add weighted random package

rand/random.go

@@ -0,0 +1,47 @@
+package rand
+
+import (
+	"math/rand"
+	"sort"
+	"time"
+)
+
+type WeightedRandom struct {
+	rand    *rand.Rand
+	choices []RandChoice
+}
+
+type RandChoice struct {
+	Chance float64
+	Value  interface{}
+}
+
+func NewWeightedRandom(choices []RandChoice, seed int64) *WeightedRandom {
+	if seed == 0 {
+		seed = time.Hour.Nanoseconds()
+	}
+
+	sort.Slice(choices, func(i, j int) bool { return choices[i].Chance < choices[j].Chance })
+
+	return &WeightedRandom{
+		rand:    rand.New(rand.NewSource(seed)),
+		choices: choices[:],
+	}
+}
+
+func (wr WeightedRandom) Get() interface{} {
+	var total float64
+	for _, v := range wr.choices {
+		total += v.Value.(float64)
+	}
+
+	rand := wr.rand.Float64() * total
+	for _, v := range wr.choices {
+		if v.Chance > rand {
+			return v.Value
+		}
+
+		rand -= v.Chance
+	}
+	return nil
+}