make-pi - Experimental approach to calculate Pi with up to 15 decimal places using Euler method

#!/bin/sh

BASE=4
RES=0
RES_PREV=0
ROUND=3    # number of digits used for rounding
PREC=1     # holds number of iterations with identical results bofore exiting
X=2

if [ -n "${1##*[!0-9]*}" ] && [ $1 -le 15 ] && [ $1 -gt 0 ]; then
    DIGIT=$(($1+$ROUND))
else
    DIGIT=18
fi

while [ $((DIGIT=$DIGIT-1)) -ge 0 ]; do
    BASE=$(($BASE*10))
done
ROUND=$(printf 1'%'$ROUND's' | tr ' ' 0)
while [ $PREC -ge 0 ]; do
    RES=$(($RES+($BASE/($X*$((X=$X+1))*$((X=$X+1))))-($BASE/($X*$((X=$X+1))*$((X=$X+1))))))
    if [ $RES_PREV -eq $RES ]; then
        PREC=$(($PREC-1))
    else
        ITERATION=$(($ITERATION+1))
        RES_PREV=$RES
        PREC=$(($ITERATION/10+10))
        printf "\r3."$((($RES+$ROUND*10/18)/$ROUND))
    fi
done
printf "\nUsed iterations: $ITERATION\n"