update

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)
+}