refactor: Extract changelog_helper functions into focused modules

Reorganize changelog_helper.py by extracting functions into separate
modules for better code organization and maintainability:

- Created cli/ module with argument parsing functionality
- Created formatters/ module with HTML and layer output generation
- Extracted core classes (AttackChangesEncoder, DiffStix, DomainStatistics)
  into separate files within core/ directory

The changelog_helper.py file now serves as the main entry point,
containing only main() and get_new_changelog_md() functions, while
re-exporting all previously exported functions for backward compatibility.

All 133 tests pass successfully.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jondricek

mitreattack/diffStix/changelog_helper.py

@@ -0,0 +1,5 @@
+"""CLI utilities for changelog_helper."""
+
+from mitreattack.diffStix.cli.argument_parser import get_parsed_args
+
+__all__ = ["get_parsed_args"]

mitreattack/diffStix/cli/__init__.py

@@ -0,0 +1,146 @@
+"""Command-line argument parser for changelog_helper."""
+
+import argparse
+
+from loguru import logger
+from tqdm import tqdm
+
+from mitreattack.diffStix.utils.constants import LAYER_DEFAULTS as layer_defaults
+
+
+def get_parsed_args():
+    """Create argument parser and parse arguments."""
+    parser = argparse.ArgumentParser(
+        description=(
+            "Create changelog reports on the differences between two versions of the ATT&CK content. "
+            "Takes STIX bundles as input. For default operation, put "
+            "enterprise-attack.json, mobile-attack.json, and ics-attack.json bundles "
+            "in 'old' and 'new' folders for the script to compare."
+        )
+    )
+
+    parser.add_argument(
+        "--old",
+        type=str,
+        # Default is really "old", set below
+        default=None,
+        help="Directory to load old STIX data from.",
+    )
+
+    parser.add_argument(
+        "--new",
+        type=str,
+        default="new",
+        help="Directory to load new STIX data from.",
+    )
+
+    parser.add_argument(
+        "--domains",
+        type=str,
+        nargs="+",
+        choices=["enterprise-attack", "mobile-attack", "ics-attack"],
+        default=["enterprise-attack", "mobile-attack", "ics-attack"],
+        help="Which domains to report on. Choices (and defaults) are %(choices)s",
+    )
+
+    parser.add_argument(
+        "--markdown-file",
+        type=str,
+        help="Create a markdown file reporting changes.",
+    )
+
+    parser.add_argument(
+        "--html-file",
+        type=str,
+        help="Create HTML page from markdown content.",
+    )
+
+    parser.add_argument(
+        "--html-file-detailed",
+        type=str,
+        help="Create an HTML file reporting detailed changes.",
+    )
+
+    parser.add_argument(
+        "--json-file",
+        type=str,
+        help="Create a JSON file reporting changes.",
+    )
+
+    parser.add_argument(
+        "--layers",
+        type=str,
+        nargs="*",
+        help=f"""
+            Create layer files showing changes in each domain
+            expected order of filenames is 'enterprise', 'mobile', 'ics', 'pre attack'.
+            If values are unspecified, defaults to {", ".join(layer_defaults)}
+            """,
+    )
+
+    parser.add_argument(
+        "--site_prefix",
+        type=str,
+        default="",
+        help="Prefix links in markdown output, e.g. [prefix]/techniques/T1484",
+    )
+
+    parser.add_argument(
+        "--unchanged",
+        action="store_true",
+        help="Show objects without changes in the markdown output",
+    )
+
+    parser.add_argument(
+        "--use-mitre-cti",
+        action="store_true",
+        help="Use content from the MITRE CTI repo for the -old data",
+    )
+
+    parser.add_argument(
+        "--show-key",
+        action="store_true",
+        help="Add a key explaining the change types to the markdown",
+    )
+
+    parser.add_argument(
+        "--contributors",
+        action="store_true",
+        help="Show new contributors between releases",
+    )
+    parser.add_argument(
+        "--no-contributors",
+        dest="contributors",
+        action="store_false",
+        help="Do not show new contributors between releases",
+    )
+    parser.set_defaults(contributors=True)
+
+    parser.add_argument(
+        "-v",
+        "--verbose",
+        action="store_true",
+        help="Print status messages",
+    )
+
+    args = parser.parse_args()
+
+    # the default loguru logger logs up to Debug by default
+    logger.remove()
+    if args.verbose:
+        logger.add(lambda msg: tqdm.write(msg, end=""), colorize=True)
+    else:
+        logger.add(lambda msg: tqdm.write(msg, end=""), colorize=True, level="INFO")
+
+    if args.use_mitre_cti and args.old:
+        parser.error("--use-mitre-cti and -old cannot be used together")
+
+    # set a default directory that doesn't conflict with use_mitre_cti
+    if not args.old:
+        args.old = "old"
+
+    if args.layers is not None:
+        if len(args.layers) not in [0, 3]:
+            parser.error("-layers requires exactly three files to be specified or none at all")
+
+    return args

mitreattack/diffStix/cli/argument_parser.py

@@ -0,0 +1,18 @@
+"""Custom JSON encoder for ATT&CK changelog generation."""
+
+import json
+
+from mitreattack.diffStix.utils.version_utils import AttackObjectVersion
+
+
+# TODO: Implement a custom decoder as well. Possible solution at this link
+# https://alexisgomes19.medium.com/custom-json-encoder-with-python-f52c91b48cd2
+class AttackChangesEncoder(json.JSONEncoder):
+    """Custom JSON encoder for changes made to ATT&CK between releases."""
+
+    def default(self, o):
+        """Handle custom object types so they can be serialized to JSON."""
+        if isinstance(o, AttackObjectVersion):
+            return str(o)
+
+        return json.JSONEncoder.default(self, o)