tweaks: morph a function into what it is actually used for

Since the previous commit, mbwidth() is used only to determine whether
a character is either double width or zero width.  There is no need to
return the actual width of the character; a simple yes or no is enough.

Transforming mbwidth() into is_doublewidth() also allows streamlining
it and is_zerowidth() a bit, so that they become slightly faster.

src/chars.c

@@ -225,31 +225,34 @@ int mbtowide(wchar_t *wc, const char *c)
 	return 1;
 }
 
-/* Return the width in columns of the given (multibyte) character. */
-int mbwidth(const char *c)
+/* Return TRUE when the given character occupies two cells. */
+bool is_doublewidth(const char *ch)
 {
-	/* Only characters beyond U+02FF can be other than one column wide. */
-	if ((unsigned char)*c > 0xCB) {
-		wchar_t wc;
-		int width;
+	wchar_t wc;
 
-		if (mbtowide(&wc, c) < 0)
-			return 1;
+	/* Only from U+1100 can code points have double width. */
+	if ((unsigned char)*ch < 0xE1 || !use_utf8)
+		return FALSE;
 
-		width = wcwidth(wc);
+	if (mbtowide(&wc, ch) < 0)
+		return FALSE;
 
-		if (width < 0)
-			return 1;
-
-		return width;
-	} else
-		return 1;
+	return (wcwidth(wc) == 2);
 }
 
 /* Return TRUE when the given character occupies zero cells. */
 bool is_zerowidth(const char *ch)
 {
-	return (use_utf8 && mbwidth(ch) == 0);
+	wchar_t wc;
+
+	/* Only from U+0300 can code points have zero width. */
+	if ((unsigned char)*ch < 0xCC || !use_utf8)
+		return FALSE;
+
+	if (mbtowide(&wc, ch) < 0)
+		return FALSE;
+
+	return (wcwidth(wc) == 0);
 }
 #endif /* ENABLE_UTF8 */
 

src/prototypes.h

@@ -205,7 +205,7 @@ bool is_word_char(const char *c, bool allow_punct);
 char control_mbrep(const char *c, bool isdata);
 #ifdef ENABLE_UTF8
 int mbtowide(wchar_t *wc, const char *c);
-int mbwidth(const char *c);
+bool is_doublewidth(const char *ch);
 bool is_zerowidth(const char *ch);
 #endif
 int char_length(const char *pointer);

src/winio.c

@@ -1749,7 +1749,7 @@ char *display_string(const char *text, size_t column, size_t span,
 			}
 		}
 #ifdef ENABLE_UTF8
-		else if (mbwidth(text) == 2) {
+		else if (is_doublewidth(text)) {
 			if (start_col == column) {
 				converted[index++] = ' ';
 				column++;
@@ -1765,7 +1765,7 @@ char *display_string(const char *text, size_t column, size_t span,
 
 #ifdef ENABLE_UTF8
 #define ISO8859_CHAR  FALSE
-#define ZEROWIDTH_CHAR  (mbwidth(text) == 0)
+#define ZEROWIDTH_CHAR  (is_zerowidth(text))
 #else
 #define ISO8859_CHAR  ((unsigned char)*text > 0x9F)
 #define ZEROWIDTH_CHAR  FALSE
@@ -1873,10 +1873,10 @@ char *display_string(const char *text, size_t column, size_t span,
 #ifdef ENABLE_UTF8
 		do {
 			index = step_left(converted, index);
-		} while (mbwidth(converted + index) == 0);
+		} while (is_zerowidth(converted + index));
 
 		/* Display the left half of a two-column character as '['. */
-		if (mbwidth(converted + index) == 2)
+		if (is_doublewidth(converted + index))
 			converted[index++] = '[';
 #else
 		index--;