#!/bin/sh
countd() { # count decimal places
  m="$1"; n=${2:-0}; while [ "$m" ]; do
    : $((n+=1))
    next="${m%?}"
    [ "${m#$next}" = "." ] && {
      : $((n-=1))
      break
    } || {
      m="$next"
    }
  done
  echo "$n"
}
ad(){ # add decimal to $1 based on $2
  m="$1"; n=0; while [ "$m" ]; do
    next="${m%?}"
    [ "$n" -eq "$2" ] && break
    e="${m#$next}${e}"
    m="$next"; : $((n+=1))
  done
  w="${1%$e}"; echo "${w:-0}.${e%0}"
}
[ -z "${1##*.*}" -o -z "${2##*.*}" ] || { # if not floats
  echo "$(($1*$2))"
  exit 0
}
if [ ! -z "${1##*.*}" ]; then
  set -- "${1}.0" "$2"; d="${1#*.}"; d2="${2#*.}"
elif [ ! -z "${2##*.*}" ]; then
  set -- "$1" "${2}.0"; d="${2#*.}"; d2="${1#*.}"
fi # if $1/$2 is not a float; convert to float
dn="$(countd $2 $(countd $1))"; m1="${1%.*}${1#*.}"; m2="${2%.*}${2#*.}"
until [ "${m1#0}" = "$m1" ]; do
  m1="${m1#0}"
done
until [ "${m2#0}" = "$m2" ]; do
  m2="${m2#0}"
done # dash has issues when numbers have leading 0's

# zsh,bash,etc don't need this at all
# and really dash shouldn't either but /shrug
#printf '%s\n%s\n' "${m1}*${m2}" "9223372036854775807"
ad "$((m1*m2))" "$dn"