1 files changed, 100 insertions, 0 deletions
diff --git a/util/lamport/persisted_clock.go b/util/lamport/persisted_clock.go
new file mode 100644
index 00000000..e70b01ef
--- /dev/null
+++ b/util/lamport/persisted_clock.go
@@ -0,0 +1,100 @@
+package lamport
+
+import (
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+)
+
+var ErrClockNotExist = errors.New("clock doesn't exist")
+
+type PersistedClock struct {
+	*MemClock
+	filePath string
+}
+
+// NewPersistedClock create a new persisted Lamport clock
+func NewPersistedClock(filePath string) (*PersistedClock, error) {
+	clock := &PersistedClock{
+		MemClock: NewMemClock(),
+		filePath: filePath,
+	}
+
+	dir := filepath.Dir(filePath)
+	err := os.MkdirAll(dir, 0777)
+	if err != nil {
+		return nil, err
+	}
+
+	err = clock.Write()
+	if err != nil {
+		return nil, err
+	}
+
+	return clock, nil
+}
+
+// LoadPersistedClock load a persisted Lamport clock from a file
+func LoadPersistedClock(filePath string) (*PersistedClock, error) {
+	clock := &PersistedClock{
+		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 {
+	content, err := ioutil.ReadFile(pc.filePath)
+	if os.IsNotExist(err) {
+		return ErrClockNotExist
+	}
+	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 ioutil.WriteFile(pc.filePath, data, 0644)
+}