# Design: Community Policy Engine

## Overview

This is a pure backend feature. No UI. Design covers data structures, module layout, control flow, and integration points with existing code.

---

## Architecture

The community policy engine follows the same pattern as the existing `AgentPolicy` / `PolicyEvaluator` pairing, but operates at the community (schema) layer rather than the per-session layer.

### Dependency chain placement

```
schema/validation/community_policy.rs   ← CommunityPolicy, CommunityContext, PolicyEntry
schema/validation/builders.rs           ← SchemaBuilder::community_policy() method
schema/validation/mod.rs               ← re-exports CommunityPolicy, CommunityContext
schema/mod.rs                          ← re-exports from validation
db/community.rs                        ← TidalDb::signal_with_community_policy() + evaluator call
ranking/executor/context.rs            ← CommunityContext threading, suppressed_signals set
query/retrieve/types.rs                ← RetrieveBuilder::community() method
```

No new crate dependencies. All types live in existing modules.

---

## Data Structures

### `CommunityPolicy` (schema/validation/community_policy.rs)

```rust
#[derive(Debug, Clone)]
pub struct CommunityPolicy {
    pub allowed_write_signals: Vec<String>,  // empty = no writes allowed
    pub denied_write_signals:  Vec<String>,  // always blocked
    pub allowed_read_signals:  Vec<String>,  // empty = all readable
    pub denied_read_signals:   Vec<String>,  // always suppressed from scoring
}
```

Write semantics: deny > allow > default-allow.
- Deny list takes precedence.
- If allow list non-empty, signal must be in it.
- If both empty: all writes allowed (admin-like default).

Read semantics: deny > allow > default-allow.
- Same precedence, but applied to ranking score suppression.

### `CommunityContext` (schema/validation/community_policy.rs)

```rust
#[derive(Debug, Clone)]
pub struct CommunityContext {
    pub community_id: String,  // for tracing; not validated against stored communities
    pub role: String,          // must match a CommunityPolicy name in schema
}
```

The database does not maintain a community registry. `community_id` is advisory — used for structured logging only. `role` is validated against registered `CommunityPolicy` names at call time.

### Internal policy entry (schema/validation/community_policy.rs)

```rust
pub(super) struct CommunityPolicyEntry {
    pub(super) name: String,
    pub(super) policy: CommunityPolicy,
}
```

Stored in `SchemaBuilder::community_policies: Vec<CommunityPolicyEntry>`, converted to `HashMap<String, CommunityPolicy>` at `build()` time and stored in `Schema`.

### `Schema` additions (schema/validation/mod.rs)

```rust
pub struct Schema {
    // existing fields ...
    community_policies: HashMap<String, CommunityPolicy>,
}

impl Schema {
    pub fn community_policy(&self, role: &str) -> Option<&CommunityPolicy> {
        self.community_policies.get(role)
    }
    pub fn community_policy_count(&self) -> usize {
        self.community_policies.len()
    }
}
```

### `PolicyViolationKind` additions (session/policy.rs)

Two new variants added to the existing enum:

```rust
pub enum PolicyViolationKind {
    Expired,
    CountCap,
    Denied,
    NotAllowed,
    // New:
    CommunityWriteDenied,    // signal in denied_write_signals
    CommunityWriteNotAllowed, // signal not in non-empty allowed_write_signals
}
```

---

## Write Enforcement

### `CommunityPolicyEvaluator` (db/community.rs)

```rust
pub struct CommunityPolicyEvaluator<'a> {
    policy: &'a CommunityPolicy,
    role: &'a str,
}

impl<'a> CommunityPolicyEvaluator<'a> {
    pub fn check_write(&self, signal_type: &str) -> Result<(), PolicyViolation> {
        // 1. Deny list first
        if self.policy.denied_write_signals.iter().any(|s| s == signal_type) {
            return Err(PolicyViolation {
                kind: PolicyViolationKind::CommunityWriteDenied,
                signal_type: signal_type.to_owned(),
                policy_name: self.role.to_owned(),
                reason: format!("signal '{signal_type}' denied by community role '{}'", self.role),
            });
        }
        // 2. Allow list (empty = all allowed)
        if !self.policy.allowed_write_signals.is_empty()
            && !self.policy.allowed_write_signals.iter().any(|s| s == signal_type)
        {
            return Err(PolicyViolation {
                kind: PolicyViolationKind::CommunityWriteNotAllowed,
                signal_type: signal_type.to_owned(),
                policy_name: self.role.to_owned(),
                reason: format!("signal '{signal_type}' not in allowed writes for role '{}'", self.role),
            });
        }
        Ok(())
    }
}
```

