Initial release: full Arma 3 mod management toolchain

Pipeline: parse HTML presets, compare modlists, download from Caddy
file server, create junctions/symlinks to Arma 3 Server directory.
Includes update/sync flows, missing-mod reporting, OS compat layer,
shared config, dep checker, comprehensive test suite (71 tests).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

arma_modlist_tools/compare.py

@@ -0,0 +1,79 @@
+"""
+arma_modlist_tools.compare
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+Compare two or more Arma 3 mod presets (parsed by :mod:`arma_modlist_tools.parser`)
+and produce a breakdown of shared and preset-unique mods.
+
+Typical usage::
+
+    from arma_modlist_tools.parser import parse_modlist_dir
+    from arma_modlist_tools.compare import compare_presets
+
+    presets = parse_modlist_dir("modlist_html")
+    result  = compare_presets(*presets)
+"""
+
+from __future__ import annotations
+
+
+def _mod_key(mod: dict) -> str:
+    """Return the identity key for a mod.
+
+    Uses ``steam_id`` when available (canonical Workshop identifier),
+    falls back to ``name`` for local mods that have no Workshop ID.
+    """
+    return mod["steam_id"] or mod["name"]
+
+
+def compare_presets(*presets: dict) -> dict:
+    """
+    Compare two or more preset dicts and return a comparison dict.
+
+    :param presets: Two or more preset dicts as returned by
+                    :func:`~arma_modlist_tools.parser.parse_modlist_html`.
+    :returns: Dict with keys:
+
+              - ``compared_presets`` — list of preset names that were compared
+              - ``shared`` — mods present in **every** preset
+                  - ``mod_count`` — number of shared mods
+                  - ``mods`` — list of mod entry dicts
+              - ``unique`` — per-preset mods not present in any other preset
+                  - keyed by ``preset_name``
+                  - each value has ``mod_count`` and ``mods``
+
+    :raises ValueError: If fewer than two presets are provided.
+    """
+    if len(presets) < 2:
+        raise ValueError("compare_presets requires at least two presets")
+
+    # Build per-preset {identity_key -> mod_entry} mappings
+    preset_maps: list[dict[str, dict]] = [
+        {_mod_key(mod): mod for mod in preset["mods"]}
+        for preset in presets
+    ]
+
+    # Shared keys = intersection across ALL presets
+    shared_keys: set[str] = set(preset_maps[0].keys())
+    for pm in preset_maps[1:]:
+        shared_keys &= pm.keys()
+
+    # Shared mods: take entries from the first preset (identical across all)
+    shared_mods = [preset_maps[0][k] for k in preset_maps[0] if k in shared_keys]
+
+    # Unique mods per preset: entries whose key is not in the shared set
+    unique: dict[str, dict] = {}
+    for preset, pm in zip(presets, preset_maps):
+        unique_mods = [mod for k, mod in pm.items() if k not in shared_keys]
+        unique[preset["preset_name"]] = {
+            "mod_count": len(unique_mods),
+            "mods": unique_mods,
+        }
+
+    return {
+        "compared_presets": [p["preset_name"] for p in presets],
+        "shared": {
+            "mod_count": len(shared_mods),
+            "mods": shared_mods,
+        },
+        "unique": unique,
+    }