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// Package sequencenumber provides a sequence number unwrapper
package sequencenumber
const (
maxSequenceNumberPlusOne = int64(65536)
breakpoint = 32768 // half of max uint16
)
// Unwrapper stores an unwrapped sequence number
type Unwrapper struct {
init bool
lastUnwrapped int64
}
func isNewer(value, previous uint16) bool {
if value-previous == breakpoint {
return value > previous
}
return value != previous && (value-previous) < breakpoint
}
// Unwrap unwraps the next sequencenumber
func (u *Unwrapper) Unwrap(i uint16) int64 {
if !u.init {
u.init = true
u.lastUnwrapped = int64(i)
return u.lastUnwrapped
}
lastWrapped := uint16(u.lastUnwrapped)
delta := int64(i - lastWrapped)
if isNewer(i, lastWrapped) {
if delta < 0 {
delta += maxSequenceNumberPlusOne
}
} else if delta > 0 && u.lastUnwrapped+delta-maxSequenceNumberPlusOne >= 0 {
delta -= maxSequenceNumberPlusOne
}
u.lastUnwrapped += delta
return u.lastUnwrapped
}
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