add basic get_head tests

specs/core/0_fork-choice.md

@@ -82,7 +82,7 @@ def get_genesis_store(genesis_state: BeaconState) -> Store:
         justified_checkpoint=justified_checkpoint,
         finalized_checkpoint=finalized_checkpoint,
         blocks={root: genesis_block},
-        block_states={root: genesis_state},
+        block_states={root: genesis_state.copy()},
         checkpoint_states={justified_checkpoint: genesis_state.copy()},
     )
 ```
@@ -101,10 +101,11 @@ def get_ancestor(store: Store, root: Hash, slot: Slot) -> Hash:
 ```python
 def get_latest_attesting_balance(store: Store, root: Hash) -> Gwei:
     state = store.checkpoint_states[store.justified_checkpoint]
-    active_indices = get_active_validator_indices(state.validator_registry, get_current_epoch(state))
+    active_indices = get_active_validator_indices(state, get_current_epoch(state))
     return Gwei(sum(
-        state.validator_registry[i].effective_balance for i in active_indices
-        if get_ancestor(store, store.latest_targets[i].root, store.blocks[root].slot) == root
+        state.validators[i].effective_balance for i in active_indices
+        if (i in store.latest_targets and
+            get_ancestor(store, store.latest_targets[i].root, store.blocks[root].slot) == root)
     ))
 ```
 

test_libs/pyspec/eth2spec/test/fork_choice/test_get_head.py

@@ -0,0 +1,102 @@
+from eth2spec.test.context import with_all_phases, with_state, bls_switch
+from eth2spec.test.helpers.block import build_empty_block_for_next_slot
+from eth2spec.test.helpers.state import state_transition_and_sign_block
+
+
+def add_block_to_store(spec, store, block):
+    pre_state = store.block_states[block.parent_root]
+    block_time = pre_state.genesis_time + block.slot * spec.SECONDS_PER_SLOT
+
+    if store.time < block_time:
+        spec.on_tick(store, block_time)
+
+    spec.on_block(store, block)
+
+
+@with_all_phases
+@with_state
+@bls_switch
+def test_genesis(spec, state):
+    # Initialization
+    store = spec.get_genesis_store(state)
+    genesis_block = spec.BeaconBlock(state_root=state.hash_tree_root())
+    assert spec.get_head(store) == spec.signing_root(genesis_block)
+
+
+@with_all_phases
+@with_state
+@bls_switch
+def test_chain_no_attestations(spec, state):
+    # Initialization
+    store = spec.get_genesis_store(state)
+    genesis_block = spec.BeaconBlock(state_root=state.hash_tree_root())
+    assert spec.get_head(store) == spec.signing_root(genesis_block)
+
+    # On receiving a block of `GENESIS_SLOT + 1` slot
+    block_1 = build_empty_block_for_next_slot(spec, state)
+    state_transition_and_sign_block(spec, state, block_1)
+    add_block_to_store(spec, store, block_1)
+
+    # On receiving a block of next epoch
+    block_2 = build_empty_block_for_next_slot(spec, state)
+    state_transition_and_sign_block(spec, state, block_2)
+    add_block_to_store(spec, store, block_2)
+
+    assert spec.get_head(store) == spec.signing_root(block_2)
+
+
+@with_all_phases
+@with_state
+@bls_switch
+def test_split_tie_breaker_no_attestations(spec, state):
+    genesis_state = state.copy()
+
+    # Initialization
+    store = spec.get_genesis_store(state)
+    genesis_block = spec.BeaconBlock(state_root=state.hash_tree_root())
+    assert spec.get_head(store) == spec.signing_root(genesis_block)
+
+    # block at slot 1
+    block_1_state = genesis_state.copy()
+    block_1 = build_empty_block_for_next_slot(spec, block_1_state)
+    state_transition_and_sign_block(spec, block_1_state, block_1)
+    add_block_to_store(spec, store, block_1)
+
+    # additional block at slot 1
+    block_2_state = genesis_state.copy()
+    block_2 = build_empty_block_for_next_slot(spec, block_2_state)
+    block_2.body.graffiti = b'\x42' * 32
+    state_transition_and_sign_block(spec, block_2_state, block_2)
+    add_block_to_store(spec, store, block_2)
+
+    highest_root = max(spec.signing_root(block_1), spec.signing_root(block_2))
+
+    assert spec.get_head(store) == highest_root
+
+
+@with_all_phases
+@with_state
+@bls_switch
+def test_longer_branch_no_attestations(spec, state):
+    genesis_state = state.copy()
+
+    # Initialization
+    store = spec.get_genesis_store(state)
+    genesis_block = spec.BeaconBlock(state_root=state.hash_tree_root())
+    assert spec.get_head(store) == spec.signing_root(genesis_block)
+
+    # build longer tree
+    long_state = genesis_state.copy()
+    for i in range(3):
+        long_block = build_empty_block_for_next_slot(spec, long_state)
+        state_transition_and_sign_block(spec, long_state, long_block)
+        add_block_to_store(spec, store, long_block)
+
+    # build short tree
+    short_state = genesis_state.copy()
+    short_block = build_empty_block_for_next_slot(spec, short_state)
+    short_block.body.graffiti = b'\x42' * 32
+    state_transition_and_sign_block(spec, short_state, short_block)
+    add_block_to_store(spec, store, short_block)
+
+    assert spec.get_head(store) == spec.signing_root(long_block)