#!/bin/sh -e

# Copyright (c) 2023 Emma Tebibyte 
# SPDX-License-Identifier: FSFAP
#
# Copying and distribution of this file, with or without modification, are
# permitted in any medium without royalty provided the copyright notice and this
# notice are preserved.	This file is offered as-is, without any warranty.

argv0="$0"

if test -z "$1"; then
	printf "Usage: %s file...\n" "$argv0"
fi

help="Commands:\n\
a – appends a line to the current buffer.\n\
c – clears the screen.\n\
d – deletes a line.\n\
e – evaluates a command in the shell environment.\n\
h – displays this help message.\n\
i – inserts a line into the buffer at position line.\n\
p – prints the current buffer.\n\
q – terminates editing.\n\
r – applies a regular expression to the current buffer.\n\
w – writes the current buffer to the file.\n\
\n\
To learn more about a command verb, type \`h verb'.
"

while test -n "$1"; do
	content="$(cat "$1")"
	cat "$1"

	while true; do
		printf ">> "
		read -r command args

		case $command in
			"a")
				read -r append
				content="$(printf "%s\n%s" "$content" "$append")"
				;;
			"c")
				clear
				;;
			"d")
				printf "%s: d: Command not implemented.\n" "$argv0"
				;;
			"e")
				eval "$args"
				;;
			"h")
				if test -z "$args"; then
					printf "$help\n"
				else
					printf "%s: Command-specific help not implemented.\n" "$argv0"
					continue
					case $args in
						"c")
							;;
						"d")
							;;
						"e")
							;;
						"h")
							;;
						"i")
							;;
						"p")
							;;
						"q")
							;;
						"r")
							;;
						"w")
							;;
					esac
				fi
				;;
			"i")
				printf "%s: i: Command not implemented.\n" "$argv0"
				;;
			"p")
				printf "%s\n" "$content"
				;;
			"q")
				if test -n "$(printf "%s\n" "$content" | diff "$1" -)"
				then
					printf "%s: %s: Unsaved changes in the buffer. Quit anyway? [y/N] " \
						"$argv0" \
						"$1"
					read quit
					if [ "$quit" = "y" ]; then
						break
					else
						continue
					fi
				else
					break
				fi
				;;
			"r")
				content="$(printf "%s" "$content" | sed "$args")"
				;;
			"w")
				printf "%s\n" "$content" > "$1"
				;;
		esac
	done

	shift
done