
+ * this directly.
+ *
+ * @param[out]  hash    An instance of RHash.
+ * @param[in]   ifnone  Arbitrary default value.
+ * @pre        `hash` must be of ::RUBY_T_HASH.
+ */
 VALUE rb_hash_set_ifnone(VALUE hash, VALUE ifnone);
 RBIMPL_SYMBOL_EXPORT_END()