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// Copyright 2019 The Chromium OS Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
use std::fs::File;
use std::io::{self, Read};
/// A simple prng based on a Linear congruential generator
/// https://en.wikipedia.org/wiki/Linear_congruential_generator
pub struct SimpleRng {
seed: u64,
}
impl SimpleRng {
/// Create a new SimpleRng
pub fn new(seed: u64) -> SimpleRng {
SimpleRng { seed }
}
/// Generate random u64
pub fn rng(&mut self) -> u64 {
// a simple Linear congruential generator
let a: u64 = 6364136223846793005;
let c: u64 = 1442695040888963407;
self.seed = a.wrapping_mul(self.seed).wrapping_add(c);
self.seed
}
}
// Uniformly samples the ASCII alphanumeric characters given a random variable. If an `Err` is
// passed in, the error is returned as `Some(Err(...))`. If the the random variable can not be used
// to uniformly sample, `None` is returned.
fn uniform_sample_ascii_alphanumeric(
b: Result<u8, std::io::Error>,
) -> Option<Result<char, std::io::Error>> {
const ASCII_CHARS: &'static [u8] = b"\
ABCDEFGHIJKLMNOPQRSTUVWXYZ\
abcdefghijklmnopqrstuvwxyz\
0123456789";
let char_index = match b {
Ok(c) => c as usize,
Err(e) => return Some(Err(e)),
};
// Throw away numbers that would cause sampling bias.
if char_index >= ASCII_CHARS.len() * 4 {
None
} else {
Some(Ok(ASCII_CHARS[char_index % ASCII_CHARS.len()] as char))
}
}
/// Samples `/dev/urandom` and generates a random ASCII string of length `len`
pub fn urandom_str(len: usize) -> io::Result<String> {
File::open("/dev/urandom")?
.bytes()
.filter_map(uniform_sample_ascii_alphanumeric)
.take(len)
.collect::<io::Result<String>>()
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn check_100() {
for i in 0..100 {
let s = urandom_str(i).unwrap();
assert_eq!(s.len(), i);
assert!(s.is_ascii());
assert!(!s.contains(' '));
assert!(!s.contains(|c: char| !c.is_ascii_alphanumeric()));
}
}
}
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