comments to document struct caseconv_entry

explain design of compact (packed) struct caseconv_entry,
in case it needs to be modified for future Unicode versions

newlib/libc/ctype/towctrans_l.c

@@ -4,8 +4,36 @@
 //#include <errno.h>
 #include "local.h"
 
-enum {EVENCAP, ODDCAP};
+/*
+   struct caseconv_entry describes the case conversion behaviour
+   of a range of Unicode characters.
+   It was designed to be compact for a minimal table size.
+   The range is first...first + diff.
+   Conversion behaviour for a character c in the respective range:
+     mode == TOLO	towlower (c) = c + delta
+     mode == TOUP	towupper (c) = c + delta
+     mode == TOBOTH	(titling case characters)
+			towlower (c) = c + 1
+			towupper (c) = c - 1
+     mode == TO1	capital/small letters are alternating
+	delta == EVENCAP	even codes are capital
+	delta == ODDCAP		odd codes are capital
+			(this correlates with an even/odd first range value
+			as of Unicode 10.0 but we do not rely on this)
+   As of Unicode 10.0, the following field lengths are sufficient
+	first: 17 bits
+	diff: 8 bits
+	delta: 17 bits
+	mode: 2 bits
+   The reserve of 4 bits (to limit the struct to 6 bytes)
+   is currently added to the 'first' field;
+   should a future Unicode version make it necessary to expand the others,
+   the 'first' field could be reduced as needed, or larger ranges could
+   be split up (reduce limit max=255 e.g. to max=127 or max=63 in 
+   script mkcaseconv, check increasing table size).
+ */
 enum {TO1, TOLO, TOUP, TOBOTH};
+enum {EVENCAP, ODDCAP};
 static struct caseconv_entry {
   unsigned int first: 21;
   unsigned short diff: 8;
@@ -71,6 +99,7 @@ toulower (wint_t c)
 	default:
 	  break;
       }
+
   return c;
 }
 
@@ -102,9 +131,10 @@ touupper (wint_t c)
 	  default:
 	    break;
 	  }
-	default:
-	  break;
+      default:
+	break;
       }
+
   return c;
 }