new feature: allow lines to be softwrapped at whitespace

Extend get_softwrap_breakpoint() to break softwrapped lines on
whitespace when a flag is set.  This flag is controlled by the new
rcfile option "atblanks".  The '>' characters marking two-column
characters at the edge of the screen are disabled when it's on.

If get_softwrap_breakpoint() can't find whitespace in screen range, it
will break the line on the screen edge.  (In this case, a blank can be
on the last column of the screen, but text can't, so that a blank on the
last column doesn't become invisible and possibly break the display.)

This fulfills https://savannah.gnu.org/bugs/index.php?49959.
Requested-by: Nicholas Boel <axxisd@gmail.com>

doc/nano.texi

@@ -657,6 +657,10 @@ When backing up files, allow the backup to succeed even if its
 permissions can't be (re)set due to special OS considerations.
 You should NOT enable this option unless you are sure you need it.
 
+@item set atblanks
+When soft line wrapping is enabled, make it wrap lines at blank characters
+(tabs and spaces) instead of always at the edge of the screen.
+
 @item set autoindent
 Use auto-indentation.
 

doc/nanorc.5

@@ -57,6 +57,10 @@ When backing up files, allow the backup to succeed even if its permissions
 can't be (re)set due to special OS considerations.  You should
 NOT enable this option unless you are sure you need it.
 .TP
+.B set atblanks
+When soft line wrapping is enabled, make it wrap lines at blank characters
+(tabs and spaces) instead of always at the edge of the screen.
+.TP
 .B set autoindent
 Use auto-indentation.
 .TP

doc/sample.nanorc.in

@@ -20,6 +20,10 @@
 ## versions of nano installed (e.g. your home directory is on NFS).
 # set quiet
 
+## When soft line wrapping is enabled, make it wrap lines at blanks
+## (tabs and spaces) instead of always at the edge of the screen.
+# set atblanks
+
 ## Use auto-indentation.
 # set autoindent
 

src/nano.h

@@ -521,7 +521,8 @@ enum
     JUSTIFY_TRIM,
     SHOW_CURSOR,
     LINE_NUMBERS,
-    NO_PAUSES
+    NO_PAUSES,
+    AT_BLANKS
 };
 
 /* Flags for the menus in which a given function should be present. */

src/rcfile.c

@@ -90,6 +90,7 @@ static const rcoption rcopts[] = {
     {"view", VIEW_MODE},
 #ifndef NANO_TINY
     {"allow_insecure_backup", INSECURE_BACKUP},
+    {"atblanks", AT_BLANKS},
     {"autoindent", AUTOINDENT},
     {"backup", BACKUP_FILE},
     {"backupdir", 0},

src/winio.c

@@ -2782,8 +2782,9 @@ int update_softwrapped_line(filestruct *fileptr)
 	    break;
 
 	/* If the line is softwrapped before its last column, add a ">" just
-	 * after its softwrap breakpoint. */
-	if (to_col - from_col < editwincols)
+	 * after its softwrap breakpoint, unless we're softwrapping at blanks
+	 * and not in the middle of a word. */
+	if (!ISSET(AT_BLANKS) && to_col - from_col < editwincols)
 	    mvwaddch(edit, row - 1, to_col - from_col, '>');
 
 	from_col = to_col;
@@ -2993,12 +2994,20 @@ void edit_scroll(scroll_dir direction, int nrows)
 size_t get_softwrap_breakpoint(const char *text, size_t leftedge,
 				bool *end_of_line)
 {
+    size_t index = 0;
+	/* Current index in text. */
     size_t column = 0;
 	/* Current column position in text. */
     size_t prev_column = 0;
 	/* Previous column position in text. */
     size_t goal_column;
 	/* Column of the last character where we can break the text. */
+    bool found_blank = FALSE;
+	/* Did we find at least one blank? */
+    size_t lastblank_index = 0;
+	/* Current index of the last blank in text. */
+    size_t lastblank_column = 0;
+	/* Current column position of the last blank in text. */
     int char_len = 0;
 	/* Length of current character, in bytes. */
 
@@ -3007,13 +3016,26 @@ size_t get_softwrap_breakpoint(const char *text, size_t leftedge,
     while (*text != '\0' && column < leftedge)
 	text += parse_mbchar(text, NULL, &column);
 
