#!/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.
#
# Never opens the real camera directly for detection. Instead it mirrors the
# real camera into a v4l2loopback virtual device (/dev/video42, set up by the
# v4l2loopback.conf files under etc-v4l2loopback/ + the DE installer) and reads
# motion off that mirror, so this daemon never holds — or contends for — the
# physical camera that other apps (video calls, howdy) also want. If the
# loopback device isn't present (module not installed/loaded yet), it falls
# back to reading the real camera directly, same as before.
#
# 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"
# Records whether the camera currently SEES the user (motion on the last tick),
# INDEPENDENT of who holds the idle lock. Unlike OWNED_FLAG this is set even when
# a manual caffeine session already owns the lock (so the daemon never starts its
# own inhibitor). presence-status.sh reads this so the Eww widget can show the
# presence input as a distinct signal from a manual caffeine toggle.
PRESENCE_FLAG="/tmp/presence-detected"
PRESENCE_CFG="${XDG_CONFIG_HOME:-$HOME/.config}/presence-detect.conf"

# Must match the video_nr in etc-v4l2loopback/modprobe.d/v4l2loopback.conf.
LOOPBACK_ID=42
FEEDER_PID_FILE="/tmp/presence-loopback-feeder.pid"

INTERVAL_FAST=2      # near-idle system (< LOAD_LIGHT): poll fast for responsiveness
INTERVAL_IDLE=20     # seconds between checks under normal load
INTERVAL_BUSY=120    # seconds between checks when CPU or RAM is >= LOAD_BUSY
LOAD_BUSY=0.70       # CPU-or-RAM fraction at/above which we back off to INTERVAL_BUSY
LOAD_LIGHT=0.30      # CPU-or-RAM fraction below which we speed up to INTERVAL_FAST
NPROC="$(nproc)"

# Grace window: keep reporting "present" for this long after the last detected
# motion, so brief stillness (reading, thinking, a slow moment) never drops the
# lock the instant one tick sees nothing. Any motion tick resets the clock.
GRACE_SECONDS=90

# 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
}

# Prints current CPU-or-RAM usage as a fraction (0..1): the higher of the
# core-count-normalized 1-minute load average and the used-RAM fraction. Load
# average is a cheap, sampling-free proxy for "CPU busy" — no extra measurement
# delay per tick.
_resource_usage() {
    local load1 cpu_frac mem_frac
    load1="$(awk '{print $1}' /proc/loadavg)"
    cpu_frac="$(awk -v l="$load1" -v n="$NPROC" 'BEGIN{print l/n}')"
    mem_frac="$(free | awk '/^Mem:/ {print $3/$2}')"
    awk -v c="$cpu_frac" -v m="$mem_frac" 'BEGIN{print (c>m)?c:m}'
}

# Three-tier poll interval based on current resource usage:
#   >= LOAD_BUSY   -> INTERVAL_BUSY  (heavy load: back off, stay out of the way)
#   <  LOAD_LIGHT  -> INTERVAL_FAST  (near-idle: poll fast, cheap and responsive)
#   otherwise      -> INTERVAL_IDLE  (normal load)
_next_interval() {
    local u; u="$(_resource_usage)"
    awk -v u="$u" -v busy="$LOAD_BUSY" -v light="$LOAD_LIGHT" \
        -v fb="$INTERVAL_BUSY" -v ff="$INTERVAL_FAST" -v fi="$INTERVAL_IDLE" \
        'BEGIN{ if (u >= busy) print fb; else if (u < light) print ff; else print fi }'
}

# Returns true if the loopback feeder (ffmpeg mirroring real cam -> loopback)
# is still alive.
_feeder_running() {
    [[ -f "$FEEDER_PID_FILE" ]] && kill -0 "$(cat "$FEEDER_PID_FILE")" 2>/dev/null
}

# Starts the ffmpeg mirror if the loopback device exists and nothing is
# feeding it yet. No-op (returns failure) if the loopback isn't set up —
# callers fall back to reading the real camera directly in that case.
_start_feeder() {
    _feeder_running && return 0
    [[ -e "/dev/video${LOOPBACK_ID}" ]] || return 1

    local real_cam; real_cam="$(_camera_id)"
    ffmpeg -hide_banner -loglevel error -f v4l2 -i "/dev/video${real_cam}" \
        -f v4l2 "/dev/video${LOOPBACK_ID}" &
    echo $! > "$FEEDER_PID_FILE"
    # Give ffmpeg a moment to open the real device and start writing frames
    # before the first detection read off the loopback.
    sleep 1
    _feeder_running || { rm -f "$FEEDER_PID_FILE"; return 1; }
    logger -t presence-detect "Loopback feeder started (/dev/video${real_cam} -> /dev/video${LOOPBACK_ID})"
}

_stop_feeder() {
    _feeder_running || return
    kill "$(cat "$FEEDER_PID_FILE")" 2>/dev/null
    rm -f "$FEEDER_PID_FILE"
}

# The device id handed to the Python detector: the loopback mirror whenever
# it's up, otherwise the real camera as a direct-access fallback.
_detect_device_id() {
    if _feeder_running; then
        echo "$LOOPBACK_ID"
    else
        _camera_id
    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 idle
    # lock (only if we're the one holding it) and stop mirroring the camera.
    # Also clear the presence flag — with the daemon gone, nothing is watching.
    _stop_inhibit
    _stop_feeder
    rm -f "$PRESENCE_FLAG"
    exit 0
}
# Intercept termination signals to ensure the inhibitor PID is never orphaned.
trap _cleanup SIGTERM SIGINT SIGHUP

# Epoch seconds of the last tick that saw motion; drives the GRACE_SECONDS
# window below. 0 = never seen yet, so a no-motion tick at startup releases
# immediately (nothing is held anyway).
LAST_MOTION=0

while true; do
    # Self-healing: retry the feeder every tick if it isn't up (camera
    # unplugged/replugged, ffmpeg crashed, module loaded after daemon start).
    _feeder_running || _start_feeder
    DEVICE="$(_detect_device_id)"
    # Run the OpenCV motion detector; stderr suppressed to keep the journal clean.
    python3 "$PYTHON_DETECT" "$DEVICE" 2>/dev/null
    rc=$?
    now="$(date +%s)"
    case $rc in
        # Motion: refresh the grace clock and mark presence. PRESENCE_FLAG is
        # updated *independently* of the inhibit lock — it reflects "camera sees
        # you" even during a manual caffeine session (where _start_inhibit is a
        # no-op because the lock is already held).
        0) LAST_MOTION="$now"; touch "$PRESENCE_FLAG"; _start_inhibit ;;
        # No motion: only actually release once we've been still for the whole
        # grace window. Within it, leave the flag/lock exactly as they were so a
        # brief pause in movement doesn't flicker presence off.
        1) if (( now - LAST_MOTION >= GRACE_SECONDS )); then
               rm -f "$PRESENCE_FLAG"; _stop_inhibit
           fi ;;
        # rc=2 = camera busy/unavailable — silently skip, state unchanged
    esac
    sleep "$(_next_interval)"
done