chars: add a faster version of the character-parsing function

It elides a parameter that is always NULL, and elides two ifs
that always take the same path.

src/chars.c

@@ -332,6 +332,35 @@ int parse_mbchar(const char *buf, char *chr, size_t *col)
 	return length;
 }
 
+/* Return the length (in bytes) of the character at the start of
+ * the given string, and add this character's width to *column. */
+int advance_over(const char *string, size_t *column)
+{
+	int charlen;
+
+#ifdef ENABLE_UTF8
+	if ((signed char)*string < 0) {
+		charlen = mblen(string, MAXCHARLEN);
+		if (charlen <= 0)
+			charlen = 1;
+	} else
+#endif
+		charlen = 1;
+
+	if (*string == '\t')
+		*column += tabsize - *column % tabsize;
+	else if (is_cntrl_mbchar(string))
+		*column += 2;
+	else if (charlen == 1)
+		*column += 1;
+#ifdef ENABLE_UTF8
+	else
+		*column += mbwidth(string);
+#endif
+
+	return charlen;
+}
+
 /* Return the index in buf of the beginning of the multibyte character
  * before the one at pos. */
 size_t step_left(const char *buf, size_t pos)

src/proto.h

@@ -217,6 +217,7 @@ char *make_mbchar(long chr, int *chr_mb_len);
 int char_length(const char *pointer);
 size_t mbstrlen(const char *s);
 int parse_mbchar(const char *buf, char *chr, size_t *col);
+int advance_over(const char *string, size_t *column);
 size_t step_left(const char *buf, size_t pos);
 size_t step_right(const char *buf, size_t pos);
 int mbstrcasecmp(const char *s1, const char *s2);

src/text.c

@@ -1579,7 +1579,7 @@ ssize_t break_line(const char *line, ssize_t goal, bool snap_at_nl)
 				break;
 		}
 
-		charlen = parse_mbchar(line, NULL, &column);
+		charlen = advance_over(line, &column);
 		line += charlen;
 		index += charlen;
 	}

src/utils.c

@@ -379,7 +379,7 @@ size_t actual_x(const char *text, size_t column)
 		/* The current accumulated span, in columns. */
 
 	while (*text != '\0') {
-		int charlen = parse_mbchar(text, NULL, &width);
+		int charlen = advance_over(text, &width);
 
 		if (width > column)
 			break;
@@ -400,7 +400,7 @@ size_t wideness(const char *text, size_t maxlen)
 		return 0;
 
 	while (*text != '\0') {
-		size_t charlen = parse_mbchar(text, NULL, &width);
+		size_t charlen = advance_over(text, &width);
 
 		if (maxlen <= charlen)
 			break;
@@ -418,7 +418,7 @@ size_t breadth(const char *text)
 	size_t span = 0;
 
 	while (*text != '\0')
-		text += parse_mbchar(text, NULL, &span);
+		text += advance_over(text, &span);
 
 	return span;
 }

src/winio.c

@@ -3045,7 +3045,7 @@ size_t get_softwrap_breakpoint(const char *text, size_t leftedge,
 
 	/* First find the place in text where the current chunk starts. */
 	while (*text != '\0' && column < leftedge)
-		text += parse_mbchar(text, NULL, &column);
+		text += advance_over(text, &column);
 
 	/* Now find the place in text where this chunk should end. */
 	while (*text != '\0' && column <= goal_column) {
@@ -3056,7 +3056,7 @@ size_t get_softwrap_breakpoint(const char *text, size_t leftedge,
 		}
 
 		breaking_col = (*text == '\t' ? goal_column : column);
-		text += parse_mbchar(text, NULL, &column);
+		text += advance_over(text, &column);
 	}
 
 	/* If we didn't overshoot the limit, we've found a breaking point;
@@ -3069,7 +3069,7 @@ size_t get_softwrap_breakpoint(const char *text, size_t leftedge,
 	/* If we're softwrapping at blanks and we found at least one blank, break
 	 * after that blank -- if it doesn't overshoot the screen's edge. */
 	if (farthest_blank != NULL) {
-		parse_mbchar(farthest_blank, NULL, &last_blank_col);
+		advance_over(farthest_blank, &last_blank_col);
 
 		if (last_blank_col <= goal_column)
 			return last_blank_col;