#!/bin/sh
# term-background: print "light" or "dark" for the current terminal.
#
# Asks the terminal for its background color with the OSC 11 escape sequence,
# converts the reply to perceived luminance, and prints "light" or "dark".
# Prints "dark" (the safe default) when the terminal does not answer.
#
# All terminal I/O happens on /dev/tty, so this is safe inside $(...).
#
# Works locally and over SSH (the query reaches whichever real terminal draws
# the screen). Inside tmux/screen the multiplexer may answer instead of the
# outer terminal, so detection can be wrong there; override it with
#   export TERM_BACKGROUND=light   # or dark
# in ~/.environment.local on that machine.
#
# Tune the response wait via TERM_BACKGROUND_TIMEOUT (tenths of a second).

# Need a controlling terminal to talk to.
[ -t 0 ] || { echo dark; exit 0; }
exec 3<>/dev/tty || { echo dark; exit 0; }

# Raw mode with a short read timeout (min 0 time N = up to N*0.1s) so an
# unresponsive terminal costs us at most that long.
saved=$(stty -g <&3 2>/dev/null) || { echo dark; exit 0; }
stty raw -echo min 0 time "${TERM_BACKGROUND_TIMEOUT:-4}" <&3 2>/dev/null

printf '\033]11;?\033\\' >&3            # OSC 11 query
answer=$(dd bs=64 count=1 <&3 2>/dev/null)

stty "$saved" <&3 2>/dev/null           # restore before anything else
exec 3>&-

# Expected reply: ESC ] 11 ; rgb:RRRR/GGGG/BBBB (ESC \ | BEL)
case "$answer" in
    *rgb:*) ;;
    *) echo dark; exit 0 ;;
esac

rgb=${answer#*rgb:}                      # RRRR/GGGG/BBBB<terminator>
r=${rgb%%/*}; rgb=${rgb#*/}
g=${rgb%%/*}; rgb=${rgb#*/}
b=$rgb

# Keep the most-significant two hex digits of each component.
r=${r%"${r#??}"}
g=${g%"${g#??}"}
b=${b%"${b#??}"}

case "$r$g$b" in
    ""|*[!0-9a-fA-F]*) echo dark; exit 0 ;;
esac

r=$(( 0x$r )); g=$(( 0x$g )); b=$(( 0x$b ))

# Rec. 601 perceived luminance, 0-255. Over half => light.
lum=$(( (r * 299 + g * 587 + b * 114) / 1000 ))
if [ "$lum" -gt 127 ]; then
    echo light
else
    echo dark
fi