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diff --git a/internal/gtsmodel/interactionpolicy.go b/internal/gtsmodel/interactionpolicy.go
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+++ b/internal/gtsmodel/interactionpolicy.go
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+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package gtsmodel
+
+// A policy URI is GoToSocial's internal representation of
+// one ActivityPub URI for an Actor or a Collection of Actors,
+// specific to the domain of enforcing interaction policies.
+//
+// A PolicyValue can be stored in the database either as one
+// of the Value constants defined below (to save space), OR as
+// a full-fledged ActivityPub URI.
+//
+// A PolicyValue should be translated to the canonical string
+// value of the represented URI when federating an item, or
+// from the canonical string value of the URI when receiving
+// or retrieving an item.
+//
+// For example, if the PolicyValue `followers` was being
+// federated outwards in an interaction policy attached to an
+// item created by the actor `https://example.org/users/someone`,
+// then it should be translated to their followers URI when sent,
+// eg., `https://example.org/users/someone/followers`.
+//
+// Likewise, if GoToSocial receives an item with an interaction
+// policy containing `https://example.org/users/someone/followers`,
+// and the item was created by `https://example.org/users/someone`,
+// then the followers URI would be converted to `followers`
+// for internal storage.
+type PolicyValue string
+
+const (
+	// Stand-in for ActivityPub magic public URI,
+	// which encompasses every possible Actor URI.
+	PolicyValuePublic PolicyValue = "public"
+	// Stand-in for the Followers Collection of
+	// the item owner's Actor.
+	PolicyValueFollowers PolicyValue = "followers"
+	// Stand-in for the Following Collection of
+	// the item owner's Actor.
+	PolicyValueFollowing PolicyValue = "following"
+	// Stand-in for the Mutuals Collection of
+	// the item owner's Actor.
+	//
+	// (TODO: Reserved, currently unused).
+	PolicyValueMutuals PolicyValue = "mutuals"
+	// Stand-in for Actor URIs tagged in the item.
+	PolicyValueMentioned PolicyValue = "mentioned"
+	// Stand-in for the Actor URI of the item owner.
+	PolicyValueAuthor PolicyValue = "author"
+)
+
+// FeasibleForVisibility returns true if the PolicyValue could feasibly
+// be set in a policy for an item with the given visibility, otherwise
+// returns false.
+//
+// For example, PolicyValuePublic could not be set in a policy for an
+// item with visibility FollowersOnly, but could be set in a policy
+// for an item with visibility Public or Unlocked.
+//
+// This is not prescriptive, and should be used only to guide policy
+// choices. Eg., if a remote instance wants to do something wacky like
+// set "anyone can interact with this status" for a Direct visibility
+// status, that's their business; our normal visibility filtering will
+// prevent users on our instance from actually being able to interact
+// unless they can see the status anyway.
+func (p PolicyValue) FeasibleForVisibility(v Visibility) bool {
+	switch p {
+
+	// Mentioned and self Values are
+	// feasible for any visibility.
+	case PolicyValueAuthor,
+		PolicyValueMentioned:
+		return true
+
+	// Followers/following/mutual Values
+	// are only feasible for items with
+	// followers visibility and higher.
+	case PolicyValueFollowers,
+		PolicyValueFollowing:
+		return v == VisibilityFollowersOnly ||
+			v == VisibilityPublic ||
+			v == VisibilityUnlocked
+
+	// Public policy Value only feasible
+	// for items that are To or CC public.
+	case PolicyValuePublic:
+		return v == VisibilityUnlocked ||
+			v == VisibilityPublic
+
+	// Any other combo
+	// is probably fine.
+	default:
+		return true
+	}
+}
+
+type PolicyValues []PolicyValue
+
+// PolicyResult represents the result of
+// checking an Actor URI and interaction
+// type against the conditions of an
+// InteractionPolicy to determine if that
+// interaction is permitted.
+type PolicyResult int
+
+const (
+	// Interaction is forbidden for this
+	// PolicyValue + interaction combination.
+	PolicyResultForbidden PolicyResult = iota
+	// Interaction is conditionally permitted
+	// for this PolicyValue + interaction combo,
+	// pending approval by the item owner.
+	PolicyResultWithApproval
+	// Interaction is permitted for this
+	// PolicyValue + interaction combination.
+	PolicyResultPermitted
+)
+
+// An InteractionPolicy determines which
+// interactions will be accepted for an
+// item, and according to what rules.
+type InteractionPolicy struct {
+	// Conditions in which a Like
+	// interaction will be accepted
+	// for an item with this policy.
+	CanLike PolicyRules
+	// Conditions in which a Reply
+	// interaction will be accepted
+	// for an item with this policy.
+	CanReply PolicyRules
+	// Conditions in which an Announce
+	// interaction will be accepted
+	// for an item with this policy.
+	CanAnnounce PolicyRules
+}
+
+// PolicyRules represents the rules according
+// to which a certain interaction is permitted
+// to various Actor and Actor Collection URIs.
+type PolicyRules struct {
+	// Always is for PolicyValues who are
+	// permitted to do an interaction
+	// without requiring approval.
+	Always PolicyValues
+	// WithApproval is for PolicyValues who
+	// are conditionally permitted to do
+	// an interaction, pending approval.
+	WithApproval PolicyValues
+}
+
+// Returns the default interaction policy
+// for the given visibility level.
