update

utils/random/number.go

@@ -0,0 +1,24 @@
+package random
+
+import (
+	"crypto/rand"
+	"encoding/binary"
+	"math/big"
+
+	"golang.org/x/exp/constraints"
+)
+
+// Int64N returns a random int64 between 0 and max.
+// This is a reimplementation of math/rand/v2.Int64N using a cryptographically secure random number generator.
+func Int64N[T constraints.Integer](max T) int64 {
+	rnd, _ := rand.Int(rand.Reader, big.NewInt(int64(max)))
+	return rnd.Int64()
+}
+
+// Uint64 returns a random uint64.
+// This is a reimplementation of math/rand/v2.Uint64 using a cryptographically secure random number generator.
+func Uint64() uint64 {
+	buffer := make([]byte, 8)
+	_, _ = rand.Read(buffer)
+	return binary.BigEndian.Uint64(buffer)
+}

utils/random/number_test.go

@@ -0,0 +1,24 @@
+package random_test
+
+import (
+	"testing"
+
+	"github.com/navidrome/navidrome/utils/random"
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+func TestRandom(t *testing.T) {
+	RegisterFailHandler(Fail)
+	RunSpecs(t, "Random Suite")
+}
+
+var _ = Describe("number package", func() {
+	Describe("Int64N", func() {
+		It("should return a random int64", func() {
+			for i := 0; i < 10000; i++ {
+				Expect(random.Int64N(100)).To(BeNumerically("<", 100))
+			}
+		})
+	})
+})

utils/random/weighted_random_chooser.go

@@ -0,0 +1,70 @@
+package random
+
+import (
+	"errors"
+	"slices"
+)
+
+// WeightedChooser allows to randomly choose an entry based on their weights
+// (higher weight = higher chance of being chosen). Based on the subtraction method described in
+// https://eli.thegreenplace.net/2010/01/22/weighted-random-generation-in-python/
+type WeightedChooser[T any] struct {
+	entries     []T
+	weights     []int
+	totalWeight int
+}
+
+func NewWeightedChooser[T any]() *WeightedChooser[T] {
+	return &WeightedChooser[T]{}
+}
+
+func (w *WeightedChooser[T]) Add(value T, weight int) {
+	w.entries = append(w.entries, value)
+	w.weights = append(w.weights, weight)
+	w.totalWeight += weight
+}
+
+// Pick choose a random entry based on their weights, and removes it from the list
+func (w *WeightedChooser[T]) Pick() (T, error) {
+	var empty T
+	if w.totalWeight == 0 {
+		return empty, errors.New("cannot choose from zero weight")
+	}
+	i, err := w.weightedChoice()
+	if err != nil {
+		return empty, err
+	}
+	entry := w.entries[i]
+	_ = w.Remove(i)
+	return entry, nil
+}
+
+func (w *WeightedChooser[T]) weightedChoice() (int, error) {
+	if len(w.entries) == 0 {
+		return 0, errors.New("cannot choose from empty list")
+	}
+	rnd := Int64N(w.totalWeight)
+	for i, weight := range w.weights {
+		rnd -= int64(weight)
+		if rnd < 0 {
+			return i, nil
+		}
+	}
+	return 0, errors.New("internal error - code should not reach this point")
+}
+
+func (w *WeightedChooser[T]) Remove(i int) error {
+	if i < 0 || i >= len(w.entries) {
+		return errors.New("index out of bounds")
+	}
+
+	w.totalWeight -= w.weights[i]
+
+	w.weights = slices.Delete(w.weights, i, i+1)
+	w.entries = slices.Delete(w.entries, i, i+1)
+	return nil
+}
+
+func (w *WeightedChooser[T]) Size() int {
+	return len(w.entries)
+}

utils/random/weighted_random_chooser_test.go

@@ -0,0 +1,72 @@
+package random
+
+import (
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("WeightedChooser", func() {
+	var w *WeightedChooser[int]
+	BeforeEach(func() {
+		w = NewWeightedChooser[int]()
+		for i := 0; i < 10; i++ {
+			w.Add(i, i+1)
+		}
+	})
+
+	It("selects and removes a random item", func() {
+		Expect(w.Size()).To(Equal(10))
+		_, err := w.Pick()
+		Expect(err).ToNot(HaveOccurred())
+		Expect(w.Size()).To(Equal(9))
+	})
+
+	It("removes items", func() {
+		Expect(w.Size()).To(Equal(10))
+		for i := 0; i < 10; i++ {
+			Expect(w.Remove(0)).To(Succeed())
+		}
+		Expect(w.Size()).To(Equal(0))
+	})
+
+	It("returns error if trying to remove an invalid index", func() {
+		Expect(w.Size()).To(Equal(10))
+		Expect(w.Remove(-1)).ToNot(Succeed())
+		Expect(w.Remove(10000)).ToNot(Succeed())
+		Expect(w.Size()).To(Equal(10))
+	})
+
+	It("returns the sole item", func() {
+		ws := NewWeightedChooser[string]()
+		ws.Add("a", 1)
+		Expect(ws.Pick()).To(Equal("a"))
+	})
+
+	It("returns all items from the list", func() {
+		for i := 0; i < 10; i++ {
+			Expect(w.Pick()).To(BeElementOf(0, 1, 2, 3, 4, 5, 6, 7, 8, 9))
+		}
+		Expect(w.Size()).To(Equal(0))
+	})
+
+	It("fails when trying to choose from empty set", func() {
+		w = NewWeightedChooser[int]()
+		w.Add(1, 1)
+		w.Add(2, 1)
+		Expect(w.Pick()).To(BeElementOf(1, 2))
+		Expect(w.Pick()).To(BeElementOf(1, 2))
+		_, err := w.Pick()
+		Expect(err).To(HaveOccurred())
+	})
+
+	It("chooses based on weights", func() {
+		counts := [10]int{}
+		for i := 0; i < 200000; i++ {
+			c, _ := w.weightedChoice()
+			counts[c] = counts[c] + 1
+		}
+		for i := 0; i < 9; i++ {
+			Expect(counts[i]).To(BeNumerically("<", counts[i+1]))
+		}
+	})
+})