#!/bin/bash
set -e
set -f
set -o pipefail
shopt -s lastpipe
PROGNAME=${0##*/}

BOT_SIZE=8
BOT_TITLE=input
BOT_CMD=start
TOP_CMD='PS1="$TOP_TITLE\$ " bash --noprofile --norc -i'
TOP_TITLE=output

TWOPANE=$(mktemp -d)
export TWOPANE TOP_CMD BOT_CMD TOP_TITLE BOT_TITLE BOT_SIZE
trap 'rm -r "$TWOPANE"' EXIT
STY=twopane.${TWOPANE##*/}

save_file()
{
        cat > "$TWOPANE"/"${1:?$PROGNAME: Error: filename cannot be empty string}"
}
save_screenrc()
{
        save_file screenrc"${1:+.$1}"
}

save_screenrc <<'.'
unbindall
escape \0\0
caption string '%t'
layout new
split
focus bottom
resize $BOT_SIZE
screen -ln -t "$BOT_TITLE" 0 bash --noprofile --rcfile "$TWOPANE"/bashrc -i

source "$TWOPANE"/screenrc.startpane
.

save_screenrc 'startpane' <<'.'
focus top
screen -ln -t "$TOP_TITLE" 1 sh -c "$TOP_CMD"
exec .!. socat -u UNIX-RECV:"$TWOPANE"/socket STDOUT
title 'output'
focus bottom
layout save 0
.

save_screenrc 'restart' <<.
layout new
split
focus bottom
resize $BOT_SIZE
select 0
title 'input'

source "$TWOPANE"/screenrc.startpane
.

restart()
{
        screen -X source "$TWOPANE"/screenrc.restart
}
sendp()
{
        trap "kill -SIGUSR1 $1" EXIT
        socat -u STDIN UNIX-SENDTO:"$TWOPANE"/socket || restart
}
sendc()
{
        [ "${COPROC[1]}" ] || coproc { sendp $$; }
        printf '%s' "$*" >&${COPROC[1]}
}
send()
{
        [ "${COPROC[1]}" ] || coproc { sendp $$; }
        printf '%s' "$*"$'\n' >&${COPROC[1]}
}

sendcc() { sendc $'\003'; }

raw()
{
        stty_opts=(
                cbreak
                raw
                -isig
                -iexten
                -nl
                intr    undef
                quit    undef
               #erase   undef
               #kill    undef
                eof     undef
                start   undef
                stop    undef
                susp    undef
                lnext   undef
        )
        stty "${stty_opts[@]}"
}

start()
{
        trap ': SIGPIPE ${COPROC[@]@A}' SIGPIPE
        trap ': SIGCHLD ${COPROC[@]@A}' SIGCHLD
        trap ': SIGUSR1 ${COPROC[@]@A}; COPROC=()' SIGUSR1
        raw
        while read -r -N1 || [ $? -gt 128 ]
        do
                if [ "$REPLY" ]
                then
                        sendc "$REPLY"
                fi
        done
}

save_file bashrc <<.
set -f
set -b
set -o pipefail
shopt -s lastpipe
trap "screen -X quit" EXIT
$(declare -f)
export PS1="$BOT_TITLE\$ "
\$BOT_CMD
.

main()
{
        screen -c "$TWOPANE"/screenrc -m -S "$STY" -ln
}

main "$@"