randutil.String doesn't pick the last character in a set

[antifuchs]

I wanted to test for any biases the string generator might have; sadly, it looks like the generator is strongly biased against the last character in a set - it never gets picked.

I wrote this test program:

package main

import (
    "os"
    "strconv"

    "fmt"

    "github.com/jmcvetta/randutil"
)

const (
    PasswordCharacters = "}~"
)

func main() {
    howmany, err := strconv.Atoi(os.Args[1])
    if err != nil {
        fmt.Printf("Need a password length, error %s\n", err)
        os.Exit(1)
    }
    length, err := strconv.Atoi(os.Args[2])
    if err != nil {
        fmt.Printf("Need a password count, error %s\n", err)
        os.Exit(1)
    }

    charWeights := make(map[rune]int)
    for _, c := range PasswordCharacters {
        charWeights[c] = 0
    }

    for i := 0; i < howmany; i++ {
        onePass, err := randutil.String(length, PasswordCharacters)
        if err != nil {
            fmt.Printf("Got error generating pass: %v\n", err)
        }
        for _, c := range onePass {
            charWeights[c]++
        }
    }
    for _, c := range PasswordCharacters {
        count := charWeights[c]
        fmt.Printf("%d\t%c\n", count, c)
    }
}

Then I ran this:

go run ./main.go 45 200

and get this result:

9000    }
0       ~

-- Expanding the character set, it looks like it's selecting every character but the last one.