package event

import (
	"hash/fnv"
	"strconv"
)

// PartitionOf maps a stable key (sessionId / listId / sendID, stringified) to a
// partition index in [0, n) for the track.partition.{N} ingest scheme
// (slask-tracking shell-rebuild §8). fnv1a-64 over the key bytes, so producers
// (slask-finder's track.* publisher, slask-tracking's HTTP beacon edge) and
// consumers agree on the routing without sharing app code. n <= 1 is a single
// partition (index 0).
func PartitionOf(key string, n int) int {
	if n <= 1 {
		return 0
	}
	h := fnv.New64a()
	_, _ = h.Write([]byte(key))
	return int(h.Sum64() % uint64(n))
}

// PartitionRoutingKey is the queue AND routing key for partition i: the queue
// track.partition.{i} binds to the catalog exchange with the same key, so a
// producer routing to track.partition.{i} lands on exactly that queue. This is
// the shared name both the slask-finder producer and the slask-tracking
// consumer use, so the partition→queue→consumer map is observable and stable.
func PartitionRoutingKey(i int) string {
	return "track.partition." + strconv.Itoa(i)
}