### `TidalDb::signal_with_community_policy()` (db/community.rs)

```rust
pub fn signal_with_community_policy(
    &self,
    signal_type: &str,
    entity_id: EntityId,
    weight: f64,
    timestamp: Timestamp,
    ctx: CommunityContext,
) -> crate::Result<()> {
    // Resolve policy
    let schema = self.schema();
    let policy = schema
        .community_policy(&ctx.role)
        .ok_or_else(|| TidalError::NotFound(format!("community role '{}'", ctx.role)))?;

    // Check write rules
    let evaluator = CommunityPolicyEvaluator { policy, role: &ctx.role };
    evaluator.check_write(signal_type).map_err(|v| TidalError::PolicyViolation {
        reason: v.reason,
    })?;

    // Delegate to existing signal write path
    self.signal(signal_type, entity_id, weight, timestamp)
}
```

`TidalError` already has a `PolicyViolation` variant (from session policy). Reuse it.

---

## Read Enforcement

### Threading `CommunityContext` through retrieval

`RetrieveBuilder` gains a `community(ctx: CommunityContext)` method. The `Retrieve` struct gains `community: Option<CommunityContext>`.

The `ProfileExecutor` receives `CommunityContext` via `ExecutorContext`. Before the scoring loop:

```rust
let suppressed: HashSet<SignalTypeId> = if let Some(ctx) = &executor_ctx.community {
    let policy = schema.community_policy(&ctx.role)
        .ok_or_else(|| QueryError::NotFound(format!("community role '{}'", ctx.role)))?;
    build_suppressed_set(policy, schema)
} else {
    HashSet::new()
};
```

Where `build_suppressed_set` resolves signal names from the deny/allow read lists to `SignalTypeId` values using `schema.resolve_signal_type()`.

In the scoring loop:

```rust
for signal_type_id in signal_contributions {
    if suppressed.contains(&signal_type_id) {
        continue; // skip this signal's contribution to the score
    }
    // ... normal scoring ...
}
```

Fast path: if `suppressed.is_empty()` (no community context, or policy suppresses nothing), skip the `contains` check entirely with a branch on `suppressed.is_empty()`.

---

## Schema Validation

Added to `SchemaBuilder::build()` after signal and agent policy validation:

```rust
let mut seen_community_names = HashSet::new();
let mut community_policies = HashMap::new();
for entry in self.community_policies {
    // Name validation (reuses existing is_valid_signal_name)
    if !is_valid_signal_name(&entry.name) {
        return Err(SchemaError::InvalidCommunityPolicyName(entry.name));
    }
    // Duplicate check
    if !seen_community_names.insert(entry.name.clone()) {
        return Err(SchemaError::DuplicateCommunityPolicyName(entry.name));
    }
    // All referenced signals must exist
    for sig in all_signal_refs(&entry.policy) {
        if !signals.contains_key(sig) {
            return Err(SchemaError::CommunityPolicySignalNotInSchema {
                policy: entry.name.clone(),
                signal: sig.to_owned(),
            });
        }
    }
    // Write allow/deny conflict
    for sig in &entry.policy.allowed_write_signals {
        if entry.policy.denied_write_signals.contains(sig) {
            return Err(SchemaError::CommunityPolicySignalConflict {
                policy: entry.name.clone(),
                signal: sig.clone(),
            });
        }
    }
    // Read allow/deny conflict
    for sig in &entry.policy.allowed_read_signals {
        if entry.policy.denied_read_signals.contains(sig) {
            return Err(SchemaError::CommunityPolicySignalConflict {
                policy: entry.name.clone(),
                signal: sig.clone(),
            });
        }
    }
    community_policies.insert(entry.name, entry.policy);
}
```

