Rename Ci to Ii and move it to an I/Invalid category group as it is not part of the UCD C/Other category group.

src/categories.cpp

 	if (c <= 0x0FFFFF) return Cn; // 0FFFFE..0FFFFF : Plane 15 Private Use
 	if (c <= 0x10FFFD) return Co; // 100000..10FFFD : Plane 16 Private Use
 	if (c <= 0x10FFFF) return Cn; // 10FFFE..10FFFF : Plane 16 Private Use
-	return Ci;
+	return Ii; // Invalid Unicode Codepoint
 }
 
 ucd::category_group ucd::lookup_category_group(codepoint_t c)
 {
 	switch (lookup_category(c))
 	{
-	case Cc: case Cf: case Ci: case Cn: case Co: case Cs:
+	case Cc: case Cf: case Cn: case Co: case Cs:
 		return C;
 	case Ll: case Lm: case Lo: case Lt: case Lu:
 		return L;
 		return S;
 	case Zl: case Zp: case Zs:
 		return Z;
+	case Ii:
+		return I;
 	}
 }

src/ctype.cpp

 {
 	switch (lookup_category(c))
 	{
-	case Cc: case Cf: case Ci: case Cn: case Co: case Cs:
+	case Cc: case Cf: case Cn: case Co: case Cs:
 	case Zl: case Zp: case Zs:
+	case Ii:
 		return 0;
 	}
 	return 1;
 {
 	switch (lookup_category(c))
 	{
-	case Cc: case Cf: case Ci: case Cn: case Co: case Cs:
+	case Cc: case Cf: case Cn: case Co: case Cs:
+	case Ii:
 		return 0;
 	}
 	return 1;

src/include/ucd/ucd.h

 	enum category_group
 	{
 		C, /**< @brief Other */
+		I, /**< @brief Invalid */
 		L, /**< @brief Letter */
 		M, /**< @brief Mark */
 		N, /**< @brief Number */
 	{
 		Cc, /**< @brief Control Character */
 		Cf, /**< @brief Format Control Character */
-		Ci, /**< @brief Invalid Unicode Character */
 		Cn, /**< @brief Unassigned */
 		Co, /**< @brief Private Use */
 		Cs, /**< @brief Surrogate Code Point */
 
+		Ii, /**< @brief Invalid Unicode Codepoint */
+
 		Ll, /**< @brief Lower Case Letter */
 		Lm, /**< @brief Letter Modifier */
 		Lo, /**< @brief Other Letter */

tools/categories.py

 			sys.stdout.write('\t\tconst ucd::category *table = categories_%s_%s[(c - 0x%s) / 256];\n' % (codepoints.first, codepoints.last, codepoints.first))
 			sys.stdout.write('\t\treturn table ? table[c % 256] : Cn;\n')
 			sys.stdout.write('\t}\n')
-	sys.stdout.write('\treturn Ci;\n')
+	sys.stdout.write('\treturn Ii; // Invalid Unicode Codepoint\n')
 	sys.stdout.write('}\n')
 
 	sys.stdout.write("""
 {
 	switch (lookup_category(c))
 	{
-	case Cc: case Cf: case Ci: case Cn: case Co: case Cs:
+	case Cc: case Cf: case Cn: case Co: case Cs:
 		return C;
 	case Ll: case Lm: case Lo: case Lt: case Lu:
 		return L;
 		return S;
 	case Zl: case Zp: case Zs:
 		return Z;
+	case Ii:
+		return I;
 	}
 }
 """)