Consolidate common PPcomp diff (#1714)

* Consolidate common PPcomp diff

Where a line is italic, frequently get solitary underscore in
diffs, then the closing underscore on the end of the next line.
Particularly happens in poetry. Consolidate those two diffs
into one.

Fixes #1711

* Further improvement

Increment line number of underscore diff from italic verse

Author: windymilla

src/guiguts/tools/ppcomp.py

@@ -553,10 +553,46 @@ def flush_changes():
     if cur_line and line_has_change:
         lines.append((join_tokens(cur_line), cur_linenum))
 
+    underscore_reg = (
+        f"[{FLAG_CH_HTML_L}{FLAG_CH_TEXT_L}]_[{FLAG_CH_HTML_R}{FLAG_CH_TEXT_R}]"
+    )
     # ---- Send to the dialog ----
-    for text, line_pair in lines:
+    for lnum, (text, line_pair) in enumerate(lines):
         if line_pair is None:
             continue
+        # If there is another line after this one, look for the following circumstances:
+        # 1. Next line is also the next line in the text file
+        # 2. This line is just an underscore (plus flags)
+        # 3. Next line has an underscore at the end
+        if lnum < len(lines) - 1:
+            next_text, next_line_pair = lines[lnum + 1]
+            if (
+                next_line_pair is not None
+                and line_pair[1] + 1 == next_line_pair[1]  # Next line in text file?
+                and re.fullmatch(
+                    underscore_reg,  # Just an underscore
+                    text,
+                )
+                and re.search(f"{underscore_reg}$", next_text)  # Ends in underscore
+            ):
+                # Bring info up from next line onto this one
+                text = f"{text} {next_text}"
+                line_pair = next_line_pair
+                lines[lnum] = (text, line_pair)
+                # Nullify next line so it is ignore next time round loop
+                lines[lnum + 1] = (next_text, None)
+        # If there is a line before this one, look for the following circumstances:
+        # 1. This line is just an underscore (plus flags)
+        # 2. Next line has an underscore at the end
+        if lnum > 0:
+            prev_text = lines[lnum - 1][0]
+            if re.fullmatch(underscore_reg, text) and re.search(
+                f"{underscore_reg}$", prev_text
+            ):
+                # Increment text line number
+                line_pair = (line_pair[0], line_pair[1] + 1)
+                lines[lnum] = (text, line_pair)
+
         a_line, b_line = line_pair
         if html_ln:
             if a_line is not None: