#!/bin/ksh
# to_base n m
# Convert n from decimal to base m
#
# William Robertson - www.williamrobertson.net
# Originally a prototype for equivalent function in PL/SQL when no database was available.
# Since writing this I have discovered the extended typeset -i syntax, so most of this could
# actually be achieved by:
# typeset -i$2 v_result=$1

if (( $# != 2 )); then
	# Two parameters required, else return error and usage message:
	print -u2 "$0: Usage: to_base n m\nConverts n from decimal to base n"
	exit 1
fi

# Easy way using subsequently discovered typeset built-in:
typeset -i${2} v_result_string=$1
# print ${v_result_string#*#}

# The Hard Way: 
typeset -i p_decimal=$1 v_decimal_remaining=$1 p_base=${2:-10} v_digit

if (( p_base > 36 ))
then
	print -u2 $0: Maximum base is 36
	exit 1
fi

function to_single_digit
{
	typeset -i v_decimal=$1
	
	if (( v_decimal < 10 )); then
		v_result=${v_decimal}
	else
		case ${v_decimal} in
			10 ) v_result=A ;;
			11 ) v_result=B ;;
			12 ) v_result=C ;;
			13 ) v_result=D ;;
			14 ) v_result=E ;;
			15 ) v_result=F ;;
			16 ) v_result=G ;;
			17 ) v_result=H ;;
			18 ) v_result=I ;;
			19 ) v_result=J ;;
			20 ) v_result=K ;;
			21 ) v_result=L ;;
			22 ) v_result=M ;;
			23 ) v_result=N ;;
			24 ) v_result=O ;;
			25 ) v_result=P ;;
			26 ) v_result=Q ;;
			27 ) v_result=R ;;
			28 ) v_result=S ;;
			29 ) v_result=T ;;
			30 ) v_result=U ;;
			31 ) v_result=V ;;
			32 ) v_result=W ;;
			33 ) v_result=X ;;
			34 ) v_result=Y ;;
			35 ) v_result=Z ;;
			*  ) print -u2 Digit ${v_decimal} too large; exit 1 ;;
		esac
	fi

	print ${v_result} 
}

while (( v_decimal_remaining > 0 ))
do
	(( v_digit = v_decimal_remaining % p_base ))
	v_result=$(to_single_digit ${v_digit} )${v_result}
	(( v_decimal_remaining = v_decimal_remaining / p_base ))
done 

# Self-diagnostic: ksh can convert base n to decimal using syntax n#value, e.g.
# print $(( 16#f ))
# 15
# so we can check results by converting back. This can be skipped unless decimal
# is over around 10**31 (pdksh on Apple Mac hits a limit and returns empty string).
if (( p_decimal = $(( ${p_base}#${v_result} )) ))
then
	print ${v_result}
else
	print -u2 "ERROR: result '${v_result}' is incorrect"
	exit 2
fi