+func DefaultInteractionPolicyFor(v Visibility) *InteractionPolicy {
+	switch v {
+	case VisibilityPublic:
+		return DefaultInteractionPolicyPublic()
+	case VisibilityUnlocked:
+		return DefaultInteractionPolicyUnlocked()
+	case VisibilityFollowersOnly, VisibilityMutualsOnly:
+		return DefaultInteractionPolicyFollowersOnly()
+	case VisibilityDirect:
+		return DefaultInteractionPolicyDirect()
+	default:
+		panic("visibility " + v + " not recognized")
+	}
+}
+
+// Returns the default interaction policy
+// for a post with visibility of public.
+func DefaultInteractionPolicyPublic() *InteractionPolicy {
+	// Anyone can like.
+	canLikeAlways := make(PolicyValues, 1)
+	canLikeAlways[0] = PolicyValuePublic
+
+	// Unused, set empty.
+	canLikeWithApproval := make(PolicyValues, 0)
+
+	// Anyone can reply.
+	canReplyAlways := make(PolicyValues, 1)
+	canReplyAlways[0] = PolicyValuePublic
+
+	// Unused, set empty.
+	canReplyWithApproval := make(PolicyValues, 0)
+
+	// Anyone can announce.
+	canAnnounceAlways := make(PolicyValues, 1)
+	canAnnounceAlways[0] = PolicyValuePublic
+
+	// Unused, set empty.
+	canAnnounceWithApproval := make(PolicyValues, 0)
+
+	return &InteractionPolicy{
+		CanLike: PolicyRules{
+			Always:       canLikeAlways,
+			WithApproval: canLikeWithApproval,
+		},
+		CanReply: PolicyRules{
+			Always:       canReplyAlways,
+			WithApproval: canReplyWithApproval,
+		},
+		CanAnnounce: PolicyRules{
+			Always:       canAnnounceAlways,
+			WithApproval: canAnnounceWithApproval,
+		},
+	}
+}
+
+// Returns the default interaction policy
+// for a post with visibility of unlocked.
+func DefaultInteractionPolicyUnlocked() *InteractionPolicy {
+	// Same as public (for now).
+	return DefaultInteractionPolicyPublic()
+}
+
+// Returns the default interaction policy for
+// a post with visibility of followers only.
+func DefaultInteractionPolicyFollowersOnly() *InteractionPolicy {
+	// Self, followers and mentioned can like.
+	canLikeAlways := make(PolicyValues, 3)
+	canLikeAlways[0] = PolicyValueAuthor
+	canLikeAlways[1] = PolicyValueFollowers
+	canLikeAlways[2] = PolicyValueMentioned
+
+	// Unused, set empty.
+	canLikeWithApproval := make(PolicyValues, 0)
+
+	// Self, followers and mentioned can reply.
+	canReplyAlways := make(PolicyValues, 3)
+	canReplyAlways[0] = PolicyValueAuthor
+	canReplyAlways[1] = PolicyValueFollowers
+	canReplyAlways[2] = PolicyValueMentioned
+
+	// Unused, set empty.
+	canReplyWithApproval := make(PolicyValues, 0)
+
+	// Only self can announce.
+	canAnnounceAlways := make(PolicyValues, 1)
+	canAnnounceAlways[0] = PolicyValueAuthor
+
+	// Unused, set empty.
+	canAnnounceWithApproval := make(PolicyValues, 0)
+
+	return &InteractionPolicy{
+		CanLike: PolicyRules{
+			Always:       canLikeAlways,
+			WithApproval: canLikeWithApproval,
+		},
+		CanReply: PolicyRules{
+			Always:       canReplyAlways,
+			WithApproval: canReplyWithApproval,
+		},
+		CanAnnounce: PolicyRules{
+			Always:       canAnnounceAlways,
+			WithApproval: canAnnounceWithApproval,
+		},
+	}
+}
+
+// Returns the default interaction policy
+// for a post with visibility of direct.
+func DefaultInteractionPolicyDirect() *InteractionPolicy {
+	// Mentioned and self can always like.
+	canLikeAlways := make(PolicyValues, 2)
+	canLikeAlways[0] = PolicyValueAuthor
+	canLikeAlways[1] = PolicyValueMentioned
+
+	// Unused, set empty.
+	canLikeWithApproval := make(PolicyValues, 0)
+
+	// Mentioned and self can always reply.
+	canReplyAlways := make(PolicyValues, 2)
+	canReplyAlways[0] = PolicyValueAuthor
+	canReplyAlways[1] = PolicyValueMentioned
+
+	// Unused, set empty.
+	canReplyWithApproval := make(PolicyValues, 0)
+
+	// Only self can announce.
+	canAnnounceAlways := make(PolicyValues, 1)
+	canAnnounceAlways[0] = PolicyValueAuthor
+
+	// Unused, set empty.
+	canAnnounceWithApproval := make(PolicyValues, 0)
+
+	return &InteractionPolicy{
+		CanLike: PolicyRules{
+			Always:       canLikeAlways,
+			WithApproval: canLikeWithApproval,
+		},
+		CanReply: PolicyRules{
+			Always:       canReplyAlways,
+			WithApproval: canReplyWithApproval,
+		},
+		CanAnnounce: PolicyRules{
+			Always:       canAnnounceAlways,
+			WithApproval: canAnnounceWithApproval,
+		},
+	}
+}