1 files changed, 53 insertions, 0 deletions
diff --git a/internal/rollout/rollout.go b/internal/rollout/rollout.go
new file mode 100644
index 00000000..ec592c38
--- /dev/null
+++ b/internal/rollout/rollout.go
@@ -0,0 +1,53 @@
+package rollout
+
+import (
+	"errors"
+	"hash/fnv"
+	"math/rand"
+)
+
+// Rollout returns true and no error when during this run something should
+// happen for given actor according to the stickiness and likelyhood passed
+// as a percentage value to this function. It returns false rollout and an
+// error if the percentage value is negative or higher than 100.
+func Rollout(actor string, percentage int, stickiness string) (bool, error) {
+	if percentage < 0 || percentage > 100 {
+		return false, errors.New("Rollout value should be between 0 and 100 inclusive")
+	}
+
+	if percentage == 0 {
+		return false, nil
+	}
+
+	if percentage == 100 {
+		return true, nil
+	}
+
+	switch stickiness {
+	case "random":
+		return random(percentage), nil
+	default:
+		return forActor(actor, percentage), nil
+	}
+}
+
+// random guarantees no stickiness. For every call it will yield a random
+// true/false based on the provided rollout percentage.
+func random(percentage int) bool {
+	return rand.Intn(100) < percentage
+}
+
+// forActor provides "stickines", i.e. guarantees that the same actor
+// gets the same result every time. It also assures that an actor which is
+// among the first 10% will also be among the first 20%.
+func forActor(actor string, percentage int) bool {
+	h := fnv.New32a()
+	h.Write([]byte(actor))
+	sum32 := h.Sum32()
+
+	if sum32 == 0 {
+		return false
+	}
+
+	return (sum32 % uint32(100)) < uint32(percentage)
+}