ENH: _writer: Add right alignment and centering

This patch changes the generate_appearance_stream code
so that it can deal with right alignment and centered
text.

Note that both require correct font metrics in order
to work.

Author: PJBrs

pypdf/_writer.py

@@ -943,7 +943,7 @@ def _update_field_annotation(
             if font_size >= 0
             else float(font_properties[font_properties.index("Tf") - 1])
         )
-        formatting = {
+        formatting : dict[str, Union[bool, int]] = {
             "wrap" : False,
             "scale" : False,
         }
@@ -959,6 +959,7 @@ def _update_field_annotation(
                 font_height = rct.height - 2
                 formatting["scale"] = True
         font_properties[font_properties.index("Tf") - 1] = str(font_height)
+        formatting["align"] = field.get("/Q", 0)
 
         # Retrieve font information from local DR ...
         dr: Any = cast(
@@ -3554,7 +3555,7 @@ def generate_appearance_stream(
     font_descriptor: FontDescriptor,
     rct: RectangleObject,
     font_height: float,
-    formatting: dict[str, bool],
+    formatting: dict[str, Union[bool, int]],
 ) -> bytes:
     # Only wrap text for non-choice fields, otherwise we break matching sel and line later on.
     if sel:
@@ -3567,7 +3568,7 @@ def generate_appearance_stream(
                             # takes one extra point (see below, under "line_number == 0:")
             rct.height - 3,  # One point margin for top and bottom, one point extra for the first line (see y_offset)
             txt,
-            formatting["wrap"],
+            bool(formatting["wrap"]),
         )
         font_properties[1] = str(font_height)
     else:
@@ -3576,7 +3577,11 @@ def generate_appearance_stream(
     y_offset = rct.height - 1 - font_height
     da = " ".join(font_properties)
     ap_stream = f"q\n/Tx BMC \nq\n1 1 {rct.width - 1} {rct.height - 1} re\nW\nBT\n{da}\n".encode()
+
+    current_x_pos : float = 0  # Initial virtual position within the text object.
     for line_number, line in enumerate(lines):
+        # Calculate line width
+        line_w = calculate_text_width(font_descriptor.character_widths, float(font_properties[1]), line)
         # Escape parentheses (PDF 1.7 reference, table 3.2, Literal Strings)
         line = line.replace("\\", "\\\\").replace("(", r"\(").replace(")", r"\)")
         if line in sel:
@@ -3585,11 +3590,33 @@ def generate_appearance_stream(
                 f"1 {y_offset - (line_number * font_height * 1.4) - 1} {rct.width - 2} {font_height + 2} re\n"
                 f"0.5 0.5 0.5 rg s\n{da}\n"
             ).encode()
+
+        # Calculate the desired absolute starting X for the current line
+        desired_abs_x_start : float = 0
+        if formatting["align"] == 2:  # Right aligned
+            desired_abs_x_start = rct.width - 2 - line_w
+        elif formatting["align"] == 1:  # Centered
+            desired_abs_x_start = (rct.width - line_w) / 2
+        else:  # Left aligned; default
+            desired_abs_x_start = 2
+        # Calculate x_rel_offset: how much to move from the current_x_pos
+        # to reach the desired_abs_x_start.
+        x_rel_offset = desired_abs_x_start - current_x_pos
+
+        # Y-offset:
+        y_rel_offset: float = 0
         if line_number == 0:
-            ap_stream += f"2 {y_offset} Td\n".encode()
+            y_rel_offset = y_offset # Initial vertical position
         else:
-            # Td is a relative translation
-            ap_stream += f"0 {- font_height * 1.4} Td\n".encode()
+            y_rel_offset = - font_height * 1.4 # Move down by line height
+
+        # Td is a relative translation (Tx and Ty).
+        # It updates the current text position.
+        ap_stream += f"{x_rel_offset} {y_rel_offset} Td\n".encode()
+        # Update current_x_pos based on the Td operation for the next iteration.
+        # This is the X position where the *current line* will start.
+        current_x_pos = desired_abs_x_start
+
         enc_line: list[bytes] = [
             font_full_rev.get(c, c.encode("utf-16-be")) for c in line
         ]