#!/usr/bin/env bash

# Esoteric random number generator
# <http://dilbert.com/strip/2001-10-25>
self=rndn

# Define usage function
usage() {
    printf 'USAGE: %s [-h | -s SEED]\n' "$self"
}

# Parse options
while getopts 'hs:' opt ; do
    case $opt in

        # -h for help
        h)
            usage
            exit 0
            ;;

        # -s to set seed manually
        s)
            seed=$OPTARG
            ;;

        # Unknown option, print usage and fail
        \?)
            usage >&2
            exit 2
            ;;
    esac
done
shift "$((OPTIND-1))"

# If no seed given, get one from Bash's $RANDOM
: "${seed:=$((RANDOM ** 2))}"

# Truncate the seed
seed=${seed:0:32}

# Check seed meets algorithm conditions
if [[ $seed == *[^0-9]* ]] || ((seed < 0)) ; then
    printf >&2 '%s: error: seed must be non-negative integer\n' "$self"
    exit 2
fi

# Apply algorithm
for ((seed += 10, i = 0; i < ${#seed}; i++)) ; do
    ((sum += ${seed:i:1}))
done
for ((red = seed - sum; ${#red} > 1; red = redn)) ; do
    for ((j = 0, redn = 0; j < ${#red}; j++)) ; do
        ((redn += ${red:j:1}))
    done
done

# Print result
printf '%u\n' "$red"