prevent overflowing when delta is minus

compressor.go

@@ -14,9 +14,9 @@ const (
 // Link to the paper: https://www.vldb.org/pvldb/vol8/p1816-teller.pdf
 type Compressor struct {
  bw *bitWriter
- header uint32
- t uint32
- tDelta uint32
+ header int32
+ t int32
+ tDelta int32
  leadingZeros uint8
  trailingZeros uint8
  value uint64
@@ -26,7 +26,7 @@ type Compressor struct {
 // at the end of compressing.
 func NewCompressor(w io.Writer, header uint32) (c *Compressor, finish func() error, err error) {
  c = &Compressor{
- header: header,
+ header: int32(header),
  bw: newBitWriter(w),
  leadingZeros: math.MaxUint8,
  }
@@ -40,8 +40,14 @@ func NewCompressor(w io.Writer, header uint32) (c *Compressor, finish func() err
 func (c *Compressor) Compress(t uint32, v float64) error {
  // First time to compress.
  if c.t == 0 {
- delta := t - c.header
- c.t = t
+ if int32(t)-c.header < 0 {
+ // Prevent overflowing of uint64(delta).
+ // TODO: Consider the better way to handle the case that
+ // `t` is smaller than `c.header`.
+ t = uint32(c.header)
+ }
+ delta := int32(t) - c.header
+ c.t = int32(t)
  c.tDelta = delta
  c.value = math.Float64bits(v)
 
@@ -68,11 +74,9 @@ func (c *Compressor) compress(t uint32, v float64) error {
 }
 
 func (c *Compressor) compressTimestamp(t uint32) error {
- // If t < c.t, delta is overflowed because it is uint32.
- // And it causes unexpected EOF during decoding.
- delta := t - c.t
+ delta := int32(t) - c.t
  dod := int64(delta) - int64(c.tDelta) // delta of delta
- c.t = t
+ c.t = int32(t)
  c.tDelta = delta
 
  // | DoD | Header bits | Value bits | Total bits |

gorilla_test.go

@@ -20,11 +20,16 @@ func Test_Compress_Decompress(t *testing.T) {
  header := uint32(time.Now().Unix())
 
  const dataLen = 50000
- valueFuzz := fuzz.New().NilChance(0)
  expected := make([]data, dataLen)
+ valueFuzz := fuzz.New().NilChance(0)
  ts := header
+ rand.Seed(time.Now().UnixNano())
  for i := 0; i < dataLen; i++ {
- ts += uint32(rand.Int31n(10000))
+ if 0 < i && i%10 == 0 {
+ ts -= uint32(rand.Intn(100))
+ } else {
+ ts += uint32(rand.Int31n(100))
+ }
  var v float64
  valueFuzz.Fuzz(&v)
  expected[i] = data{ts, v}