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// Copyright 2018 The gVisor Authors.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// Package seqnum defines the types and methods for TCP sequence numbers such
// that they fit in 32-bit words and work properly when overflows occur.
package seqnum
// Value represents the value of a sequence number.
type Value uint32
// Size represents the size (length) of a sequence number window.
type Size uint32
// LessThan checks if v is before w, i.e., v < w.
func (v Value) LessThan(w Value) bool {
return int32(v-w) < 0
// LessThanEq returns true if v==w or v is before i.e., v < w.
func (v Value) LessThanEq(w Value) bool {
if v == w {
return true
return v.LessThan(w)
// InRange checks if v is in the range [a,b), i.e., a <= v < b.
func (v Value) InRange(a, b Value) bool {
return v-a < b-a
// InWindow checks if v is in the window that starts at 'first' and spans 'size'
// sequence numbers.
func (v Value) InWindow(first Value, size Size) bool {
return v.InRange(first, first.Add(size))
// Add calculates the sequence number following the [v, v+s) window.
func (v Value) Add(s Size) Value {
return v + Value(s)
// Size calculates the size of the window defined by [v, w).
func (v Value) Size(w Value) Size {
return Size(w - v)
// UpdateForward updates v such that it becomes v + s.
func (v *Value) UpdateForward(s Size) {
*v += Value(s)