New `SchemaError` variants:

```rust
InvalidCommunityPolicyName(String),
DuplicateCommunityPolicyName(String),
CommunityPolicySignalNotInSchema { policy: String, signal: String },
CommunityPolicySignalConflict { policy: String, signal: String },
```

---

## Control Flow Diagram

### Write path with community context

```
TidalDb::signal_with_community_policy(signal_type, entity_id, weight, ts, ctx)
  │
  ├─ schema.community_policy(ctx.role)
  │     → None → TidalError::NotFound
  │     → Some(policy)
  │
  ├─ CommunityPolicyEvaluator::check_write(signal_type, policy)
  │     → Err(violation) → TidalError::PolicyViolation
  │     → Ok(())
  │
  └─ self.signal(signal_type, entity_id, weight, ts)   [existing path]
```

### Read path with community context

```
TidalDb::retrieve(Retrieve { community: Some(ctx), ... })
  │
  └─ QueryExecutor::execute(...)
       │
       └─ ProfileExecutor::score_candidates(ctx, candidates)
            │
            ├─ schema.community_policy(ctx.role)
            │     → None → QueryError::NotFound
            │     → Some(policy)
            │
            ├─ build_suppressed_set(policy, schema) → HashSet<SignalTypeId>
            │
            └─ for each candidate:
                 for each signal_type_id:
                   if suppressed.contains(signal_type_id): skip
                   else: apply score contribution
```

---

## File Changes Summary

| File | Change |
|---|---|
| `tidal/src/schema/validation/community_policy.rs` | New: `CommunityPolicy`, `CommunityContext`, `CommunityPolicyEntry` |
| `tidal/src/schema/validation/builders.rs` | Add `community_policies` field + `community_policy()` method + build validation |
| `tidal/src/schema/validation/mod.rs` | Add `community_policies` to `Schema`; re-export `CommunityPolicy`, `CommunityContext` |
| `tidal/src/schema/error.rs` | Add 4 new `SchemaError` variants |
| `tidal/src/schema/mod.rs` | Re-export `CommunityPolicy`, `CommunityContext` |
| `tidal/src/session/policy.rs` | Add 2 new `PolicyViolationKind` variants |
| `tidal/src/db/community.rs` | New: `CommunityPolicyEvaluator::check_write()` |
| `tidal/src/query/retrieve/types.rs` | Add `community: Option<CommunityContext>` to `Retrieve`; `RetrieveBuilder::community()` |
| `tidal/src/ranking/executor/` | Scoring loop: check `suppressed` set per signal contribution |
| `tidal/tests/m10_community_policy.rs` | New: integration test suite (9 scenarios from spec) |

---

## Testing Strategy

Unit tests live in `community_policy.rs` and `builders.rs`:
- `CommunityPolicyEvaluator::check_write` for all 3 outcomes (allow, deny-list, allow-list miss)
- Schema validation rejection for each new error variant

Integration tests in `tidal/tests/m10_community_policy.rs`:
- All 9 scenarios from the spec test matrix
- Specifically: suppressed read signals produce lower ranking scores than unsuppressed, verified by scoring two candidates identically configured except one has a suppressed signal contribution

---

## Risks and Mitigations

| Risk | Mitigation |
|---|---|
| `suppressed_signals` lookup adds latency to scoring hot path | Build set once per query, not per candidate; `HashSet::contains` is O(1) |
| `signal_with_community_policy` API confusion vs `signal` | Clear doc comments; `signal` remains preferred for non-community use cases |
| `CommunityContext.community_id` field adds overhead without being validated | Keep it `String`, used only for `tracing::instrument` span attribute |
| Read suppression silently changes ranking without caller awareness | `QueryStats` should log suppressed signal count (future enhancement; out of scope here) |