implement method to get first index in amt

amt.go

@@ -295,6 +295,49 @@ func (n *Node) forEachAt(ctx context.Context, bs cbor.IpldStore, height int, sta
 
 }
 
+func (r *Root) FirstSetIndex(ctx context.Context) (uint64, error) {
+	return r.Node.firstSetIndex(ctx, r.store, int(r.Height))
+}
+
+var errNoVals = fmt.Errorf("no values")
+
+func (n *Node) firstSetIndex(ctx context.Context, bs cbor.IpldStore, height int) (uint64, error) {
+	if height == 0 {
+		n.expandValues()
+		for i, v := range n.expVals {
+			if v != nil {
+				return uint64(i), nil
+			}
+		}
+		// Would be really weird if we ever actually hit this
+		return 0, errNoVals
+	}
+
+	if n.cache == nil {
+		n.expandLinks()
+	}
+
+	for i := 0; i < width; i++ {
+		ok, _ := n.getBit(uint64(i))
+		if ok {
+			subn, err := n.loadNode(ctx, bs, uint64(i), false)
+			if err != nil {
+				return 0, err
+			}
+
+			ix, err := subn.firstSetIndex(ctx, bs, height-1)
+			if err != nil {
+				return 0, err
+			}
+
+			subCount := nodesForHeight(width, height)
+			return ix + (uint64(i) * subCount), nil
+		}
+	}
+
+	return 0, errNoVals
+}
+
 func (n *Node) expandValues() {
 	if len(n.expVals) == 0 {
 		n.expVals = make([]*cbg.Deferred, width)

amt_test.go

@@ -651,3 +651,45 @@ func TestForEachAt(t *testing.T) {
 		}
 	}
 }
+
+func TestFirstSetIndex(t *testing.T) {
+	bs := cbor.NewCborStore(newMockBlocks())
+	ctx := context.Background()
+
+	vals := []uint64{0, 1, 5, width, width + 1, 276, 1234, 62881923}
+	for _, v := range vals {
+		a := NewAMT(bs)
+		if err := a.Set(ctx, v, fmt.Sprint(v)); err != nil {
+			t.Fatal(err)
+		}
+
+		fsi, err := a.FirstSetIndex(ctx)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if fsi != v {
+			t.Fatal("got wrong index out", fsi, v)
+		}
+
+		rc, err := a.Flush(ctx)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		after, err := LoadAMT(ctx, bs, rc)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		fsi, err = after.FirstSetIndex(ctx)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if fsi != v {
+			t.Fatal("got wrong index out after serialization")
+		}
+	}
+
+}