Re: rigorously predictable random numbers

[Roman Perepelitsa <roman.perepelitsa@xxxxxxxxx>]

On Fri, May 3, 2024 at 5:48 AM Ray Andrews <rayandrews@xxxxxxxxxxx> wrote:
>
> How does one obtain a genuinely random number then?

There are several ways. I use this:
https://gist.github.com/romkatv/a6cede40714ec77d4da73605c5ddb36a

  # Returns a random 32-bit number.
  # If /dev/urandom is cryptographically secure, so is srand32.
  #
  # If zsh is compiled with 64-bit number support, the result
  # is non-negative. Otherwise it may be negative and the value
  # is governed by the rules of unsigned-to-signed conversion in C.
  #
  # Examples:
  #
  #   % print -r -- $(( srand32() ))
  #   263510467
  #
  #   % local -i x='srand32()'
  #   % typeset -p x
  #   typeset -i x=858148951
  #
  # Performance: ~30us per call.
  #
  #   % time ( repeat 1000000 srand32 )
  #   user=19.95s system=6.95s cpu=99% total=26.934
  #
  #   % uname -srm
  #   Linux 5.4.0-58-generic x86_64
  #
  #   % grep 'model name' /proc/cpuinfo | head -1
  #   model name      : AMD Ryzen 7 3700X 8-Core Processor
  function srand32() {
    emulate -L zsh -o no_multi_byte
    local bytes
    while (( $#bytes < 4 )); do
      sysread -s$((4-$#bytes)) 'bytes[$#bytes+1]' </dev/urandom || return
    done
    local b1=$bytes[1] b2=$bytes[2] b3=$bytes[3] b4=$bytes[4]
    return '#b1 << 24 | #b2 << 16 | #b3 << 8 | #b4'
  }
  functions -M srand32 0 0

Roman