Simplify code for getting a unicode codepoint's canonical class.

Three places of unicode_norm.c use a similar logic for getting the
combining class from a codepoint.  Commit 2991ac5 has added the function
get_canonical_class() for this purpose, but it was only called by the
backend.  This commit refactors the code to use this function in all
the places where the combining class is retrieved from a given
codepoint.

Author: John Naylor
Discussion: https://postgr.es/m/CAFBsxsHUV7s7YrOm6hFz-Jq8Sc7K_yxTkfNZxsDV-DuM-k-gwg@mail.gmail.com

Author: michaelpq

src/common/unicode_norm.c

@@ -105,6 +105,23 @@ get_code_entry(pg_wchar code)
 #endif
 }
 
+/*
+ * Get the combining class of the given codepoint.
+ */
+static uint8
+get_canonical_class(pg_wchar code)
+{
+	const pg_unicode_decomposition *entry = get_code_entry(code);
+
+	/*
+	 * If no entries are found, the character used is either an Hangul
+	 * character or a character with a class of 0 and no decompositions.
+	 */
+	if (!entry)
+		return 0;
+	else
+		return entry->comb_class;
+}
 
 /*
  * Given a decomposition entry looked up earlier, get the decomposed
@@ -430,16 +447,8 @@ unicode_normalize(UnicodeNormalizationForm form, const pg_wchar *input)
 		pg_wchar	prev = decomp_chars[count - 1];
 		pg_wchar	next = decomp_chars[count];
 		pg_wchar	tmp;
-		const pg_unicode_decomposition *prevEntry = get_code_entry(prev);
-		const pg_unicode_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;
+		const uint8 prevClass = get_canonical_class(prev);
+		const uint8 nextClass = get_canonical_class(next);
 
 		/*
 		 * Per Unicode (https://www.unicode.org/reports/tr15/tr15-18.html)
@@ -449,10 +458,10 @@ unicode_normalize(UnicodeNormalizationForm form, const pg_wchar *input)
 		 * combining class for the second, and the second is not a starter.  A
 		 * character is a starter if its combining class is 0.
 		 */
-		if (nextEntry->comb_class == 0x0 || prevEntry->comb_class == 0x0)
+		if (prevClass == 0 || nextClass == 0)
 			continue;
 
-		if (prevEntry->comb_class <= nextEntry->comb_class)
+		if (prevClass <= nextClass)
 			continue;
 
 		/* exchange can happen */
@@ -489,8 +498,7 @@ unicode_normalize(UnicodeNormalizationForm form, const pg_wchar *input)
 	for (count = 1; count < decomp_size; count++)
 	{
 		pg_wchar	ch = decomp_chars[count];
-		const pg_unicode_decomposition *ch_entry = get_code_entry(ch);
-		int			ch_class = (ch_entry == NULL) ? 0 : ch_entry->comb_class;
+		int			ch_class = get_canonical_class(ch);
 		pg_wchar	composite;
 
 		if (last_class < ch_class &&
@@ -527,17 +535,6 @@ unicode_normalize(UnicodeNormalizationForm form, const pg_wchar *input)
 /* We only need this in the backend. */
 #ifndef FRONTEND
 
-static uint8
-get_canonical_class(pg_wchar ch)
-{
-	const pg_unicode_decomposition *entry = get_code_entry(ch);
-
-	if (!entry)
-		return 0;
-	else
-		return entry->comb_class;
-}
-
 static const pg_unicode_normprops *
 qc_hash_lookup(pg_wchar ch, const pg_unicode_norminfo *norminfo)
 {