Refactor random functions

utils/random/number.go

@@ -0,0 +1,11 @@
+package random
+
+import (
+	"crypto/rand"
+	"math/big"
+)
+
+func Int64(max int64) int64 {
+	rnd, _ := rand.Int(rand.Reader, big.NewInt(max))
+	return rnd.Int64()
+}

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("Int64", func() {
+		It("should return a random int64", func() {
+			for i := 0; i < 10000; i++ {
+				Expect(random.Int64(100)).To(BeNumerically("<", 100))
+			}
+		})
+	})
+})

utils/random/weighted_random_chooser.go

@@ -0,0 +1,65 @@
+package random
+
+import (
+	"errors"
+)
+
+type WeightedChooser struct {
+	entries     []interface{}
+	weights     []int
+	totalWeight int
+}
+
+func NewWeightedRandomChooser() *WeightedChooser {
+	return &WeightedChooser{}
+}
+
+func (w *WeightedChooser) Add(value interface{}, weight int) {
+	w.entries = append(w.entries, value)
+	w.weights = append(w.weights, weight)
+	w.totalWeight += weight
+}
+
+// GetAndRemove choose a random entry based on their weights, and removes it from the list
+func (w *WeightedChooser) GetAndRemove() (interface{}, error) {
+	if w.totalWeight == 0 {
+		return nil, errors.New("cannot choose from zero weight")
+	}
+	i, err := w.weightedChoice()
+	if err != nil {
+		return nil, err
+	}
+	entry := w.entries[i]
+	w.Remove(i)
+	return entry, nil
+}
+
+// Based on https://eli.thegreenplace.net/2010/01/22/weighted-random-generation-in-python/
+func (w *WeightedChooser) weightedChoice() (int, error) {
+	if w.totalWeight == 0 {
+		return 0, errors.New("no choices available")
+	}
+	rnd := Int64(int64(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) Remove(i int) {
+	w.totalWeight -= w.weights[i]
+
+	w.weights[i] = w.weights[len(w.weights)-1]
+	w.weights = w.weights[:len(w.weights)-1]
+
+	w.entries[i] = w.entries[len(w.entries)-1]
+	w.entries[len(w.entries)-1] = nil
+	w.entries = w.entries[:len(w.entries)-1]
+}
+
+func (w *WeightedChooser) Size() int {
+	return len(w.entries)
+}

utils/random/weighted_random_chooser_test.go

@@ -0,0 +1,50 @@
+package random
+
+import (
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("WeightedRandomChooser", func() {
+	var w *WeightedChooser
+	BeforeEach(func() {
+		w = NewWeightedRandomChooser()
+		for i := 0; i < 10; i++ {
+			w.Add(i, i)
+		}
+	})
+
+	It("removes a random item", func() {
+		Expect(w.Size()).To(Equal(10))
+		_, err := w.GetAndRemove()
+		Expect(err).ToNot(HaveOccurred())
+		Expect(w.Size()).To(Equal(9))
+	})
+
+	It("returns the sole item", func() {
+		w = NewWeightedRandomChooser()
+		w.Add("a", 1)
+		Expect(w.GetAndRemove()).To(Equal("a"))
+	})
+
+	It("fails when trying to choose from empty set", func() {
+		w = NewWeightedRandomChooser()
+		w.Add("a", 1)
+		w.Add("b", 1)
+		Expect(w.GetAndRemove()).To(BeElementOf("a", "b"))
+		Expect(w.GetAndRemove()).To(BeElementOf("a", "b"))
+		_, err := w.GetAndRemove()
+		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]))
+		}
+	})
+})