package routing

import (
	"crypto/sha256"
	"encoding/binary"
)

// CanonicalHash returns a deterministic 64-bit hash of (system, user).
// Used to make sample-band routing decisions reproducible: identical input
// strings produce the same hash on every call, independent of process state.
//
// Inputs are joined with a 0x00 byte separator before hashing — distinguishes
// (system="ab", user="cd") from (system="abcd", user="").
func CanonicalHash(system, user string) uint64 {
	h := sha256.New()
	h.Write([]byte(system))
	h.Write([]byte{0})
	h.Write([]byte(user))
	sum := h.Sum(nil)
	return binary.BigEndian.Uint64(sum[:8])
}