Merge pull request #54 from owenfarrell/offset

Ensure even distribution of versions across interval/range

lord/time_lord.go

@@ -19,7 +19,7 @@ type TimeLord struct {
 
 func (tl TimeLord) Check(now time.Time) bool {
 
-	start, stop := tl.latestRangeBefore(now)
+	start, stop := tl.LatestRangeBefore(now)
 
 	if !tl.daysMatch(now) {
 		return false
@@ -54,7 +54,7 @@ func (tl TimeLord) Latest(reference time.Time) time.Time {
 		refInLoc = refInLoc.AddDate(0, 0, -1)
 	}
 
-	start, stop := tl.latestRangeBefore(refInLoc)
+	start, stop := tl.LatestRangeBefore(refInLoc)
 
 	if tl.PreviousTime.IsZero() && !reference.Before(stop) {
 		return time.Time{}
@@ -85,7 +85,7 @@ func (tl TimeLord) List(reference time.Time) []time.Time {
 	if tl.Interval == nil {
 
 		if start.IsZero() {
-			refRangeStart, refRangeEnd := tl.latestRangeBefore(reference)
+			refRangeStart, refRangeEnd := tl.LatestRangeBefore(reference)
 			if !reference.Before(refRangeEnd) {
 				return versions
 			}
@@ -122,7 +122,7 @@ func (tl TimeLord) List(reference time.Time) []time.Time {
 	var dailyStart, dailyEnd time.Time
 	for dailyInterval := start; !dailyStart.After(reference); dailyInterval = dailyInterval.AddDate(0, 0, 1) {
 		if tl.daysMatch(dailyInterval) {
-			dailyStart, dailyEnd = tl.latestRangeBefore(dailyInterval)
+			dailyStart, dailyEnd = tl.LatestRangeBefore(dailyInterval)
 			if dailyStart.After(reference) {
 				break
 			}
@@ -148,7 +148,7 @@ func (tl TimeLord) daysMatch(now time.Time) bool {
 	return false
 }
 
-func (tl TimeLord) latestRangeBefore(reference time.Time) (time.Time, time.Time) {
+func (tl TimeLord) LatestRangeBefore(reference time.Time) (time.Time, time.Time) {
 
 	tlStart := DEFAULT_TIME_OF_DAY
 	if tl.Start != nil {

offset.go

@@ -0,0 +1,48 @@
+package resource
+
+import (
+	"fmt"
+	"hash/fnv"
+	"math"
+	"os"
+	"time"
+
+	"github.com/concourse/time-resource/lord"
+)
+
+const BUILD_TEAM_NAME = "BUILD_TEAM_NAME"
+const BUILD_PIPELINE_NAME = "BUILD_PIPELINE_NAME"
+const offsetHashFormat = "%s/%s"
+
+const maxHashValue = int64(math.MaxUint32)
+
+var msPerMinute = time.Minute.Milliseconds()
+
+func Offset(tl lord.TimeLord, reference time.Time) time.Time {
+	str := fmt.Sprintf(offsetHashFormat, os.Getenv(BUILD_TEAM_NAME), os.Getenv(BUILD_PIPELINE_NAME))
+	hasher := fnv.New32a()
+	if _, err := hasher.Write([]byte(str)); err != nil {
+		fmt.Fprintln(os.Stderr, "hash error:", err.Error())
+		os.Exit(1)
+	}
+	hash := int64(hasher.Sum32())
+
+	start, stop := tl.LatestRangeBefore(reference)
+	rangeDuration := stop.Sub(start)
+
+	if tl.Interval != nil {
+		if intervalDuration := time.Duration(*tl.Interval); intervalDuration < rangeDuration {
+			rangeDuration = intervalDuration
+			start = reference.Truncate(rangeDuration)
+		}
+	}
+
+	if rangeDuration <= time.Minute {
+		return start
+	}
+
+	hashPerMinute := maxHashValue / (rangeDuration.Milliseconds() / msPerMinute)
+	offsetDuration := time.Duration(hash/hashPerMinute) * time.Minute
+
+	return start.Add(offsetDuration)
+}