-    /* Use a full screen row for text. */
-    goal_column = column + editwincols;
+    /* Use a full screen row for text, or, if we're softwrapping at blanks, use
+     * a full screen row less one column for text and reserve the last column
+     * for blanks.  The latter case is to ensure that we have enough room for
+     * blanks exactly on the last column of the screen. */
+    if (ISSET(AT_BLANKS) && editwincols > 2)
+	goal_column = column + (editwincols - 1);
+    else
+	goal_column = column + editwincols;
 
     while (*text != '\0' && column <= goal_column) {
+	if (ISSET(AT_BLANKS) && editwincols > 2 && is_blank_mbchar(text)) {
+	    found_blank = TRUE;
+	    lastblank_index = index;
+	    lastblank_column = column;
+	}
+
 	prev_column = column;
 	char_len = parse_mbchar(text, NULL, &column);
 	text += char_len;
+	index += char_len;
     }
 
     /* If the text displays within goal_column, we've reached the end of the
@@ -3023,6 +3045,14 @@ size_t get_softwrap_breakpoint(const char *text, size_t leftedge,
 	return column;
     }
 
+    /* If we're softwrapping at blanks and we found at least one blank, move
+     * the pointer back to the last blank, step beyond it, and we're done. */
+    if (found_blank) {
+	text = text - index + lastblank_index;
+	parse_mbchar(text, NULL, &lastblank_column);
+	return lastblank_column;
+    }
+
     /* Otherwise, return the column of the last character before goal_column,
      * since we can't break the text anywhere else. */
     return (editwincols > 2) ? prev_column : column - 1;

syntax/nanorc.nanorc

@@ -7,7 +7,7 @@ comment "#"
 icolor brightred "^[[:space:]]*((un)?(bind|set)|include|syntax|header|comment|magic|linter|i?color|extendsyntax).*$"
 
 # Keywords
-icolor brightgreen "^[[:space:]]*(set|unset)[[:space:]]+(allow_insecure_backup|autoindent|backup|backwards|boldtext|casesensitive|constantshow|cut|fill[[:space:]]+-?[[:digit:]]+|historylog|justifytrim|linenumbers|locking|morespace|mouse|multibuffer|noconvert|nohelp|nopauses|nonewlines|nowrap|positionlog|preserve|quickblank|quiet|rebinddelete|rebindkeypad|regexp|showcursor|smarthome|smooth|softwrap|suspend|tabsize[[:space:]]+[1-9][0-9]*|tabstospaces|tempfile|unix|view|wordbounds)\>"
+icolor brightgreen "^[[:space:]]*(set|unset)[[:space:]]+(allow_insecure_backup|atblanks|autoindent|backup|backwards|boldtext|casesensitive|constantshow|cut|fill[[:space:]]+-?[[:digit:]]+|historylog|justifytrim|linenumbers|locking|morespace|mouse|multibuffer|noconvert|nohelp|nopauses|nonewlines|nowrap|positionlog|preserve|quickblank|quiet|rebinddelete|rebindkeypad|regexp|showcursor|smarthome|smooth|softwrap|suspend|tabsize[[:space:]]+[1-9][0-9]*|tabstospaces|tempfile|unix|view|wordbounds)\>"
 icolor yellow "^[[:space:]]*set[[:space:]]+((function|key|number|status|title)color)[[:space:]]+(bright)?(white|black|red|blue|green|yellow|magenta|cyan)?(,(white|black|red|blue|green|yellow|magenta|cyan))?\>"
 icolor brightgreen "^[[:space:]]*set[[:space:]]+(backupdir|brackets|functioncolor|keycolor|matchbrackets|numbercolor|operatingdir|punct|quotestr|speller|statuscolor|titlecolor|whitespace|wordchars)[[:space:]]+"
 icolor brightgreen "^[[:space:]]*bind[[:space:]]+((\^([[:alpha:]]|[]0-9\^_]|Space)|M-([[:alpha:]]|[]!"#$%&'()*+,./0-9:;<=>?@\^_`{|}~-]|Space))|F([1-9]|1[0-6])|Ins|Del)[[:space:]]+[[:alpha:]]+[[:space:]]+(all|main|search|replace(with)?|gotoline|writeout|insert|ext(ernal)?cmd|help|spell|linter|browser|whereisfile|gotodir)([[:space:]]+#|[[:space:]]*$)"