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package lamport
import (
"errors"
"fmt"
"io/ioutil"
"os"
"github.com/go-git/go-billy/v5"
"github.com/go-git/go-billy/v5/util"
)
var ErrClockNotExist = errors.New("clock doesn't exist")
type PersistedClock struct {
*MemClock
root billy.Filesystem
filePath string
}
// NewPersistedClock create a new persisted Lamport clock
func NewPersistedClock(root billy.Filesystem, filePath string) (*PersistedClock, error) {
clock := &PersistedClock{
MemClock: NewMemClock(),
root: root,
filePath: filePath,
}
err := clock.Write()
if err != nil {
return nil, err
}
return clock, nil
}
// LoadPersistedClock load a persisted Lamport clock from a file
func LoadPersistedClock(root billy.Filesystem, filePath string) (*PersistedClock, error) {
clock := &PersistedClock{
root: root,
filePath: filePath,
}
err := clock.read()
if err != nil {
return nil, err
}
return clock, nil
}
// Increment is used to return the value of the lamport clock and increment it afterwards
func (pc *PersistedClock) Increment() (Time, error) {
time, err := pc.MemClock.Increment()
if err != nil {
return 0, err
}
return time, pc.Write()
}
// Witness is called to update our local clock if necessary after
// witnessing a clock value received from another process
func (pc *PersistedClock) Witness(time Time) error {
// TODO: rework so that we write only when the clock was actually updated
err := pc.MemClock.Witness(time)
if err != nil {
return err
}
return pc.Write()
}
func (pc *PersistedClock) read() error {
f, err := pc.root.Open(pc.filePath)
if os.IsNotExist(err) {
return ErrClockNotExist
}
if err != nil {
return err
}
defer f.Close()
content, err := ioutil.ReadAll(f)
if err != nil {
return err
}
var value uint64
n, err := fmt.Sscanf(string(content), "%d", &value)
if err != nil {
return err
}
if n != 1 {
return fmt.Errorf("could not read the clock")
}
pc.MemClock = NewMemClockWithTime(value)
return nil
}
func (pc *PersistedClock) Write() error {
data := []byte(fmt.Sprintf("%d", pc.counter))
return util.WriteFile(pc.root, pc.filePath, data, 0644)
}
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