Update per-slot processing to v0.4.0

eth2/state_processing/src/per_slot_processing.rs

@@ -0,0 +1,58 @@
+use crate::*;
+use types::{BeaconState, BeaconStateError, ChainSpec, Hash256};
+
+#[derive(Debug, PartialEq)]
+pub enum Error {
+    BeaconStateError(BeaconStateError),
+    EpochProcessingError(EpochProcessingError),
+}
+
+/// Advances a state forward by one slot, performing per-epoch processing if required.
+///
+/// Spec v0.4.0
+pub fn per_slot_processing(
+    state: &mut BeaconState,
+    previous_block_root: Hash256,
+    spec: &ChainSpec,
+) -> Result<(), Error> {
+    if (state.slot + 1) % spec.slots_per_epoch == 0 {
+        per_epoch_processing(state, spec)?;
+        state.advance_caches();
+    }
+
+    state.slot += 1;
+
+    update_block_roots(state, previous_block_root, spec);
+
+    Ok(())
+}
+
+/// Updates the state's block roots as per-slot processing is performed.
+///
+/// Spec v0.4.0
+pub fn update_block_roots(state: &mut BeaconState, previous_block_root: Hash256, spec: &ChainSpec) {
+    state.latest_block_roots[(state.slot.as_usize() - 1) % spec.latest_block_roots_length] =
+        previous_block_root;
+
+    if state.slot.as_usize() % spec.latest_block_roots_length == 0 {
+        let root = merkle_root(&state.latest_block_roots[..]);
+        state.batched_block_roots.push(root);
+    }
+}
+
+fn merkle_root(_input: &[Hash256]) -> Hash256 {
+    // TODO: implement correctly.
+    Hash256::zero()
+}
+
+impl From<BeaconStateError> for Error {
+    fn from(e: BeaconStateError) -> Error {
+        Error::BeaconStateError(e)
+    }
+}
+
+impl From<EpochProcessingError> for Error {
+    fn from(e: EpochProcessingError) -> Error {
+        Error::EpochProcessingError(e)
+    }
+}