diff --git a/save/bitstorage.go b/save/bitstorage.go
index a87ca75..d6de313 100644
--- a/save/bitstorage.go
+++ b/save/bitstorage.go
@@ -5,6 +5,8 @@ import (
 	"math"
 )
 
+// BitStorage implement the compacted data array used in chunk storage.
+// https://wiki.vg/Chunk_Format
 // This implement the format since Minecraft 1.16
 type BitStorage struct {
 	data []uint64
@@ -14,7 +16,11 @@ type BitStorage struct {
 	valuesPerLong int
 }
 
-// NewBitStorage create a new BitStorage, // TODO: document
+// NewBitStorage create a new BitStorage.
+// bits is the number of bits per value.
+// size is the number of values.
+// arrl is optional data for initializing.
+// It's length must match the bits and size if it's not nil.
 func NewBitStorage(bits, size int, arrl []uint64) (b *BitStorage) {
 	b = &BitStorage{
 		mask:          1<<bits - 1,
@@ -25,7 +31,7 @@ func NewBitStorage(bits, size int, arrl []uint64) (b *BitStorage) {
 	dataLen := (size + b.valuesPerLong - 1) / b.valuesPerLong
 	if arrl != nil {
 		if len(arrl) != dataLen {
-			panic(fmt.Errorf("invalid length given for storage, got: %d but expected: %d", len(arrl), dataLen))
+			panic(initBitStorageErr{ArrlLen: len(arrl), WantLen: dataLen})
 		}
 		b.data = arrl
 	} else {
@@ -34,11 +40,21 @@ func NewBitStorage(bits, size int, arrl []uint64) (b *BitStorage) {
 	return
 }
 
+type initBitStorageErr struct {
+	ArrlLen int
+	WantLen int
+}
+
+func (i initBitStorageErr) Error() string {
+	return fmt.Sprintf("invalid length given for storage, got: %d but expected: %d", i.ArrlLen, i.WantLen)
+}
+
 func (b *BitStorage) cellIndex(n int) int {
 	elemPerLong := 64 / b.bits
 	return n / elemPerLong
 }
 
+// Swap sets v into [i], and return the previous [i] value.
 func (b *BitStorage) Swap(i, v int) (old int) {
 	if i < 0 || i > b.size-1 ||
 		v < 0 || uint64(v) > b.mask {
@@ -52,6 +68,7 @@ func (b *BitStorage) Swap(i, v int) (old int) {
 	return
 }
 
+// Set sets v into [i]
 func (b *BitStorage) Set(i, v int) {
 	if i < 0 || i > b.size-1 ||
 		v < 0 || uint64(v) > b.mask {
@@ -63,6 +80,7 @@ func (b *BitStorage) Set(i, v int) {
 	b.data[c] = l&(b.mask<<offset^math.MaxUint64) | (uint64(v)&b.mask)<<offset
 }
 
+// Get gets [i] value.
 func (b *BitStorage) Get(i int) int {
 	if i < 0 || i > b.size-1 {
 		panic("out of bounds")