#!/bin/sh
# IMPORTANT NOTE: The "local" keyword is not defined in POSIX, so I've decided to work without it.
#                 For this reason, I do "$X/$f" instead of assigning local variables.
#                 Another thing to consider is that in recursive functions, backtracking logic
#                 CANNOT depend on variables, which are changed somewhere in the body before
#                 the recursive step.

cd "${0%/*}"

[ -z "$HOSTNAME" ] && HOSTNAME="$(hostname)"
if [ -z "$HOSTNAME" ]
then
	echo "Set HOSTNAME variable to the system's name!" >/dev/stderr
	exit 1
fi
if [ ! -f "./SYSTEM/$HOSTNAME" ]
then
	echo "No configuration in ./SYSTEM for '$HOSTNAME'!" >/dev/stderr
	exit 1
fi

# __merge_link <.FILES_DIR> <HOME_DIR>
__merge_link() {
	while read -r f
	do
		# Output file doesn't exist -> Create link
		# This is also true on broken symlinks
		if [ ! -e "$2/$f" ]
		then
			if [ -h "$2/$f" ]
			then
				echo "Warn: '$2/$f' is a broken link to '$(readlink "$2/$f")', updating!"
				rm "$2/$f"
			fi
			ln -s "$1/$f" "$2/$f"
		# Output file is already linked to current -> Do nothing
		elif [ -h "$2/$f" -a "$(readlink $2/$f)" = "$1/$f" ]
		then
			:
		# Current file is a file (and output exists) -> Create link but also update current
		elif [ -f "$1/$f" ]
		then
			if [ -f "$2/$f" ]
			then
				# When output is NOT a link inside .files,
				# update current file with the contents of the output
				# Otherwise,
				# replace output file with (link to) current
				if ! cmp -s "$1/$f" "$2/$f"
				then
					if [ -h "$2/$f" ]
					then
						real="$(readlink "$2/$f")"
						if [ "${real#$PWD}" = "$real" ]
						then
							cp "$real" "$1/$f" 2>/dev/null
							echo "Warn: '$2/$f' already existed, '$1/$f' updated with its contents!" >/dev/stderr
						fi
					else
						cp "$2/$f" "$1/$f"
						echo "Warn: '$2/$f' already existed, '$1/$f' updated with its contents!" >/dev/stderr
					fi
				fi
				rm "$2/$f"

				ln -s "$1/$f" "$2/$f"
			else
				echo "Error: '$2/$f' already exists and is not a regular file!"
			fi
		# Current file is a directory (and output exists) -> Link current's files & dirs in output directory
		elif [ -d "$1/$f" ]
		then
			if [ -d "$2/$f" ]
			then
				# If output is a link to directory in .files, continuing with merge
				# will place links inside a directory in .files, which is bad.
				# Instead, we'll make the link to directory become a directory with
				# links to all inner files.
				if [ -h "$2/$f" ]
				then
					real="$(realpath "$2/$f")"
					if [ "${real#$PWD}" != "$real" ]
					then
						rm "$2/$f"
						mkdir "$2/$f"
						while read -r ff
						do
							ln -s "$real/$ff" "$2/$f/$ff"
						done <<FILES
$(ls -A1 "$real")
FILES
					fi
				fi
				__merge_link "$1/$f" "$2/$f"
			else
				echo "Error: '$2/$f' already exists and is not a directory!"
			fi
		else
			echo "Error: '$1/$f' is neither a file or directory!"
		fi
	done <<FILES
$(ls -A1 "$1")
FILES
}

link() {
	while read -r dir
	do
		dir="${dir%%*/}"
		if [ ! -d "./$dir" ]
		then
			echo "Unknown directory '$dir'!" >/dev/stderr
			continue
		fi

		__merge_link "$PWD/$dir" "$HOME"
		echo "Linked $dir"
	done <"./SYSTEM/$HOSTNAME"
}

# __remove_link <.FILES_DIR> <HOME_DIR>
__remove_link() {
	while read -r f
	do
		if [ -h "$2/$f" ]
		then
			real="$(realpath "$2/$f")"
			# Output file exists and is a link to current -> Remove output
			if [ "$real" = "$1/$f" ]
			then
				rm "$2/$f"
				continue
			# Output file exists and is a link in .files -> Do nothing (do not remove from .files)
			# This is possible in the case when we have .files/X/A and .files/Y/A, and we linked
			# .files/X but are now unlinking .files/Y
			elif [ "${real#$PWD}" != "$real" ]
			then
				continue
			fi
		fi
		if [ -d "$1/$f" -a -d "$2/$f" ]
		then
			__remove_link "$1/$f" "$2/$f"
		fi
	done <<FILES
$(ls -A1 "$1")
FILES
	# See IMPORTANT NOTE, since we're aiming for full POSIX compatibility,
	# the variable f will be overwritten by recursive calls, so this loop
	# cannot be merged with the loop above (remove dirs on backtracking)
	while read -r f
	do
		# Output file exists and is an empty directory -> Remove it
		# We're assuming this directory was created by the script and/or the user
		# will not care for empty dirs
		if [ -d "$2/$f" -a -z "$(ls -A "$2/$f" 2>/dev/null)" ]
		then
			rm -r "$2/$f"
		fi
	done <<FILES
$(ls -A1 "$1")
FILES
}

unlink() {
	while read -r dir
	do
		dir="${dir%%*/}"
		if [ ! -d "./$dir" ]
		then
			echo "Unknown directory '$dir'!" >/dev/stderr
			continue
		fi

		__remove_link "$PWD/$dir" "$HOME"
		echo "Unlinked $dir"
	done <"./SYSTEM/$HOSTNAME"
}

case "$1" in
	(unlink) unlink ;;
	(link|*) link ;;
esac