update

utils/hasher/hasher.go

@@ -0,0 +1,74 @@
+package hasher
+
+import (
+	"hash/maphash"
+	"strconv"
+	"sync"
+
+	"github.com/navidrome/navidrome/utils/random"
+)
+
+var instance = NewHasher()
+
+func Reseed(id string) {
+	instance.Reseed(id)
+}
+
+func SetSeed(id string, seed string) {
+	instance.SetSeed(id, seed)
+}
+
+func CurrentSeed(id string) string {
+	instance.mutex.RLock()
+	defer instance.mutex.RUnlock()
+	return instance.seeds[id]
+}
+
+func HashFunc() func(id, str string) uint64 {
+	return instance.HashFunc()
+}
+
+type Hasher struct {
+	seeds    map[string]string
+	mutex    sync.RWMutex
+	hashSeed maphash.Seed
+}
+
+func NewHasher() *Hasher {
+	h := new(Hasher)
+	h.seeds = make(map[string]string)
+	h.hashSeed = maphash.MakeSeed()
+	return h
+}
+
+// SetSeed sets a seed for the given id
+func (h *Hasher) SetSeed(id string, seed string) {
+	h.mutex.Lock()
+	defer h.mutex.Unlock()
+	h.seeds[id] = seed
+}
+
+// Reseed generates a new random seed for the given id
+func (h *Hasher) Reseed(id string) {
+	_ = h.reseed(id)
+}
+
+func (h *Hasher) reseed(id string) string {
+	seed := strconv.FormatUint(random.Uint64(), 36)
+	h.SetSeed(id, seed)
+	return seed
+}
+
+// HashFunc returns a function that hashes a string using the seed for the given id
+func (h *Hasher) HashFunc() func(id, str string) uint64 {
+	return func(id, str string) uint64 {
+		h.mutex.RLock()
+		seed, ok := h.seeds[id]
+		h.mutex.RUnlock()
+		if !ok {
+			seed = h.reseed(id)
+		}
+
+		return maphash.Bytes(h.hashSeed, []byte(seed+str))
+	}
+}

utils/hasher/hasher_test.go

@@ -0,0 +1,68 @@
+package hasher_test
+
+import (
+	"strconv"
+	"testing"
+
+	"github.com/navidrome/navidrome/utils/hasher"
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+func TestHasher(t *testing.T) {
+	RegisterFailHandler(Fail)
+	RunSpecs(t, "Hasher Suite")
+}
+
+var _ = Describe("HashFunc", func() {
+	const input = "123e4567e89b12d3a456426614174000"
+
+	It("hashes the input and returns the sum", func() {
+		hashFunc := hasher.HashFunc()
+		sum := hashFunc("1", input)
+		Expect(sum > 0).To(BeTrue())
+	})
+
+	It("hashes the input, reseeds and returns a different sum", func() {
+		hashFunc := hasher.HashFunc()
+		sum := hashFunc("1", input)
+		hasher.Reseed("1")
+		sum2 := hashFunc("1", input)
+		Expect(sum).NotTo(Equal(sum2))
+	})
+
+	It("keeps different hashes for different ids", func() {
+		hashFunc := hasher.HashFunc()
+		sum := hashFunc("1", input)
+		sum2 := hashFunc("2", input)
+
+		Expect(sum).NotTo(Equal(sum2))
+
+		Expect(sum).To(Equal(hashFunc("1", input)))
+		Expect(sum2).To(Equal(hashFunc("2", input)))
+	})
+
+	It("keeps the same hash for the same id and seed", func() {
+		id := "1"
+		hashFunc := hasher.HashFunc()
+		hasher.SetSeed(id, "original_seed")
+		sum := hashFunc(id, input)
+		Expect(sum).To(Equal(hashFunc(id, input)))
+
+		hasher.Reseed(id)
+		Expect(sum).NotTo(Equal(hashFunc(id, input)))
+
+		hasher.SetSeed(id, "original_seed")
+		Expect(sum).To(Equal(hashFunc(id, input)))
+	})
+
+	It("does not cause race conditions", func() {
+		for i := 0; i < 1000; i++ {
+			go func() {
+				hashFunc := hasher.HashFunc()
+				sum := hashFunc(strconv.Itoa(i), input)
+				Expect(sum).ToNot(BeZero())
+			}()
+		}
+	})
+})