util: add a Lamport clock implementation as well as a persistable one

1 files changed, 81 insertions, 0 deletions

diff --git a/util/persisted_lamport.go b/util/persisted_lamport.go
new file mode 100644
index 00000000..43a2863c
--- /dev/null
+++ b/util/persisted_lamport.go
@@ -0,0 +1,81 @@
+package util
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+)
+
+type PersistedLamport struct {
+	LamportClock
+	filePath string
+}
+
+func NewPersistedLamport(filePath string) *PersistedLamport {
+	clock := &PersistedLamport{
+		filePath: filePath,
+	}
+	return clock
+}
+
+func LoadPersistedLamport(filePath string) (*PersistedLamport, error) {
+	clock := &PersistedLamport{
+		filePath: filePath,
+	}
+
+	err := clock.read()
+	if err != nil {
+		return nil, err
+	}
+
+	return clock, nil
+}
+
+func (c *PersistedLamport) Witness(time LamportTime) error {
+	c.LamportClock.Witness(time)
+	return c.Write()
+}
+
+func (c *PersistedLamport) Time() LamportTime {
+	// Equivalent to:
+	//
+	// res = c.LamportClock.Time()
+	// bugClock.Increment()
+	//
+	// ... but thread safe
+	return c.Increment() - 1
+}
+
+func (c *PersistedLamport) read() error {
+	content, err := ioutil.ReadFile(c.filePath)
+	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")
+	}
+
+	c.LamportClock = NewLamportClockWithTime(value)
+
+	return nil
+}
+
+func (c *PersistedLamport) Write() error {
+	dir := filepath.Dir(c.filePath)
+	err := os.MkdirAll(dir, 0777)
+	if err != nil {
+		return err
+	}
+
+	data := []byte(fmt.Sprintf("%d", c.LamportClock.Time()))
+	return ioutil.WriteFile(c.filePath, data, 0644)
+}