Generate Any One of Given Numbers According to Given Probabilities

package main

import (
      "math/rand"
      "time"
)

var r *rand.Rand // Rand for this package.

func Random(input []int, prob []int) int {
      // check sanity
      if len(input) != len(prob) {
              panic("length of input and prob is not equal")
      }
      sum := 0
      for _, p := range prob {
              sum += p
      }
      if sum != 100 {
              panic("sum of prob must be 100")
      }

      probSum := make([]int, len(input))
      for i, p := range prob {
              if i == 0 {
                      probSum[0] = p
              } else {
                      probSum[i] = probSum[i-1] + p
              }
      }

      ri := r.Intn(100)

      for i, ps := range probSum {
              if ri < ps {
                      return input[i]
              }
      }

      panic("should not run here")
}

func init() {
      r = rand.New(rand.NewSource(time.Now().UnixNano()))
}

func main() {
      input := []int{1, 2, 3, 4, 5}
      prob := []int{30, 10, 20, 15, 25}

      m := make(map[int]int)
      for _, n := range input {
              m[n] = 0
      }
      for i := 0; i < 1000000; i++ {
              m[Random(input, prob)] += 1
      }
      for k, v := range m {
              print(k)
              print(" : ")
              println(v)
      }
}

1 : 299989
2 : 99405
3 : 200718
4 : 149958
5 : 249930