pg_sasl_prepare: Implement decomposition of Hangul characters

The Unicode documentation is rather specific on the matter with that:
http://unicode.org/reports/tr15/tr15-18.html#Hangul

pg_sasl_prepare/pg_sasl_prepare.c

@@ -93,7 +93,6 @@ get_code_entry(uint32 code)
 					sizeof(pg_utf_decomposition),
 					conv_compare);
-	Assert(entry != NULL);
 	return entry;
 }
@@ -108,6 +107,26 @@ get_decomposed_size(uint32 code)
 	int		size = 0;
 	int		i;
+	/*
+	 * Fast path for Hangul characters not stored in tables to save memory
+	 * as decomposition is algorithmic.
+	 * See http://unicode.org/reports/tr15/tr15-18.html, annex 10 for details
+	 * on the matter.
+	 */
+	if (code >= 0xAC00 && code < 0xD7A4)
+	{
+		uint32	l, v, t, hindex;
+
+		hindex = code - 0xAC00;
+		l = 0x1100 + hindex / (21 * 28);
+		v = 0x1161 + (hindex % (21 * 28)) / 28;
+		t = hindex % 28;
+
+		if (t != 0)
+			return 3;
+		return 2;
+	}
+
 	entry = get_code_entry(code);
 	/* Just count current code if no other decompositions */
@@ -143,6 +162,36 @@ decompose_code(uint32 code, int **result, int *current)
 	pg_utf_decomposition *entry;
 	int		i;
+	/*
+	 * Fast path for Hangul characters not stored in tables to save memory
+	 * as decomposition is algorithmic.
+	 * See http://unicode.org/reports/tr15/tr15-18.html, annex 10 for details
+	 * on the matter.
+	 */
+	if (code >= 0xAC00 && code < 0xD7A4)
+	{
+		uint32	l, v, t, hindex;
+		int	   *res = *result;
+
+		hindex = code - 0xAC00;
+		l = 0x1100 + hindex / (21 * 28);
+		v = 0x1161 + (hindex % (21 * 28)) / 28;
+		t = hindex % 28;
+
+		res[*current] = l;
+		(*current)++;
+		res[*current] = v;
+		(*current)++;
+
+		if (t != 0)
+		{
+			res[*current] = 0x11A7 + t;
+			(*current)++;
+		}
+
+		return;
+    }
+
 	entry = get_code_entry(code);
 	/*
@@ -233,6 +282,14 @@ pg_sasl_prepare(PG_FUNCTION_ARGS)
 		pg_utf_decomposition *prevEntry = get_code_entry(prev);
 		pg_utf_decomposition *nextEntry = get_code_entry(next);
+		/*
+		 * If no entries are found, the character used is either an Hangul
+		 * character or a character with a class of 0 and no decompositions,
+		 * so move to next result.
+		 */
+		if (prevEntry == NULL || nextEntry == NULL)
+			continue;
+
 		/*
 		 * Per Unicode (http://unicode.org/reports/tr15/tr15-18.html) annex 4,
 		 * a sequence of two adjacent characters in a string is an exchangeable