#!/bin/sh

set -eu

realpath_canonical() {
	# resolve symlink (if there is one) and use $@ to detect symlink loops
	while [ -L "$1" ] && target=$(readlink -- "$1"); do
		REPLY=$(dirname "$1")
		# Resolve relative to symlink's directory
		if [ "$REPLY" = "." ]; then
			REPLY=$target
		else
			case "$target" in
				/*)
					REPLY=$target;;
				*)
					REPLY=$REPLY/$target;;
			esac
		fi
		# Break out if we found a symlink loop
		for target in "$@"; do [ "$REPLY" = "$target" ] && break 2; done
		# Add to the loop-detect list and tail-recurse
		set -- "$REPLY" "$@"
	done
	# recurse unless root
	REPLY=$(dirname "$1")
	if [ "$REPLY" != "/" ] && [ "$REPLY" != "." ]; then
		REPLY=$(realpath_canonical "$REPLY");
	fi
	# set $1 to dirname and $2 to basename
	set -- "$REPLY" "$(basename "$1")"
	# combine canon parent w/basename
	REPLY=$PWD
	while [ $# -gt 0 ]; do
		# handle case of component starting with double slash and no
		# other slashes separately because it cannot be solved by
		# globbing
		if echo "$1" | grep -E '^//[^/]*$' >/dev/null 2>&1; then
			REPLY=//
			first=$(echo "$1" | cut -c3-)
			shift
			set -- "$first" "$@"
			continue
		fi
		# all other cases can be expressed by globbing
		case $1 in
			/*)
				REPLY=/
				first=$(echo "$1" | sed -e 's/^\/\+//')
				shift
				set -- "$first" "$@"
				;;
			*/*)
				front=${1%%/*}
				back=$(echo "$1" | sed -e "s/^$front\\/\\+//")
				shift
				set -- "$front" "$back" "$@"
				;;
			''|.)
				shift
				;;
			..)
				REPLY=$(dirname "$REPLY")
				shift
				;;
			*)
				REPLY="${REPLY%/}/$1"
				shift
				;;
		esac
	done
	echo "$REPLY"
}

if [ $# -ne 1 ]; then
	echo "usage: $0 path"
	exit 1
fi

realpath_canonical "$1"