Deterministic pagination in random albums sort (#1841)

* Deterministic pagination in random albums sort

* Reseed on first random page

* Add unit tests

* Use rand in Subsonic API

* Use different seeds per user on SEEDEDRAND() SQLite3 function

* Small refactor

* Fix id mismatch

* Add seeded random to media_file (subsonic endpoint `getRandomSongs`)

* Refactor

* Remove unneeded import

---------

Co-authored-by: Deluan <deluan@navidrome.org>

utils/hasher/hasher.go

@@ -0,0 +1,44 @@
+package hasher
+
+import "hash/maphash"
+
+var instance = NewHasher()
+
+func Reseed(id string) {
+	instance.Reseed(id)
+}
+
+func HashFunc() func(id, str string) uint64 {
+	return instance.HashFunc()
+}
+
+type hasher struct {
+	seeds map[string]maphash.Seed
+}
+
+func NewHasher() *hasher {
+	h := new(hasher)
+	h.seeds = make(map[string]maphash.Seed)
+	return h
+}
+
+// Reseed generates a new seed for the given id
+func (h *hasher) Reseed(id string) {
+	h.seeds[id] = maphash.MakeSeed()
+}
+
+// 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 {
+		var hash maphash.Hash
+		var seed maphash.Seed
+		var ok bool
+		if seed, ok = h.seeds[id]; !ok {
+			seed = maphash.MakeSeed()
+			h.seeds[id] = seed
+		}
+		hash.SetSeed(seed)
+		_, _ = hash.WriteString(str)
+		return hash.Sum64()
+	}
+}

utils/hasher/hasher_test.go

@@ -0,0 +1,36 @@
+package hasher_test
+
+import (
+	"github.com/navidrome/navidrome/utils/hasher"
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+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)))
+	})
+})