mpt: implement (*Trie).Collapse()

Because trie size is rather big, it can't be stored in memory.
Thus some form of caching should also be implemented. To avoid
marshaling/unmarshaling of items which are close to root and are used
very frequenly we can save them across the persists.
This commit implements pruning items at the specified depth,
replacing them by hash nodes.

pkg/core/mpt/trie.go

@@ -355,3 +355,35 @@ func (t *Trie) getFromStore(h util.Uint256) (Node, error) {
 	}
 	return n.Node, nil
 }
+
+// Collapse compresses all nodes at depth n to the hash nodes.
+// Note: this function does not perform any kind of storage flushing so
+// `Flush()` should be called explicitly before invoking function.
+func (t *Trie) Collapse(depth int) {
+	if depth < 0 {
+		panic("negative depth")
+	}
+	t.root = collapse(depth, t.root)
+}
+
+func collapse(depth int, node Node) Node {
+	if _, ok := node.(*HashNode); ok {
+		return node
+	} else if depth == 0 {
+		return NewHashNode(node.Hash())
+	}
+
+	switch n := node.(type) {
+	case *BranchNode:
+		for i := range n.Children {
+			n.Children[i] = collapse(depth-1, n.Children[i])
+		}
+	case *ExtensionNode:
+		n.next = collapse(depth-1, n.next)
+	case *LeafNode:
+	case *HashNode:
+	default:
+		panic("invalid MPT node type")
+	}
+	return node
+}

pkg/core/mpt/trie_test.go

@@ -371,3 +371,76 @@ func TestTrie_PanicInvalidRoot(t *testing.T) {
 	require.Panics(t, func() { _, _ = tr.Get([]byte{1}) })
 	require.Panics(t, func() { _ = tr.Delete([]byte{1}) })
 }
+
+func TestTrie_Collapse(t *testing.T) {
+	t.Run("PanicNegative", func(t *testing.T) {
+		tr := newTestTrie(t)
+		require.Panics(t, func() { tr.Collapse(-1) })
+	})
+	t.Run("Depth=0", func(t *testing.T) {
+		tr := newTestTrie(t)
+		h := tr.root.Hash()
+
+		_, ok := tr.root.(*HashNode)
+		require.False(t, ok)
+
+		tr.Collapse(0)
+		_, ok = tr.root.(*HashNode)
+		require.True(t, ok)
+		require.Equal(t, h, tr.root.Hash())
+	})
+	t.Run("Branch,Depth=1", func(t *testing.T) {
+		b := NewBranchNode()
+		e := NewExtensionNode([]byte{0x01}, NewLeafNode([]byte("value1")))
+		he := e.Hash()
+		b.Children[0] = e
+		hb := b.Hash()
+
+		tr := NewTrie(b, newTestStore())
+		tr.Collapse(1)
+
+		newb, ok := tr.root.(*BranchNode)
+		require.True(t, ok)
+		require.Equal(t, hb, newb.Hash())
+		require.IsType(t, (*HashNode)(nil), b.Children[0])
+		require.Equal(t, he, b.Children[0].Hash())
+	})
+	t.Run("Extension,Depth=1", func(t *testing.T) {
+		l := NewLeafNode([]byte("value"))
+		hl := l.Hash()
+		e := NewExtensionNode([]byte{0x01}, l)
+		h := e.Hash()
+		tr := NewTrie(e, newTestStore())
+		tr.Collapse(1)
+
+		newe, ok := tr.root.(*ExtensionNode)
+		require.True(t, ok)
+		require.Equal(t, h, newe.Hash())
+		require.IsType(t, (*HashNode)(nil), newe.next)
+		require.Equal(t, hl, newe.next.Hash())
+	})
+	t.Run("Leaf", func(t *testing.T) {
+		l := NewLeafNode([]byte("value"))
+		tr := NewTrie(l, newTestStore())
+		tr.Collapse(10)
+		require.Equal(t, NewLeafNode([]byte("value")), tr.root)
+	})
+	t.Run("Hash", func(t *testing.T) {
+		t.Run("Empty", func(t *testing.T) {
+			tr := NewTrie(new(HashNode), newTestStore())
+			require.NotPanics(t, func() { tr.Collapse(1) })
+			hn, ok := tr.root.(*HashNode)
+			require.True(t, ok)
+			require.True(t, hn.IsEmpty())
+		})
+
+		h := random.Uint256()
+		hn := NewHashNode(h)
+		tr := NewTrie(hn, newTestStore())
+		tr.Collapse(10)
+
+		newRoot, ok := tr.root.(*HashNode)
+		require.True(t, ok)
+		require.Equal(t, NewHashNode(h), newRoot)
+	})
+}