From b19e24e9e0808f9801e5e81a74a34a016172992c Mon Sep 17 00:00:00 2001
From: terence tsao <terence@prysmaticlabs.com>
Date: Wed, 17 Oct 2018 16:00:23 -0700
Subject: [PATCH] Implemented Update Ancestor Hashes Function (#660)

---
 beacon-chain/types/block.go      | 12 ++++++++++++
 beacon-chain/types/block_test.go | 27 +++++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/beacon-chain/types/block.go b/beacon-chain/types/block.go
index 90fa7b8c5..b3ab12c7a 100644
--- a/beacon-chain/types/block.go
+++ b/beacon-chain/types/block.go
@@ -210,6 +210,18 @@ func (b *Block) IsValid(
 	return true
 }
 
+// UpdateAncestorHashes updates the skip list of ancestor block hashes.
+// i'th item is 2**i'th ancestor for i = 0, ..., 31.
+func UpdateAncestorHashes(parentAncestorHashes [][32]byte, parentSlotNum uint64, parentHash [32]byte) [][32]byte {
+	newAncestorHashes := parentAncestorHashes
+	for i := range parentAncestorHashes {
+		if (parentSlotNum % (1 << uint64(i))) == 0 {
+			newAncestorHashes[i] = parentHash
+		}
+	}
+	return newAncestorHashes
+}
+
 // isAttestationValid validates an attestation in a block.
 // Attestations are cross-checked against validators in CrystallizedState.ShardAndCommitteesForSlots.
 // In addition, the signature is verified by constructing the list of parent hashes using ActiveState.RecentBlockHashes.
diff --git a/beacon-chain/types/block_test.go b/beacon-chain/types/block_test.go
index 946206466..9cb5d84a0 100644
--- a/beacon-chain/types/block_test.go
+++ b/beacon-chain/types/block_test.go
@@ -161,3 +161,30 @@ func TestIsAttestationSlotNumberValid(t *testing.T) {
 		t.Errorf("attestation slot number could be less than or equal to parent block's slot number")
 	}
 }
+
+func TestUpdateAncestorHashes(t *testing.T) {
+	parentHashes := make([][32]byte, 32)
+	for i := 0; i < 32; i++ {
+		parentHashes[i] = hashutil.Hash([]byte{byte(i)})
+	}
+
+	tests := []struct {
+		a uint64
+		b [32]byte
+		c int
+	}{
+		{a: 1, b: [32]byte{'a'}, c: 0},
+		{a: 2, b: [32]byte{'b'}, c: 1},
+		{a: 4, b: [32]byte{'c'}, c: 2},
+		{a: 8, b: [32]byte{'d'}, c: 3},
+		{a: 16, b: [32]byte{'e'}, c: 4},
+		{a: 1 << 29, b: [32]byte{'f'}, c: 29},
+		{a: 1 << 30, b: [32]byte{'g'}, c: 30},
+		{a: 1 << 31, b: [32]byte{'h'}, c: 31},
+	}
+	for _, tt := range tests {
+		if UpdateAncestorHashes(parentHashes, tt.a, tt.b)[tt.c] != tt.b {
+			t.Errorf("Failed to update ancestor hash at index %d. Wanted: %v, got: %v", tt.c, tt.b, UpdateAncestorHashes(parentHashes, tt.a, tt.b)[tt.c])
+		}
+	}
+}