#!/bin/bash
# Webcam presence detection daemon.
# Checks for motion at an adaptive interval (20s normally, backed off to 120s
# under heavy system load) and shares caffeine's systemd-inhibit idle lock
# while the user is detected, so hypridle never fires during an active session.
#
# Camera selection: set PRESENCE_DETECT_CAMERA env var or write
#   CAMERA=<id>  to ~/.config/presence-detect.conf
#
# Exit codes from python helper: 0=motion, 1=no motion, 2=camera error (busy/unavailable)
# On camera error the daemon silently skips the tick and leaves the inhibit
# state exactly as it was on the previous run.

# Resolve the script's real directory so the Python helper path stays valid
# even when invoked via a symlink or from a different cwd.
SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
PYTHON_DETECT="$SCRIPT_DIR/python/presence_detect.py"
# Shared with caffeine.sh: both the manual toggle and this daemon drive the
# same systemd-inhibit lock, so caffeine-status.sh reflects either source.
PID_FILE="/tmp/caffeine-inhibit.pid"
# Marks that *this daemon* (not the manual caffeine toggle) currently holds
# the lock, so a "no motion" tick never releases a manually-started session.
OWNED_FLAG="/tmp/presence-inhibit-owned"
PRESENCE_CFG="${XDG_CONFIG_HOME:-$HOME/.config}/presence-detect.conf"

INTERVAL_IDLE=20     # seconds between checks under normal load
INTERVAL_BUSY=120    # seconds between checks when CPU or RAM is >= LOAD_THRESHOLD
LOAD_THRESHOLD=0.70
NPROC="$(nproc)"

# Resolve camera ID: env var takes highest priority, then config file, then default 0.
_camera_id() {
    if [[ -n "$PRESENCE_DETECT_CAMERA" ]]; then
        echo "$PRESENCE_DETECT_CAMERA"
    elif [[ -f "$PRESENCE_CFG" ]]; then
        # -oP 'CAMERA=\K[0-9]+': Perl-style look-behind strips "CAMERA=" prefix.
        grep -oP 'CAMERA=\K[0-9]+' "$PRESENCE_CFG" 2>/dev/null || echo 0
    else
        echo 0
    fi
}

# True if 1-minute load average (normalized by core count) or RAM usage is
# at/above LOAD_THRESHOLD. Load average is a cheap, sampling-free proxy for
# "CPU busy" — avoids adding an extra measurement delay to every tick.
_system_busy() {
    local load1 cpu_busy mem_frac mem_busy
    load1="$(awk '{print $1}' /proc/loadavg)"
    cpu_busy="$(awk -v l="$load1" -v n="$NPROC" -v t="$LOAD_THRESHOLD" \
        'BEGIN{print (l/n >= t) ? 1 : 0}')"
    [[ "$cpu_busy" == "1" ]] && return 0

    mem_frac="$(free | awk '/^Mem:/ {print $3/$2}')"
    mem_busy="$(awk -v m="$mem_frac" -v t="$LOAD_THRESHOLD" 'BEGIN{print (m >= t) ? 1 : 0}')"
    [[ "$mem_busy" == "1" ]]
}

_next_interval() {
    if _system_busy; then echo "$INTERVAL_BUSY"; else echo "$INTERVAL_IDLE"; fi
}

# Returns true if the inhibitor sentinel process is still alive.
_inhibit_running() {
    [[ -f "$PID_FILE" ]] && kill -0 "$(cat "$PID_FILE")" 2>/dev/null
}

_start_inhibit() {
    # Guard: don't start a second inhibitor if one is already active — whether
    # it's ours from a previous tick or a manually-started caffeine session.
    _inhibit_running && return
    # --what=idle: target the logind idle-inhibit lock that hypridle polls.
    # "sleep infinity" is the sentinel; its PID is saved so we can kill it later.
    systemd-inhibit --what=idle --who="presence-detect" \
        --why="User presence detected" --mode=block \
        sleep infinity &
    echo $! > "$PID_FILE"
    touch "$OWNED_FLAG"
    # logger writes to the system journal — visible via `journalctl -t presence-detect`.
    logger -t presence-detect "Motion detected — idle inhibited"
}

_stop_inhibit() {
    _inhibit_running || return
    # Never release a lock we didn't start — that would be a manual caffeine
    # session, which must persist regardless of presence.
    [[ -f "$OWNED_FLAG" ]] || return
    # Killing the sleep process releases the systemd-inhibit lock automatically.
    kill "$(cat "$PID_FILE")" 2>/dev/null
    rm -f "$PID_FILE" "$OWNED_FLAG"
    logger -t presence-detect "No motion — idle inhibit released"
}

_cleanup() {
    # On daemon stop (systemd unit stop, user logout, etc.), release the lock
    # cleanly, but only if we're the one holding it.
    _stop_inhibit
    exit 0
}
# Intercept termination signals to ensure the inhibitor PID is never orphaned.
trap _cleanup SIGTERM SIGINT SIGHUP

while true; do
    CAMERA="$(_camera_id)"
    # Run the OpenCV motion detector; stderr suppressed to keep the journal clean.
    python3 "$PYTHON_DETECT" "$CAMERA" 2>/dev/null
    rc=$?
    case $rc in
        0) _start_inhibit ;;
        1) _stop_inhibit  ;;
        # rc=2 = camera busy/unavailable — silently skip, state unchanged
    esac
    sleep "$(_next_interval)"
done