Custom bitfield type for custom serialization/deserialization

Without touching the generated code. This is a slightly breaking change,
but it shouldn't affect anyone.

Makefile

@@ -8,6 +8,10 @@ test:
 	go test ./...
 .PHONY: test
 
+gen:
+	go run ./gen
+.PHONY: gen
+
 coverage:
 	go test -coverprofile=coverage.out ./...
 	go tool cover -html=coverage.out

bitfield.go

@@ -0,0 +1,26 @@
+package hamt
+
+import (
+	"io"
+	"math/big"
+
+	cbg "github.com/whyrusleeping/cbor-gen"
+)
+
+type Bitfield struct {
+	big.Int
+}
+
+func (b *Bitfield) MarshalCBOR(w io.Writer) error {
+	return cbg.WriteByteArray(w, b.Bytes())
+}
+
+func (b *Bitfield) UnmarshalCBOR(r io.Reader) error {
+	bytes, err := cbg.ReadByteArray(r, cbg.ByteArrayMaxLen)
+	if err != nil {
+		return err
+	}
+	b.SetBytes(bytes)
+
+	return nil
+}

hamt.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"context"
 	"fmt"
-	"math/big"
 	"sort"
 
 	cid "github.com/ipfs/go-cid"
@@ -81,7 +80,7 @@ type HashFunc func([]byte) []byte
 //		pointers [Pointer]
 //	} representation tuple
 type Node struct {
-	Bitfield *big.Int
+	Bitfield Bitfield
 	Pointers []*Pointer
 
 	bitWidth int
@@ -237,7 +236,6 @@ func (n *Node) Delete(ctx context.Context, k string) (bool, error) {
 // Constructs a new node value.
 func newNode(cs cbor.IpldStore, hashFn HashFunc, bitWidth int) *Node {
 	nd := &Node{
-		Bitfield: big.NewInt(0),
 		Pointers: make([]*Pointer, 0),
 		bitWidth: bitWidth,
 		hash:     hashFn,
@@ -776,7 +774,7 @@ func (n *Node) modifyValue(ctx context.Context, hv *hashBits, k []byte, v *cbg.D
 // bucket containing the single key/value pair at that position.
 func (n *Node) insertKV(idx int, k []byte, v *cbg.Deferred) error {
 	i := n.indexForBitPos(idx)
-	n.Bitfield.SetBit(n.Bitfield, idx, 1)
+	n.Bitfield.SetBit(&n.Bitfield.Int, idx, 1)
 
 	p := &Pointer{KVs: []*KV{{Key: k, Value: v}}}
 
@@ -798,7 +796,7 @@ func (n *Node) setPointer(i byte, p *Pointer) error {
 func (n *Node) rmPointer(i byte, idx int) error {
 	copy(n.Pointers[i:], n.Pointers[i+1:])
 	n.Pointers = n.Pointers[:len(n.Pointers)-1]
-	n.Bitfield.SetBit(n.Bitfield, idx, 0)
+	n.Bitfield.SetBit(&n.Bitfield.Int, idx, 0)
 
 	return nil
 }
@@ -824,7 +822,7 @@ func (n *Node) getPointer(i byte) *Pointer {
 func (n *Node) Copy() *Node {
 	// TODO(rvagg): clarify what situations this method is actually useful for.
 	nn := newNode(n.store, n.hash, n.bitWidth)
-	nn.Bitfield.Set(n.Bitfield)
+	nn.Bitfield.Set(&n.Bitfield.Int)
 	nn.Pointers = make([]*Pointer, len(n.Pointers))
 
 	for i, p := range n.Pointers {

uhamt.go

@@ -13,7 +13,7 @@ import (
 // associated array. Indexes `[1]` and `[2]` are not present, but index `[3]`
 // is at the second position of our Pointers array.
 func (n *Node) indexForBitPos(bp int) int {
-	return indexForBitPos(bp, n.Bitfield)
+	return indexForBitPos(bp, &n.Bitfield.Int)
 }
 
 func indexForBitPos(bp int, bitfield *big.Int) int {