shtikl/lists.sh

quote_list_element() {
	case "$1" in
		# The string may need either type of quoting if it contains \, {, or }
		*[\\{}]*) ;;
		# A string with a space but no \, {, or } can be brace-quoted
		# Same is true of the empty string
		*\ *|'')
			ret="{$1}"
			return
			;;
		# A non-empty string without any of the above can be left as-is
		*)
			ret="$1"
			return
			;;
	esac

	backslashed=''
	can_use_braced=yes

	after_backslash=no
	brace_nesting_depth=0

	# https://blog.dnmfarrell.com/post/how-to-split-a-string-in-posix-shell/
	# We know $1 can't be '-' since that doesn't need quoting
	OPTIND=1
	while getopts ':' _ "-$1"
	do
		char="${OPTARG:-:}"

		case "$after_backslash $brace_nesting_depth $char" in
			no\ *\ {) brace_nesting_depth=$((brace_nesting_depth + 1)) ;;
			no\ 0\ }) can_use_braced=no ;;
			no\ *\ }) brace_nesting_depth=$((brace_nesting_depth - 1)) ;;
		esac

		case "$char" in
			[\ \\{}]) backslashed="$backslashed\\" ;;
		esac
		backslashed="$backslashed$char"

		case "$char" in
			\\) after_backslash=yes ;;
			*) after_backslash=no ;;
		esac
	done

	# A brace-quoted string can't end with a backslash
	# $char must have been set, since the empty string is handled above
	case $after_backslash in
		yes) can_use_braced=no ;;
	esac

	# Strings with mismatched braces can't be brace-quoted
	case $brace_nesting_depth in
		0) ;;
		*) can_use_braced=no ;;
	esac

	# Prefer brace-quoting whenever possible
	case $can_use_braced in
		yes) ret="{$1}" ;;
		no) ret="$backslashed" ;;
	esac
}

uncons_list() {
	case "$1" in
		# Handle '-' here specially, since getopts wouldn't interpret it right
		-)
			ret='-'
			ret_rest=''
			return
			;;
	esac

	quoted_element=''
	unquoted_element=''

	parser_state=start
	after_backslash=no
	brace_nesting_depth=0

	OPTIND=1
	while getopts ':' _ "-$1"
	do
		char="${OPTARG:-:}"

		case "$parser_state $after_backslash $brace_nesting_depth $char" in
			# The character following a backslash is never special
			*\ yes\ *\ *)
				after_backslash=no
				unquoted_element="$unquoted_element$char"
				;;

			start\ no\ 0\ \ ) ;;
			start\ no\ 0\ {)
				parser_state=braced
				brace_nesting_depth=1
				;;
			start\ no\ 0\ \\)
				parser_state=non-braced
				after_backslash=yes
				;;
			start\ no\ 0\ *)
				parser_state=non-braced
				unquoted_element="$char"
				;;

			# Non-quoted space ends an element
			*\ no\ 0\ \ ) break ;;

			braced\ no\ 0\ *)
				error "unexpected '$char' directly after a list element in braces"
				;;

			# Consume the backslash in non-brace-quoted elements
			non-braced\ no\ *\ \\) after_backslash=yes ;;
			# but leave it as-is in brace-quoted
			braced\ no\ *\ \\)
				after_backslash=yes
				unquoted_element="$unquoted_element\\"
				;;

			# Consume the final closing brace of a brace-quoted element
			braced\ no\ 1\ }) brace_nesting_depth=0 ;;

			braced\ no\ *\ {)
				brace_nesting_depth=$((brace_nesting_depth + 1))
				unquoted_element="$unquoted_element{"
				;;
			braced\ no\ *\ })
				brace_nesting_depth=$((brace_nesting_depth - 1))
				unquoted_element="$unquoted_element}"
				;;

			*) unquoted_element="$unquoted_element$char" ;;
		esac

		quoted_element="$quoted_element$char"
	done

	case $brace_nesting_depth in
		0) ;;
		*) error "unmatched '{' in list" ;;
	esac

	# A trailing backslash is always included in the element
	case $after_backslash in
		yes) unquoted_element="$unquoted_element\\" ;;
	esac

	ret="$unquoted_element"
	ret_rest="${1#"$quoted_element"}"
}

list() {
	accumulated_list=''
	for list_element
	do
		quote_list_element "$list_element"
		accumulated_list="$accumulated_list $ret"
	done
	# Remove the extraneous space from the beginning of the list
	ret="${accumulated_list# }"
}

llength() {
	list_length=0

	while :
	do
		case "$1" in
			# Remaining non-whitespace characters = list not empty
			*[!\ ]*)
				uncons_list "$1"
				set -- "$ret_rest"
				list_length=$((list_length + 1))
				;;
			*) break ;;
		esac
	done

	ret=$list_length
}

lindex() {
	current_list="$1"
	shift

	while :
	do
		while :
		do
			# Get the first index, if any, out of the index list
			case "$1" in
				*[!\ ]*)
					uncons_list "$1"
					index="$ret"
					shift
					set -- "$ret_rest" "$@"
					break
					;;
				*)
					# No more remaining elements, try the next argument
					shift
					case $# in
						0) break 2 ;;
					esac
					;;
			esac
		done

		while :
		do
			uncons_list "$current_list"
			case "$index" in
				0)
					current_list="$ret"
					break
					;;
				*)
					index=$((index - 1))
					current_list="$ret_rest"
					;;
			esac
		done
	done

	ret="$current_list"
}