#!/bin/bash --noprofile

# DO ALMOST ANYTHING PUBLIC LICENSE
# Everyone is permitted to copy and distribute verbatim or modified
# copies of this code for any purpose as long as any "Credit"
# lines are not removed.

#Credit: /u/linuxversion
#<your Credit line here, if you want>

#extra strict flags
set -euo pipefail

#so I don't accidently forget whats builtin and whats not
PATH=''

#configuration options
declare -i cfg_line_numbers=1

#globals
declare -a file_map
declare -i file_offset=0
declare -i file_lines=0
declare -i term_height=0
declare -i term_width=0
declare -i textarea_height=0
declare -i textarea_line=0
declare -i command_line=0

#move the cursor to a specific row and colunm
function put_cursor() {
	echo -en "\e[${1};${2}f"
}

#basically the same as the libncurses function of the same name
function mvprintw() {
	local row="$1"; shift
	local col="$1"; shift
	
	printf "%s%s%s" $'\x1b[' "${row};${col}f" "$@"
}

#self-explanitory
function clear_screen() {
	echo -en '\e[2J'
}

#erase text from command line to bottom of screen
function clear_command_line() {
	mvprintw "$command_line" 0 $'\x1b[J'
}

function init_screen() {
	#set terminal configuration to default
	#echo -en '\ec'
	
	#get the size of the terminal
	put_cursor 999 999
	echo -en '\e[6n'
	read -s -r -d'['
	read -s -r -d';' term_height
	read -s -r -d'R' term_width
	
	#do some math to position the various graphical elements
	textarea_line=0
	textarea_height=$term_height
	command_line=$term_height
	
	clear_screen
	put_cursor "$command_line" 0
}

#draw text on the screen from memory with offset
function render_text() {
	clear_screen
	
	#figure out how many lines we can render
	local modifier="$1"
	local lines=$textarea_height
	local last_screen=$((file_lines - textarea_height))
	
	file_offset=$((file_offset + modifier))
	
	[ $file_offset -gt $last_screen ] && file_offset=$last_screen
	[ $file_offset -lt 0 ] && file_offset=0
	[ $lines -gt $file_lines ] && lines=$file_lines
	
	#draw text in the viewing area
	local fd_offset
	local term_offset
	for (( i=0; i<lines; ++i ))
	do
		fd_offset=$((i+file_offset))
		term_offset=$((i+textarea_line))
		[ $cfg_line_numbers -eq 1 ] && {
			mvprintw "${term_offset}" 0 "${fd_offset}"
			mvprintw "${term_offset}" 6 "${file_map[$fd_offset]}"
		} || {
			mvprintw "${term_offset}" 0 "${file_map[$fd_offset]}"
		}
	done
}

function main() {
	#arg check
	if (( $# > 0 )) && [ -f "$1" ]
	then 
		mapfile file_map <"$1"
	else	
		echo 'Usage: bash_pager <file>'
		return 1
	fi
	
	file_lines="${#file_map[@]}"
	
	init_screen
	render_text 0
	
	#read keys and perform actions
	while read -s -r -N1
	do
		case "$REPLY" in
		( $'\x1b' ) #ESC pressed, handle sequence
			read -s -r -N1
			case "$REPLY" in
			( $'\x1b' ) #second escape, eval user input
				read -e -r -p ': '
				set +eu
				eval "$REPLY"
				set -eu
				;;
			( '[' ) #arrow key or pgup/pgdown
				read -s -r -N1
				case "$REPLY" in
				( 'A' ) #arrow up
					render_text -1
					;;
				( 'B' ) #arrow down
					render_text 1
					;;
				( '5' ) #pgup
					render_text "-$((textarea_height/2))"
					;;
				( '6' ) #pgdown
					render_text "$((textarea_height/2))"
					;;
				esac
				;;
			esac
			;;
		( 'q' ) #quit
			return 0
			;;
		esac
		
		#input keys have been read display results
		clear_command_line
		put_cursor "$command_line" 0
	done
}

main $@