#!/bin/sh

FG () { printf '\033[38;05;%dm' $1; }
BG () { printf '\033[48;05;%dm' $1; }

reset=$(printf '\033[00m')

numcolor=false
maxcolor=256

while [ $# -gt 0 ] && { arg=$1; shift; }
do case $arg in

t)
    FG () { tput setaf $1; }
    BG () { tput setab $1; }
    reset=$(tput op)
    ;;

n)
    numcolor=true
    ;;

c)
    maxcolor=$1; shift
    case $maxcolor in *[!0-9]*)
        echo >&2 'invalid color number; setting to 256'
        maxcolor=256
    esac
    ;;

*)
    printf >&2 'ignoring command-line argument: %s\n' "$arg"

esac done

c=$(tput cols)

i=0
l=0
while [ $i -lt $maxcolor ]
do
    if $numcolor
    then printf '%s%03d%s %s  %s' "$(FG $i)" $i "$reset" "$(BG $i)" "$reset"
    else printf '%03d %s  %s' $i "$(BG $i)" "$reset"
    fi
    # The length of the resulting string is 6 (be careful if you change it)
    # Plus we'll put 2 spaces between each string
    # So this is how we decide when to put a newline:
    if [ $((8*($i+1-($c/8+$c%8/6)*$l)+6)) -gt $c ]
    # Remember all divisions are rounded down immediately
    # We add 1 to i because it started from 0
    then l=$(($l+1)); echo
    else printf '  '
    fi
    i=$(($i+1))
done
echo