blockchain: better Ancestor with skiplists

On startup, Ancestor call was taking a lot of time when the node was
loading the blockindex onto memory. This change speeds up the Ancestor
function significantly and speeds up the node during startup.

On testnet3 at blockheight ~2,500,000, the startup was around 30seconds
on current main and was 5 seconds with this change. Below is a benchstat
result showing the significant speedup.

goos: darwin
goarch: arm64
pkg: github.com/utreexo/utreexod/blockchain
           │     old.txt      │               new.txt                │
           │      sec/op      │    sec/op     vs base                │
Ancestor-8   120819.301µ ± 5%   7.013µ ± 19%  -99.99% (p=0.000 n=10)

           │  old.txt   │            new.txt             │
           │    B/op    │    B/op     vs base            │
Ancestor-8   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

           │  old.txt   │            new.txt             │
           │ allocs/op  │ allocs/op   vs base            │
Ancestor-8   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

blockchain/bench_test.go

@@ -60,3 +60,16 @@ func BenchmarkUtxoFetchSlices(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkAncestor(b *testing.B) {
+	height := 1 << 19
+	blockNodes := chainedNodes(nil, height)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		blockNodes[len(blockNodes)-1].Ancestor(0)
+		for j := 0; j <= 19; j++ {
+			blockNodes[len(blockNodes)-1].Ancestor(1 << j)
+		}
+	}
+}

blockchain/blockindex.go

@@ -74,6 +74,9 @@ type blockNode struct {
 	// parent is the parent block for this node.
 	parent *blockNode
 
+	// ancestor is a block that is more than one block back from this node.
+	ancestor *blockNode
+
 	// hash is the double sha 256 of the block.
 	hash chainhash.Hash
 
@@ -119,6 +122,7 @@ func initBlockNode(node *blockNode, blockHeader *wire.BlockHeader, parent *block
 		node.parent = parent
 		node.height = parent.height + 1
 		node.workSum = node.workSum.Add(parent.workSum, node.workSum)
+		node.buildAncestor()
 	}
 }
 
@@ -150,6 +154,26 @@ func (node *blockNode) Header() wire.BlockHeader {
 	}
 }
 
+// invertLowestOne turns the lowest 1 bit in the binary representation of a number into a 0.
+func invertLowestOne(n int32) int32 {
+	return n & (n - 1)
+}
+
+// getAncestorHeight returns a suitable ancestor for the node at the given height.
+func getAncestorHeight(height int32) int32 {
+	// We pop off two 1 bits of the height.
+	// This results in a maximum of 330 steps to go back to an ancestor
+	// from height 1<<29.
+	return invertLowestOne(invertLowestOne(height))
+}
+
+// buildAncestor sets an ancestor for the given blocknode.
+func (node *blockNode) buildAncestor() {
+	if node.parent != nil {
+		node.ancestor = node.parent.Ancestor(getAncestorHeight(node.height))
+	}
+}
+
 // Ancestor returns the ancestor block node at the provided height by following
 // the chain backwards from this node.  The returned block will be nil when a
 // height is requested that is after the height of the passed node or is less
@@ -161,9 +185,22 @@ func (node *blockNode) Ancestor(height int32) *blockNode {
 		return nil
 	}
 
+	// Traverse back until we find the desired node.
 	n := node
-	for ; n != nil && n.height != height; n = n.parent {
+	for n != nil && n.height != height {
-		// Intentionally left blank
+		// If there's an ancestor available, use it. Otherwise, just
+		// follow the parent.
+		if n.ancestor != nil {
+			// Calculate the height for this ancestor and
+			// check if we can take the ancestor skip.
+			if getAncestorHeight(n.height) >= height {
+				n = n.ancestor
+				continue
+			}
+		}
+
+		// We couldn't take the ancestor skip so traverse back to the parent.
+		n = n.parent
 	}
 
 	return n

blockchain/blockindex_test.go

@@ -0,0 +1,42 @@
+// Copyright (c) 2023 The utreexo developers
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+package blockchain
+
+import (
+	"math/rand"
+	"testing"
+)
+
+func TestAncestor(t *testing.T) {
+	height := 500_000
+	blockNodes := chainedNodes(nil, height)
+
+	for i, blockNode := range blockNodes {
+		// Grab a random node that's a child of this node
+		// and try to fetch the current blockNode with Ancestor.
+		randNode := blockNodes[rand.Intn(height-i)+i]
+		got := randNode.Ancestor(blockNode.height)
+
+		// See if we got the right one.
+		if got.hash != blockNode.hash {
+			t.Fatalf("expected ancestor at height %d "+
+				"but got a node at height %d",
+				blockNode.height, got.height)
+		}
+
+		// Gensis doesn't have ancestors so skip the check below.
+		if blockNode.height == 0 {
+			continue
+		}
+
+		// The ancestors are deterministic so check that this node's
+		// ancestor is the correct one.
+		if blockNode.ancestor.height != getAncestorHeight(blockNode.height) {
+			t.Fatalf("expected anestor at height %d, but it was at %d",
+				getAncestorHeight(blockNode.height),
+				blockNode.ancestor.height)
+		}
+	}
+}