Implement channel permissions resolution (#11)

* Add channel permission resolution

* Fix target ID handling in permission overwrite retrieval

disagreement/models.py

@@ -21,6 +21,7 @@ from .enums import (  # These enums will need to be defined in disagreement/enum
     ButtonStyle,  # Added for Button
     # SelectMenuType will be part of ComponentType or a new enum if needed
 )
+from .permissions import Permissions
 
 
 if TYPE_CHECKING:
@@ -897,6 +898,78 @@ class Channel:
     def __repr__(self) -> str:
         return f"<Channel id='{self.id}' name='{self.name}' type='{self.type.name if hasattr(self.type, 'name') else self._type_val}'>"
 
+    def permission_overwrite_for(
+        self, target: Union["Role", "Member", str]
+    ) -> Optional["PermissionOverwrite"]:
+        """Return the :class:`PermissionOverwrite` for ``target`` if present."""
+
+        if isinstance(target, str):
+            target_id = int(target)
+        else:
+            target_id = target.id
+        for overwrite in self.permission_overwrites:
+            if overwrite.id == target_id:
+                return overwrite
+        return None
+
+    @staticmethod
+    def _apply_overwrite(
+        perms: Permissions, overwrite: Optional["PermissionOverwrite"]
+    ) -> Permissions:
+        if overwrite is None:
+            return perms
+
+        perms &= ~Permissions(int(overwrite.deny))
+        perms |= Permissions(int(overwrite.allow))
+        return perms
+
+    def permissions_for(self, member: "Member") -> Permissions:
+        """Resolve channel permissions for ``member``."""
+
+        if self.guild_id is None:
+            return Permissions(~0)
+
+        if not hasattr(self._client, "get_guild"):
+            return Permissions(0)
+
+        guild = self._client.get_guild(self.guild_id)
+        if guild is None:
+            return Permissions(0)
+
+        base = Permissions(0)
+
+        everyone = guild.get_role(guild.id)
+        if everyone is not None:
+            base |= Permissions(int(everyone.permissions))
+
+        for rid in member.roles:
+            role = guild.get_role(rid)
+            if role is not None:
+                base |= Permissions(int(role.permissions))
+
+        if base & Permissions.ADMINISTRATOR:
+            return Permissions(~0)
+
+        # Apply @everyone overwrite
+        base = self._apply_overwrite(base, self.permission_overwrite_for(guild.id))
+
+        # Role overwrites
+        role_allow = Permissions(0)
+        role_deny = Permissions(0)
+        for rid in member.roles:
+            ow = self.permission_overwrite_for(rid)
+            if ow is not None:
+                role_allow |= Permissions(int(ow.allow))
+                role_deny |= Permissions(int(ow.deny))
+
+        base &= ~role_deny
+        base |= role_allow
+
+        # Member overwrite
+        base = self._apply_overwrite(base, self.permission_overwrite_for(member.id))
+
+        return base
+
 
 class TextChannel(Channel):
     """Represents a guild text channel or announcement channel."""

tests/test_channel_permissions.py

@@ -0,0 +1,128 @@
+import pytest  # pylint: disable=E0401
+
+from disagreement.models import Guild, Member, Role, TextChannel, PermissionOverwrite
+from disagreement.enums import (
+    ChannelType,
+    VerificationLevel,
+    MessageNotificationLevel,
+    ExplicitContentFilterLevel,
+    MFALevel,
+    GuildNSFWLevel,
+    PremiumTier,
+    OverwriteType,
+)
+from disagreement.permissions import Permissions
+
+
+class DummyClient:
+    def __init__(self):
+        self._guilds = {}
+
+    def get_guild(self, gid):
+        return self._guilds.get(gid)
+
+
+def _base_guild(client):
+    data = {
+        "id": "1",
+        "name": "g",
+        "owner_id": "1",
+        "afk_timeout": 60,
+        "verification_level": VerificationLevel.NONE.value,
+        "default_message_notifications": MessageNotificationLevel.ALL_MESSAGES.value,
+        "explicit_content_filter": ExplicitContentFilterLevel.DISABLED.value,
+        "roles": [],
+        "emojis": [],
+        "features": [],
+        "mfa_level": MFALevel.NONE.value,
+        "system_channel_flags": 0,
+        "premium_tier": PremiumTier.NONE.value,
+        "nsfw_level": GuildNSFWLevel.DEFAULT.value,
+    }
+    guild = Guild(data, client_instance=client)
+    client._guilds[guild.id] = guild
+    return guild
+
+
+def _member(guild, *roles):
+    data = {
+        "user": {"id": "10", "username": "u", "discriminator": "0001"},
+        "joined_at": "t",
+        "roles": [r.id for r in roles] or [guild.id],
+    }
+    member = Member(data, client_instance=None)
+    member.guild_id = guild.id
+    guild._members[member.id] = member
+    return member
+
+
+def _role(guild, rid, perms):
+    role = Role(
+        {
+            "id": rid,
+            "name": f"r{rid}",
+            "color": 0,
+            "hoist": False,
+            "position": 0,
+            "permissions": str(int(perms)),
+            "managed": False,
+            "mentionable": False,
+        }
+    )
+    guild.roles.append(role)
+    return role
+
+
+def _channel(guild, client):
+    data = {
+        "id": "100",
+        "type": ChannelType.GUILD_TEXT.value,
+        "guild_id": guild.id,
+        "permission_overwrites": [],
+    }
+    channel = TextChannel(data, client_instance=client)
+    guild._channels[channel.id] = channel
+    return channel
+
+
+def test_permissions_for_base_roles():
+    client = DummyClient()
+    guild = _base_guild(client)
+    everyone = _role(
+        guild, guild.id, Permissions.VIEW_CHANNEL | Permissions.SEND_MESSAGES
+    )
+    mod = _role(guild, "2", Permissions.MANAGE_MESSAGES)
+    member = _member(guild, everyone, mod)
+    channel = _channel(guild, client)
+
+    perms = channel.permissions_for(member)
+    assert perms & Permissions.MANAGE_MESSAGES
+    assert perms & Permissions.SEND_MESSAGES
+    assert perms & Permissions.VIEW_CHANNEL
+
+
+def test_permissions_for_with_overwrite():
+    client = DummyClient()
+    guild = _base_guild(client)
+    everyone = _role(
+        guild, guild.id, Permissions.VIEW_CHANNEL | Permissions.SEND_MESSAGES
+    )
+    mod = _role(guild, "2", Permissions.MANAGE_MESSAGES)
+    member = _member(guild, everyone, mod)
+    channel = _channel(guild, client)
+
+    channel.permission_overwrites.append(
+        PermissionOverwrite(
+            {
+                "id": mod.id,
+                "type": OverwriteType.ROLE.value,
+                "allow": "0",
+                "deny": str(int(Permissions.MANAGE_MESSAGES)),
+            }
+        )
+    )
+
+    perms = channel.permissions_for(member)
+    assert not perms & Permissions.MANAGE_MESSAGES
+    assert perms & Permissions.SEND_MESSAGES
+    assert perms & Permissions.VIEW_CHANNEL