softwrap: prepare for allowing the chunk width to vary

The new function find_softwrap_breakpoint() returns the column number
of the last position in screen range where we can wrap the given text
without breaking a two-column character in half (as was done until now).
The returned column number is the leftedge of the next softwrapped chunk.

If the end of the text is reached while searching for a wrapping point,
the parameter end_of_line is set to TRUE.

The new function get_chunk() uses find_softwrap_breakpoint() to find the
row and leftedge corresponding to a given column of a given line.

src/proto.h

@@ -663,6 +663,9 @@ int go_forward_chunks(int nrows, filestruct **line, size_t *leftedge);
 bool less_than_a_screenful(size_t was_lineno, size_t was_leftedge);
 void edit_scroll(scroll_dir direction, int nrows);
 #ifndef NANO_TINY
+size_t get_softwrap_breakpoint(const char *text, size_t leftedge,
+				bool *end_of_line);
+size_t get_chunk(filestruct *line, size_t column, size_t *leftedge);
 void ensure_firstcolumn_is_aligned(void);
 #endif
 void edit_redraw(filestruct *old_current);

src/winio.c

@@ -2974,6 +2974,71 @@ void edit_scroll(scroll_dir direction, int nrows)
 }
 
 #ifndef NANO_TINY
+/* Get the column number after leftedge where we can break the given text, and
+ * return it.  This will always be editwincols or less after leftedge.  Set
+ * end_of_line to TRUE if we reach the end of the line while searching the
+ * text.  Assume leftedge is the leftmost column of a softwrapped chunk. */
+size_t get_softwrap_breakpoint(const char *text, size_t leftedge,
+				bool *end_of_line)
+{
+    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. */
+    int char_len = 0;
+	/* Length of current character, in bytes. */
+
+    *end_of_line = FALSE;
+
+    while (*text != '\0' && column < leftedge)
+	text += parse_mbchar(text, NULL, &column);
+
+    /* Use a full screen row for text. */
+    goal_column = column + editwincols;
+
+    while (*text != '\0' && column <= goal_column) {
+	prev_column = column;
+	char_len = parse_mbchar(text, NULL, &column);
+	text += char_len;
+    }
+
+    /* If the text displays within goal_column, we've reached the end of the
+     * line, and we're done. */
+    if (column <= goal_column) {
+	*end_of_line = TRUE;
+	return 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;
+}
+
+/* Get the row of the softwrapped chunk of the given line that column is on,
+ * relative to the first row (zero-based), and return it.  If leftedge isn't
+ * NULL, return the leftmost column of the chunk in it. */
+size_t get_chunk(filestruct *line, size_t column, size_t *leftedge)
+{
+    size_t current_chunk = 0, start_col = 0, end_col;
+    bool end_of_line;
+
+    while (TRUE) {
+	end_col = get_softwrap_breakpoint(line->data, start_col, &end_of_line);
+
+	/* We reached the end of the line and/or found column, so get out. */
+	if (end_of_line || (column >= start_col && column < end_col)) {
+	    if (leftedge != NULL)
+		*leftedge = start_col;
+	    return current_chunk;
+	}
+
+	current_chunk++;
+	start_col = end_col;
+    }
+}
+
 /* Ensure that firstcolumn is at the starting column of the softwrapped chunk
  * it's on.  We need to do this when the number of columns of the edit window
  * has changed, because then the width of softwrapped chunks has changed. */