An implementation of a xorshift128+ RNG in shell code

xorshift128plus

#!/bin/bash
#set -x

# Function to generate a reliable seed for whatever method requires one
# Because 'date +%s' isn't entirely portable, we try other methods to get the epoch
generate_seed() {
    # First we check if /dev/urandom is available.
    if [ -c /dev/urandom ] && command -v od >/dev/null 2>&1; then
        # Get a string of bytes from /dev/urandom using od
        od -N 4 -A n -t uL /dev/urandom | tr -d '\n' | awk '{$1=$1+0};1'
    # Otherwise, we see if 'date' will emit an epoch timestamp
    elif date '+%s' >/dev/null 2>&1; then
        date '+%s'
    # In the rare case that we have a limited version of 'date'
    # We generate an epoch timestamp with octal correction
    # Based on http://www.etalabs.net/sh_tricks.html
    else
        local int_secs int_mins int_hours int_days int_years int_yearoffset

        int_secs=$(TZ=GMT0 date +%S)
        int_mins=$(TZ=GMT0 date +%M)
        int_hours=$(TZ=GMT0 date +%H)
        int_days=$(TZ=GMT0 date +%j | sed 's/^0*//')

        int_yearoffset=$(( $(TZ=GMT0 date +%Y) - 1600 ))

        int_years=$(( (
            int_yearoffset * 365
            + int_yearoffset / 4
            - int_yearoffset / 100
            + int_yearoffset / 400
            + int_days
            - 135140
        ) * 86400 ))

        printf '%s\n' "$(( \
            int_years \
            + (${int_hours#0} * 3600) \
            + (${int_mins#0} * 60) \
            + ${int_secs#0} \
        ))"
    fi
}

# Start assembling our vars
int_count="${1:-1}"
int_min="${2:-1}"

# Figure out our default maxRand, using 'getconf'
case "$(getconf LONG_BIT)" in
    (64)
        rand_ceiling=9223372036854775807
    ;;
    (32)
        rand_ceiling=2147483647
    ;;
    (*)
        rand_ceiling=32767
    ;;
esac

int_max="${3:-$rand_ceiling}"

rand_range=$(( int_max - int_min + 1 ))

(( rand_range > 0 )) || {
    printf -- '%s\n' "Invalid range" >&2
    exit 1
}

full_range=0
if (( int_min == 0 && int_max == rand_ceiling )); then
    full_range=1
else
    threshold=$(( (rand_ceiling - rand_range + 1) % rand_range ))
fi

# Initialise the seed state
seed="${4:-$(generate_seed)}"
(( seed == 0 )) && seed=4  # RFC 1149.5

seed0="${seed}"
seed1=$(( seed ^ 0x9e3779b9 ))

# Get the ints ready for the loop
int0="${seed0}"
int1="${seed1}"

while (( int_count > 0 )); do
    old_int0="${int0}"
    old_int1="${int1}"

    # xorshift128+ state transition
    int0="${old_int1}"

    old_int0=$(( old_int0 ^ (old_int0 << 23) ))

    int1=$(( \
        old_int0 \
        ^ old_int1 \
        ^ (old_int0 >> 17) \
        ^ (old_int1 >> 26) \
    ))

    rnd=$(( int1 + old_int1 ))

    if (( full_range )); then
        if (( rnd >= 0 )); then
            printf -- '%u\n' "$rnd"
            (( int_count-- ))
        fi
        continue
    fi

    if (( rnd >= 0 && rnd >= threshold )); then
        printf -- '%u\n' "$(( (rnd % rand_range) + int_min ))"
        (( int_count-- ))
    fi
done