Complete attestation_validation, bar tests

beacon_chain/attestation_validation/src/justified_slot.rs

@@ -0,0 +1,38 @@
+use super::types::{AttestationData, BeaconState};
+use super::{Error, Invalid, Outcome};
+
+/// Verify that an attestation's `data.justified_slot` matches the justified slot known to the
+/// `state`.
+///
+/// In the case that an attestation references a slot _before_ the latest state transition, is
+/// acceptable for the attestation to reference the previous known `justified_slot`. If this were
+/// not the case, all attestations created _prior_ to the last state recalculation would be rejected
+/// if a block was justified in that state recalculation. It is both ideal and likely that blocks
+/// will be justified during a state recalcuation.
+pub fn validate_attestation_justified_slot(
+    data: &AttestationData,
+    state: &BeaconState,
+) -> Result<Outcome, Error> {
+    let permissable_justified_slot = if data.slot >= state.latest_state_recalculation_slot {
+        state.justified_slot
+    } else {
+        state.previous_justified_slot
+    };
+    verify_or!(
+        data.justified_slot == permissable_justified_slot,
+        reject!(Invalid::JustifiedSlotImpermissable)
+    );
+    accept!()
+}
+
+#[cfg(test)]
+mod tests {
+    /*
+     * TODO: Implement tests.
+     *
+     * These tests will require the `BeaconBlock` and `BeaconBlockBody` updates, which are not
+     * yet included in the code base. Adding tests now will result in duplicated work.
+     *
+     * https://github.com/sigp/lighthouse/issues/97
+     */
+}