Describe character combining table layout

Author: eschnett

utf8proc.h

@@ -255,6 +255,38 @@ typedef struct utf8proc_property_struct {
   utf8proc_uint16_t uppercase_seqindex;
   utf8proc_uint16_t lowercase_seqindex;
   utf8proc_uint16_t titlecase_seqindex;
+  /**
+   * Character combining table.
+   *
+   * The character combining table is formally indexed by two
+   * characters, the first and second character that might form a
+   * combining pair. The table entry then contains the combined
+   * character. Most character pairs cannot be combined. There are
+   * about 1,000 characters that can be the first character in a
+   * combining pair, and for most, there are only a handful for
+   * possible second characters.
+   *
+   * The combining table is stored as `utf8proc_uint32_t
+   * utf8proc_combinations[][2]`. That is, it contains a pair `(second
+   * combining character, combined character)` for every character
+   * that can be a first combining character.
+   *
+   * - `comb_index`: Index into the combining table if this character
+   *   is the first character in a combining pair, else 0x3ff
+   *
+   * - `comb_length`: Number of table entries for this first character
+   *
+   * - `comb_is_second`: As optimization we also record whether this
+   *   characther is the second combining character in any pair. If
+   *   not, we can skip the table lookup.
+   *
+   * A table lookup starts from a given character pair. It first
+   * checks whether the first character is stored in the table
+   * (checking whether the index is 0x3ff) and whether the second
+   * index is stored in the table (looking at `comb_is_second`). If
+   * so, the `comb_length` table entries will be checked sequentially
+   * for a match.
+   */
   utf8proc_uint16_t comb_index:10;
   utf8proc_uint16_t comb_length:5;
   utf8proc_uint16_t comb_issecond:1;