Add get_start_shard unittests and update minimal config

1. Add unittests for testing `get_start_shard` with better granularity
2. Change `INITIAL_ACTIVE_SHARDS` from `4` to `2` for tight crosslinking

configs/minimal.yaml

@@ -165,7 +165,7 @@ PHASE_1_FORK_VERSION: 0x01000001
 # [customized] for testing
 PHASE_1_GENESIS_SLOT: 8
 # [customized] reduced for testing
-INITIAL_ACTIVE_SHARDS: 4
+INITIAL_ACTIVE_SHARDS: 2
 
 
 # Phase 1: General

tests/core/pyspec/eth2spec/test/phase_1/unittests/test_get_start_shard.py

@@ -0,0 +1,71 @@
+from eth2spec.test.context import (
+    PHASE0,
+    with_all_phases_except,
+    spec_state_test,
+)
+from eth2spec.test.helpers.state import next_epoch
+
+
+@with_all_phases_except([PHASE0])
+@spec_state_test
+def test_get_committee_count_delta(spec, state):
+    assert spec.get_committee_count_delta(state, 0, 0) == 0
+    assert spec.get_committee_count_at_slot(state, 0) != 0
+    assert spec.get_committee_count_delta(state, 0, 1) == spec.get_committee_count_at_slot(state, 0)
+    assert spec.get_committee_count_delta(state, 1, 2) == spec.get_committee_count_at_slot(state, 1)
+    assert spec.get_committee_count_delta(state, 0, 2) == (
+        spec.get_committee_count_at_slot(state, 0) + spec.get_committee_count_at_slot(state, 1)
+    )
+
+
+@with_all_phases_except([PHASE0])
+@spec_state_test
+def test_get_start_shard_current_epoch_start(spec, state):
+    assert state.current_epoch_start_shard == 0
+    next_epoch(spec, state)
+    active_shard_count = spec.get_active_shard_count(state)
+    assert state.current_epoch_start_shard == (
+        spec.get_committee_count_delta(state, 0, spec.SLOTS_PER_EPOCH) % active_shard_count
+    )
+    current_epoch_start_slot = spec.compute_start_slot_at_epoch(spec.get_current_epoch(state))
+
+    slot = current_epoch_start_slot
+    start_shard = spec.get_start_shard(state, slot)
+    assert start_shard == state.current_epoch_start_shard
+
+
+@with_all_phases_except([PHASE0])
+@spec_state_test
+def test_get_start_shard_next_slot(spec, state):
+    next_epoch(spec, state)
+    active_shard_count = spec.get_active_shard_count(state)
+    current_epoch_start_slot = spec.compute_start_slot_at_epoch(spec.get_current_epoch(state))
+
+    slot = current_epoch_start_slot + 1
+    start_shard = spec.get_start_shard(state, slot)
+
+    current_epoch_start_slot = spec.compute_start_slot_at_epoch(spec.get_current_epoch(state))
+    expected_start_shard = (
+        state.current_epoch_start_shard +
+        spec.get_committee_count_delta(state, start_slot=current_epoch_start_slot, stop_slot=slot)
+    ) % active_shard_count
+    assert start_shard == expected_start_shard
+
+
+@with_all_phases_except([PHASE0])
+@spec_state_test
+def test_get_start_shard_previous_slot(spec, state):
+    next_epoch(spec, state)
+    active_shard_count = spec.get_active_shard_count(state)
+    current_epoch_start_slot = spec.compute_start_slot_at_epoch(spec.get_current_epoch(state))
+
+    slot = current_epoch_start_slot - 1
+    start_shard = spec.get_start_shard(state, slot)
+
+    current_epoch_start_slot = spec.compute_start_slot_at_epoch(spec.get_current_epoch(state))
+    expected_start_shard = (
+        state.current_epoch_start_shard
+        + spec.MAX_COMMITTEES_PER_SLOT * spec.SLOTS_PER_EPOCH * active_shard_count
+        - spec.get_committee_count_delta(state, start_slot=slot, stop_slot=current_epoch_start_slot)
+    ) % active_shard_count
+    assert start_shard == expected_start_shard