# make sure to put a shebang here

readc() { # arg: <variable-name>
	# adapted from https://unix.stackexchange.com/a/464963
	if test -t 0 ; then
		# if stdin is a tty device, put it out of icanon, set min and
		# time to sane value, but don't otherwise touch other input or
		# or local settings (echo, isig, icrnl...). Take a backup of the
		# previous settings beforehand.
		saved_tty_settings=$(stty -g)
		stty -icanon min 1 time 0
	fi

	# Sanitize any variable that's passed in
	case "$1" in
		c) var=_c ;;
		var) var=_var ;;
		'') var=REPLY ;;
		*) var="$1" ;;
	esac

	# Zero-out the variable
	eval "$var="

	# Read one full *character*, not *byte*
	while :; do
		c=$(dd bs=1 count=1 2>/dev/null; echo .)
		c=${c%.}

		# Break on EOF
		test -n "$c" || break

		eval "$var=\"\${$var}$c\""

		# Break when we have a full character (wc -m)
		test "$(eval printf %s "\"\$$var\"" | wc -m)" -eq 0 || break
	done

	# Round-trip sanitized variables
	test "x$var" = x_c && eval c="\$$var"
	test "x$var" = x_var && eval var="\$$var"

	if [ -t 0 ]; then
		# restore settings saved earlier if stdin is a tty device.
		stty "$saved_tty_settings"
	fi
}

setup_terminal() {
	# Setup the terminal for the TUI.
	# '\e[?1049h': Use alternative screen buffer.
	# '\e[?7l':    Disable line wrapping.
	# '\e[?25l':   Hide the cursor.
	# '\e[2J':     Clear the screen.
	# '\e[1;Nr':   Limit scrolling to scrolling area.
	#              Also sets cursor to (0,0).
	printf '\e[?1049h\e[?7l\e[2J\e[1;%sr' "$((LINES - 1))"

	# Hide echoing of user input
	stty -echo
}

reset_terminal() {
	# Reset the terminal to a useable state (undo all changes).
	# '\e[?7h':   Re-enable line wrapping.
	# '\e[?25h':  Unhide the cursor.
	# '\e[2J':    Clear the terminal.
	# '\e[;r':    Set the scroll region to its default value.
	#             Also sets cursor to (0,0).
	# '\e[?1049l: Restore main screen buffer.
	printf '\e[?7h\e[?25h\e[2J\e[;r\e[?1049l'

	# Show user input.
	stty echo
}

printlines() { # printlines file min max
	if test -n "$DEBUG"; then
		echo --- >&2
	else
		clear
	fi
	sed -n "$2,$3p" "$1"
	notify_flush
}

notify() { # notify STRING
	notification="$notification$1"
}

notify_flush() { # notify_flush
	winch
	printf '\e[s\e[%s;%sH' 0 "$c"
	printf '%s' "$notification"
	printf '\e[u'
	notification=
}

scroll() { # scroll nlines -- MUTATES GLOBAL STATE
	arg="${1:-0}"
	case "$arg" in
		+*|-*) n=$((n+${arg#+})) ;;
		0) ;;
		*) n=$1 ;;
	esac

	if test $n -gt $((max - l))
	then
		n=$((max - l))
		notify '_'
	fi

	if test $n -lt $min
	then
		notify '^'
		n=$min
	fi

	m=$((n+l))
	if test $m -ge $max
	then
		m=$max
	fi
	# echo "min:$min max:$max n:$n m:$m l:$l" >&2
}

ctty() { # ctty
	## Get the controlling TTY interface.
	if test -t 2
	then
		# only works if not redirecting stderr
		tty <&2
	else
		echo /proc/self/fd/1 # probably only works on linux
	fi
}

winch() {
	stty size > "$wf"
	read -r l c < "$wf"
	# echo l:$l c:$c >&2
	l=$((l-2)) # last line
}

cleanup() {
	rm "$bf" "$wf"
	reset_terminal
	exit
}

vellum() { # vellum [-n NUM] < INPUT
	# Set up buffer
	bf="$(mktemp)" # buffer
	cat "${@:--}" > "$bf"
	# Redirect input to the controlling terminal
	## XXX: This assumes that you haven't redirected stderr.
	exec < "$(ctty)"
	max=$(wc -l < "$bf")
	min=1 # min = minimum accessible line num.
	n=$min # n = num. first line to show

	# Set up terminal
	setup_terminal
	wf="$(mktemp)" # winch file
	winch; trap winch WINCH
	m=$((n+l)) # m = num. last line to show

	trap cleanup EXIT INT QUIT

	# Main loop
	while :; do
		scroll
		printlines "$bf" $n $m

		readc key >/dev/null 2>&1
		case "$key" in
			' ') scroll +$l ;;
			'') scroll -$l ;;
			j) scroll +1 ;;
			k) scroll -1 ;;
			g) scroll $min ;;
			G) scroll $max ;;
			q) break ;;
		esac
	done
	break
	echo
}

test -n "$DEBUG" && { set -x; exec 2> /tmp/vellum.log; }
test -z "$SOURCE" && vellum "$@"