diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index ee37d9217..b75f14153 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -647,7 +647,7 @@ def get_previous_epoch(state: BeaconState) -> Epoch:
     Return the current epoch if it's genesis epoch.
     """
     current_epoch = get_current_epoch(state)
-    return (current_epoch - 1) if current_epoch > GENESIS_EPOCH else current_epoch
+    return GENESIS_EPOCH if current_epoch == GENESIS_EPOCH else current_epoch - 1
 ```
 
 ### `get_current_epoch`
@@ -887,7 +887,7 @@ def get_shuffled_index(index: ValidatorIndex, index_count: int, seed: Bytes32) -
     # See the 'generalized domain' algorithm on page 3
     for round in range(SHUFFLE_ROUND_COUNT):
         pivot = bytes_to_int(hash(seed + int_to_bytes(round, length=1))[0:8]) % index_count
-        flip = (pivot - index) % index_count
+        flip = (pivot + index_count - index) % index_count
         position = max(index, flip)
         source = hash(seed + int_to_bytes(round, length=1) + int_to_bytes(position // 256, length=4))
         byte = source[(position % 256) // 8]
@@ -1362,19 +1362,19 @@ def process_justification_and_finalization(state: BeaconState) -> None:
     # Process finalizations
     bitfield = state.justification_bitfield
     # The 2nd/3rd/4th most recent epochs are justified, the 2nd using the 4th as source
-    if (bitfield >> 1) % 8 == 0b111 and old_previous_justified_epoch == current_epoch - 3:
+    if (bitfield >> 1) % 8 == 0b111 and old_previous_justified_epoch + 3 == current_epoch:
         state.finalized_epoch = old_previous_justified_epoch
         state.finalized_root = get_block_root(state, state.finalized_epoch)
     # The 2nd/3rd most recent epochs are justified, the 2nd using the 3rd as source
-    if (bitfield >> 1) % 4 == 0b11 and old_previous_justified_epoch == current_epoch - 2:
+    if (bitfield >> 1) % 4 == 0b11 and old_previous_justified_epoch + 2 == current_epoch:
         state.finalized_epoch = old_previous_justified_epoch
         state.finalized_root = get_block_root(state, state.finalized_epoch)
     # The 1st/2nd/3rd most recent epochs are justified, the 1st using the 3rd as source
-    if (bitfield >> 0) % 8 == 0b111 and old_current_justified_epoch == current_epoch - 2:
+    if (bitfield >> 0) % 8 == 0b111 and old_current_justified_epoch + 2 == current_epoch:
         state.finalized_epoch = old_current_justified_epoch
         state.finalized_root = get_block_root(state, state.finalized_epoch)
     # The 1st/2nd most recent epochs are justified, the 1st using the 2nd as source
-    if (bitfield >> 0) % 4 == 0b11 and old_current_justified_epoch == current_epoch - 1:
+    if (bitfield >> 0) % 4 == 0b11 and old_current_justified_epoch + 1 == current_epoch:
         state.finalized_epoch = old_current_justified_epoch
         state.finalized_root = get_block_root(state, state.finalized_epoch)
 ```